ContinentalShelfResearch,Vol.4, No. 4, pp. 445 to 464, 1985.
Printedin GreatBritain.
0278-4343/85$3.00+ 0.00
O 1985PergamonPressLtd.
Bacterial g r o w t h a n d the d e c o m p o s i t i o n o f particulate o r g a n i c c a r b o n
c o l l e c t e d in s e d i m e n t traps
HUGH W . DUCKLOW,*~" SUZANNE M . H I L L * t a n d WILFORD D . GARDNER*
(Received 25 January 1984; in revisedform 13 July 1984; accepted 23 July 1984)
Abstract--We have studied bacterial abundance and production in samples from sediment traps
deployed for 1 and 100 days in several areas of the shelf and slope regions of the Middle Atlantic
Bight, U.S.A. By making a series of assumptions about bacterial growth at the expense of POC in
traps, we have estimated that the turnover time of organic particles collected in traps during long
deployments is slow (mean 1500 4- 300 days), if only bacterial activity is consiaered. However the
abundance and biomass of bacteria in traps is very high, ranging from 3 to 30 x 1011 cells gC -t , Le.,
0.3 to 3% of the POC is bacterial carbon. Fifteen to 88% of the particles in traps were colonized by
bacteria, but usually about half the particles had only 0 to 1 cell attached. Growth of bacteria was
observed at all scales relevant to these trap deployments; over periods ranging from hours to weeks,
at rates of 0.01 to 0.3 d -l . In spite of slow growth, bacteria appeared to be physiologically active in
that [3 H]adenine and [3 H]thymidine were incorporated more rapidly into RNA and protein than
into DNA. Total incorporation rates were high. We conclude that even relatively old (ca. I y) POC
in sediment traps supports high levels of active bacterial biomass, but that POC decomposition is
slow, so that bacteria may not be the principal agents of POC turnover following collection.
INTRODUCTION
SEDIMENTINGand resuspended particulate matter in the marine environment consists of a
diverse collection of'large' particles including phytoplankton cells, zooplankton fecal pellets
and molts, amorphous fecal matter, organic aggregates, marine snow, and mineral matter
(BISHOP et al., 1977; Rowe and GARDNER, 1979; SHANKSand TRENT, 1980; FELLOWSet al.,
1981). Any of these particle types may be falling through the water column either for the first
time (primary particles), ingested and repackaged as new fecal material, or resuspended after
deposition on the sea floor. Because of their rapid settling rates, many of these particles are
not collected efficiently in Niskin bottles (SHEI.DONet aL, 1972; SASAKIand NISHIZAWA,
1981), and, even if collected, they often escape being sampled (GARDr~R, 1977). In the past
few years, sediment traps have been used increasingly to sample large particles in the ocean
and to estimate the rates of vertical particulate flux (REYNOLDSet al., 1980). But sediment
traps are oases of rich accumulations of particulate organic matter and elevated biological
activity in the relatively impoverished water column, and the extent to which biological
processes alter trapped particles is poorly understood (GARDNERet aL, 1983).
Most of the aforementioned particle types can be colonized by bacteria and could support
bacterial activity. Bacteria are potentially major agents of mineralization of particulate
*Lamont-Doherty Geological Observatory of Columbia University, Palisades, NY 10964, U.S.A.
~"Present address: Horn Point Environmental Laboratory, Box 775, Cambridge, MD 21613, U.S.A.
445
446
H . W . DUCKLOWet al.
carbon and nitrogen (POC, PON), but their importance as mineralizers relative to
protozoans, micro- and macrozooplankton is still not established (JOHANNES, 1965). Given
the diversity of particle types, sizes, ages, and compositions, we expect a corresponding complexity of bacterial roles in decomposing particles. Investigations of bacterial relationships
with sedimenting particles have only recently been undertaken using modern techniques
(ITURRIAGA, 1979; DUCKLOWet a/., 1982; KARLand KNAUEg, 1984). Basic data on the
bacteriology of materials collected in sediment traps is required for two reasons: to
understand the fate of POC/N in traps, and to clarify the role of bacteria in decomposing
particles in traps. Such data may also be useful for estimating the rates at which particles
decompose as they settle through the water column. In this paper we report on patterns of
colonization and growth of bacteria on particles and trap surfaces from a series of trap
deployments in the Middle Atlantic Bight (MAB). In particular, we will present data on
bacterial abundance changes and nucleic acid synthesis in materials from floating sediment
traps deployed for 24 h and moored traps deployed for 3 to 6 months.
It is not the purpose of this report to relate bacteriological parameters of the different
collections to ambient conditions at each site. Rather we wish to demonstrate some of the
variability in bacterial abundance and activity in these sediment trap collections in order to
suggest preliminary answers to the questions: does sedimenting particulate matter support
bacterial growth? and, what is the role of bacteria in the turnover of this material? It is
difficult if not impossible to observe the bacterial role in decomposing sedimenting POC
directly. Our approach as described below will he to measure the abundance and apparent
growth rates of bacteria associated with the trap contents, and then use these results in a
simple model of POC decomposition to estimate POC turnover times due to bacterial
activity. By comparing these extrapolated results with observations of total POC turnover
times, we hope to learn about the role of bacteria and also how to construct better models in
the future.
M A T E R I A L S AND METHODS
All experiments and measucei~ents were made using samples of materials recovered from
moored and floating sediment traps deployed in the Middle A t l a n t i c B ~ t during I980 to
1982 (Table 1). The floating s e d ~ n t traps and M4 moored traps ~able 1) were PVC
cylinders similar to those described by ROWEand GARDNER(1979), The other moored traps
(M 1 to M3, Table 1) were butyrate cylinders 7.6 × 38 cm with 1 cm wide by 5 cm deep
baffles in the top. The samples from floating trap deployments were processed aboard R.V.
Knorr within 2 to 4 h of recovery, as described more fully in DucgLow et al. (1982).
Samples recovered at sea from moored traps were returned on ice to the laboratory at
Lamont in plastic jars and stored in the dark at 5 °C, until further processi~ or subsampling.
Subsamples from all traps were preserved in cell-free formalin immediately following
recovery (floating traps and M4) or immediately following the arrival onice at Lamont (M1
to M3), for subsequent enumeration of pa~cles and bacteria b y e p l f l u o r e ~ m i c r o s c o p y
and acridine-orange direct counting (AODC; HonmEet al., 1977).
Determinations of total particulate matter in the traps were made by splitting, drying and
weighing of the trap contents as described in ROWEand GARDNER(I979). POC determinations were made using Perkin-Elmer Model 120B CHN analyzers at Brookhaven National
Laboratories (floating traps) a n d a t Lamont (M1 to M3). Particles in the traps were
enumerated by egifluorescence microscopy and acridine-orange (AO)stained samples. We
447
Bacterial growth on POC in sediment traps
Table 1. Details of sediment trap samples used in this study
Sample
Recovery
date
Deployment
time
(days)
Latitude °N
Longitude °W
Bottom
depth
Trap
depth
Temperature
at trap(s)
(m)
(m)
(°C)
i
FI
3/10/81
F2
3/11/81
F3
3/17/81
/
/
1
1
1
40020.7 '
73053.6 '
40 ° 14.8'
73 °55.5'
40 ° 19.8'
25
10
3
20
10
3
25
10
3
112
100
112
92
112
82
108
96
108
88
108
88
1005
655
5
1290
1075
5
73053.4 '
M la
10/2/81
111
M lb
10/2/81
111
M lc
10/2/81
111
M2a
10/2/81
112
M2b
10/2/81
112
M2c
10/2/81
112
M3
9/28/81
166
M4
11/5/80
99
38 o 10.0'
73054.7 '
38 ° 10.0'
73054.7 '
38 ° 10.0'
73054.7 '
38° 18.1'
73046.3 '
38 ° 18.1'
73 °46.3'
38 ° 18.1'
73046.3 '
38005.2 '
73047.2 '
38°59.1 '
72031.5 '
7-9 (deployment)
12-13 (recovery)
6-9 (deployment)
11-12 (recovery)
enumerated any particles which stained translucent or opaque green, red, or orange with AO,
but counted bacteria separately. Bacteria were distinguished from particles by morphology
and intensity of staining. Thus 'particles' as discussed below included mineral particles,
amorphous organic matter, fecal pellets, phytoplankton cells and animal remains (molts,
carcasses, etc.) which were visible in the stained samples, but not bacteria.
Attached and free-living bacteria were enumerated separately. Bacteria were counted as
'attached' if they were seen in close contact with a stained 'particle'. Cells near, but not
obviously in contact with visible particles were counted as free cells (KmcH~AN and
MITCHELL, 1982; DUCKLOWetal., 1982). Free-living cells enter the trap when it fills following
deployment, and by detachment from particles, or perhaps as a result of particle solubilization. We distinguish between free and attached cells in order to distinguish their respective
importance in decomposing POC, using the model presented below.
Several traps from deployment M3 (Table 1) were sampled in such a way that bacteria in
the bulk contents and the bacteria colonizing the trap walls could be enumerated separately.
Traps sat on deck for 0.5 to 1 h following recovery to allow particles to settle. The supernatant water was discarded and the samples were drained into plastic jars. The mucus-like
material fouling the walls of the trap was gently scraped into a second jar. A control trap
(M3) was deployed upside down to prevent particle collection; only the wall growth from this
trap was collected. In these samples, free and attached bacteria were categorized as described
above, even though free bacteria were contained in the mucus matrix.
The contents of the six traps from deployments M 1 to M2 were incubated without shaking
in plastic jars in the dark at 5°C following recovery and return to Lamont. This procedure
448
H.W. DUCKLOWet al.
simulated storage in traps in situ. Subsamples from each jar were removed at intervals of I to
4 weeks following recovery for bacterial enumeration. The contents of traps from M4 were
also stored for six months, then processed as described below.
Besides following changes in bacterial abundance over time intervals ranging from hours
to months in incubated samples, we also estimated bacterial activity from the rates of
incorporation of the nucleic acid precursors [methyl-3H]thymidine and [2.3 H]adenine into
DNA and RNA in short-term assays. The procedures described in FUI-mMAN and AZAM
(1980) and in KARL et al. (1981) were followed exactly for [3H]thymidine or [3H] adenine
incorporation, respectively. The specific incorporation of each precursor into DNA, RNA,
and residue (protein) was determined by the sequential extraction procedure of KARL (1982).
To examine the coupling between cell division and the incorporation of labelled nucleic
acid precursors into macromolecules, the following experiment was performed on samples
F2 and M4. The experiment with sample F2 was carried out at the in situ temperature (3°C)
immediately following recovery of the sample. The experiment with sample M4 was performed at 15°C, following six months of storage at 5°C, in order to test the potential of trap
contents to support bacterial activity after prolonged storage under in situ conditions.
Samples were diluted by 1:3 (F2) or 1:4 (M4) with bacteria-free water from the same site,
and then incubated with gentle shaking for 60 h (F2) and 10 h (M4). Periodically samples
were withdrawn for bacterial counts, and for short-term (30 min) precursor incorporation
assays. This experiment can be used to estimate growth rates o f the precursor-utilizing
bacteria in the incubated samples (KmCHMANet aL, 1982), though we have not done so in the
present paper.
The radioactivity in all samples was assayed in 7 ml glass mini LSC vials with Aquasol II
(New England Nuclear) on a Beckman LSI00 liquid scintillation counter. Quenching was
estimated by the external standard ratio using a quenched [3 H]toluene standard series in
glass minivials.
D a t a analysis
All data reported are means of replicate determinations +S.E., unless noted otherwise.
Some of the data on the floating sediment trap deployments were presented in DOCKLOW
et aL (1982) and are included here for comparison with the other data.
Bacterial growth rates were calculated only from increases in cell numbers in incubated
samples by linear regression of in N vs t, where N is the abundance of cells at time t (days).
From these rate data, bacterial production was calculated as
P = gNBc,
(1)
where P is the rate of production (Bg C 1-1 dq), B is the specific growth rate (day-1 ), Nis cell
numbers (cells 1-1), and Bc is the mean mass of the bacteria (here, 1 × 10 -14 g C cell-l , or
10 fgC cell-l). Here we assume that the growth rate we observed in incubations following
trap recovery is a good estimate of the mean rate in the trap during the deployment. This is
likely to be a poor assumption in the ease of long deployments. Nonetheless the results
should provide some indication at least of the scale of the processes revolved.
If conversion efficiency of the bacteria (gC biomass produced) (gC substrate consumed) -1
is known, turnover times for the POC in the traps can be calculated from
T-
POC
D
(2)
449
Bacterial growth on POC in sediment traps
where T is the decomposition turnover time (days), D is the decomposition rate (ttg
C 1-j d-a) and POC is the concentration of POC in the sample (ttg C 1-]). D is calculated
from
D --
P(I
~
-
e)
(3)
,
where P is given in equation (1) above, and e is the conversion efficiency. For the calculations
presented below we have made the conservative assumption that only the attached cells
(defined above) decompose particles, so the rates P and D are scaled down by the proportion
of attached cells given below. This and other assumptions are treated in more detail in the
discussion section.
RESULTS
The sediment trap collections of particulate matter used in these studies were obtained
from varying depths, temperature, and distance from shore during different seasons (Table 1,
Fig. 1). Collections were made over time scales of hours (collections F 1 to F3) and months
77°39 °
76"40:
75"41 •
74" 42 =
.NEW.
..!
•
..
. . .
III
•
°
° . ; q
..•
CAPE MAY
S/
M I-3
25
0
!...... [
25
t
50
,
75
~
I00
i
KILOMETERS
Fig. 1. The continental shelf and slope of the Middle Atlantic Bight, showing areas where sediment
traps were deployed. MI to M3 in the Baltimore Canyon area; F1 to F3 near the Hudson plume;
M4 on the continental slope. See text and Table I for details of deployments.
450
H . W . DUCKLOWet al.
Table 2.
Sample
Estimates o f total particulate flux and POC f l u x into sediment traps
used in this study
Total flux
(rag m -2 d -I)
Org. C.
(%)
Organic carbon flux
(mgC m -2 d -1)
FI*
F2*
F3*
12670
3580
2540
5.8
8.8
18.0
735
315
457
Mla
Mlb
Mlc
1239
266
105
7.0
7.4
86
8
M2a
M2b
M2c
768
107
111
6.0
6.1
-
46
7
-
M3
1127
3.9
43
M4
1526
* Total fluxes as given in DUCKLOW el al. (1982). Organic carbon fluxes are
based on %POC values from G. Rowe for these samples.
(moored collections) (Table 1). The estimates of total and POC flux varied by factors of
about 10 each. There was equal variability in the flux within and between deployments (Table
2). These estimates are similar to other fluxes sampled in the New York and MAB area
(Rows and GARDNF.R, 1979; FAtXOWSKIet al., 1983).
Flux estimates were greater in traps nearer to the bottom (Mla, M2a) and in shallower
water (F 1 to F3), indicating the importance of resuapension of bottom materials in the lower
water column. The sample from trap M3 may have had a low fraction of organic carbon
(Table 2) because o f dilution from organic-poor sediments resusimaded from the sea floor
and advected to the trap location. The variability in the POC data suggest a diversity of
origins and fates of labile POC in sediment traps.
Bacterial abundance and colonization of particulate matter in traps
The density of bacteria in the trap samples (cells per mi of trap contents) was equal to or
higher than bacterial densities in ambient waters (Table 3; DUCKLOWet aL, 1982). Absolute
abundance levels in traps were high, ranging from 1.4 to 16.7 x 106 cells m1-1 in the bulk
contents collected in traps. When these levels were normalized to themass of total particulate
matter or to POC in the traps, values ranged from 0.8 to 331 x 10j° cells g-~, with highest
values for total cells per gram POC, and lowest for attached cells per gram total sediment
(Table 4). There were similar amounts of variability in all six parameters (total cells 1-1 or g-~
total particulates or POC; and attached cells 1-1 or g-I total particulates or POC) with
coefficients of variation of 0.9 to 1.0 for the data in Table 4, and 1.6 to 1.9 for the data in
Table 3.
These bacterial densities are high (see Discussion and compare levels with values in Table
8) but bacterial colonization of particles in the traps was sparse. In the bulk contents of traps,
particles were colonized by an average of 2.4 to 9:8 cells each (Table 3), similar to values for
suspended particles in the water column (DUcKLOwand KIRCHMAN, 1983). Although a wide
range for % attached cells was observed (15 to 88%, Table 3), 6 4 % of the particles in one set
Bacterial growth on POC in sediment traps
Table3.
451
Patterns of bacterial colonization of sedtment trap contents and surfaces
Average number of
Total cells
% cellsattached
attached ceilsper
(10e cells ml-t)
to particles
particle
FI
F2
F3
2.9
1.5
1.4
66
24
15
3.0
2.4
3.6
Mla
Mlb
Mlc
8.0
5.5
4.6
59
54
15
4.5
9.8
1.5
M2a
M2b
M2c
7.2
5.3
6.2
43
42
48
4.1
3.6
4.2
16.7
92.8
46.2
88
64
20
4.8
21.8
13
0.3-1.5
<20
<4
Deployment
M3
M3, wall washings
M3, wall washings from
inverted trap
Mean ambient shelf water*
* See, for example, I~CKLOWand KIRCHMAN(1983).
Table 4. Bacterial densities in sediment trap contents expressed as cells per gram total particulate matter or per
gram POC only
Attached cells only
Total cells
(10' cells g-t)
(109 cells gPOC -t)
(10~ cells g-I)
(109 cells gPOC -l)
33
45
55
570
510
310
22
11
8
380
130
40
Mla
Mlc
25
180
360
2430
14
27
210
360
M2a
M2b
40
200
670
3310
17
85
280
1390
M3
42
1070
37
950
Mean
CV
78
0.9
1154
280
0.9
468
1.0
Deployment
FI
F2
F3
1.0
o f samples had 0 o r 1 cell per particle, and only 10 to 20% o f the particles h a d 10 o r m o r e
cells (Fig. 2). These d a t a are based on a total o f 3530 particles in 36 samples from the M 2
time series (Figs 3 and 4).
W a l l growth in traps
One partial exception to these patterns concerns the slime scraped from the walls o f traps in
mooring M3. T r a p walls were occasionally coated with layers o f slime o f varying thickness,
which h a d high densities o f bacterial cells and high numbers o f cells per particle (Table 3).
H.W. DUCKLOW et al.
452
W
t.--
40
r,tt/
2
!
W
~
2
_J
n= 1071
M2c
40
Z
I.t,I
20
~J
OI
0
v
I
12-516.-10!11-49~'501
CELLS PER PARTICLE
Fig, 2. Frequency of particle colonization by bacteriain incubated sedimenttrapcontents.
~
6.6
0~
6.4-]
--
/s
I t . _ _ _ e/
6.2
6.0
'
0
¢:1
12
o
b
c
20
3o
•
•
I
i
2O
f
TOTAL C E L L S ~
~"t~
ATTACHED C~LLS . . . .
20
'
~o
'
~ ~ ~ ~ ~411
I'
oo
I
~o
DAYS
Fig. 3. Time courses of total bacterial and attached bacterial numbersin incubatedcontents from
traps M2a,b,c.
The data for attached cells refer only to particles (defmed in methods) held within the slime
matrix, not to the total number of cells in the slime; however these m a y also be considered to
be attached cells in the sense that they were not free cells suspended in water, like cells in
interstitial spaces. It is possible the dense aggregations of bacteria in the slime attached to
trap surfaces turn sediment traps into efficient reactors for P o e d e c o ~ .
We have no
data on the activity of these slimes. However slimes from seawater h ~ d s (m research vessels
have extremdy high rates of [3H]thymidine incorlx)ration (H. DUCKLOWeI al., u n p ~ d
Bacterialgrowthon POC in sedimenttraps
453
,J
4
-~)~_~__e.---e-.. "'o ....
....... ==2---=
-I
O|
"I
I
I
I
I
I
I
I
I
I
I
1
1o,, o
5.7
5.6
0
b
20 •
C
30
1 2
I
0
portl¢los ml -I
cellt partlc|e -I . . . .
•
,
,
40
1 6 1 0 1 1 1 1
80
I00
DAYS
Fig. 4. Time course of total particle concentrationand bacterial cells per particle for incubated
contentsfromtraps M2a,b,c. Materialwas incubatedwithoutshakingat 5°C.
data from R.V. Knorr, 1982). Comparison of the data from upright and inverted traps in
M3 (Table 3) suggests that the slime can attract particles and bacteria from the bulk contents. The inverted trap had lower values for all parameters even though it had copious slime.
Growth rates
The growth of bacteria was monitored in different samples over different time intervals
which were chosen to examine growth on the time scales over which sediment traps are often
deployed. Growth rates were estimated over periods of hours (M4), days (F 1 to F3) and
months (lVl1, M2). The time courses of bacterial numbers are shown in Figs 3 to 5 and 8, and
discussed in turn below.
Samples from moorings M 1 (data not shown) and M2 showed increasing numbers of total
and attached cells (Fig. 3) over the first 24 days of the experiment, and also increasing
number of cells per particle (Fig. 4). Most of the increase in total cells could be accounted for
by increases in attached cells. For example, the total and attached cells in sample M2a
increased by 2.9 and 2.6 x 106 cells ml-~ , respectively. This suggests that bacterial growth
over this time scale was sustained by the decomposition of particulate organic matter, rather
than at the expense of dissolved material in the water in the samples. Over the same 24-day
period, the total number of particles increased (Fig. 4), possibly because of fragmentation due
to biological activity. Between days 24 and 99, values of these properties stabilized, declined,
or varied irregularly. Note especially the precipitous decline in attached cells in M2a and
M2b between days 24 and 73 (Fig. 3). This may have been due to predation or autolysis but
not simply detachment, because free cells also declined.
Growth was measured over periods of 3 to 5 days in samples F 1 to F3 aboard ship
immediately following recovery. In sample F2 (Fig. 5B), growth ceased after 1.2 days. In
sample M4, we observed slow growth between 2 and 10 h of incubation (Fig. 8B). Note that
cell numbers increased and then levelled off on all 3 time scales of observation. This indicates
the importance of investigating growth processes over a variety of time scales relevant to
sediment trap deployments.
454
H.W. DUCKLOW et al.
~o~o
A
9oo
750 ~,'
soo-
~1500
I
I
i
I
40
50
I
60
I
I
I
I
I
I0
20
i
30
__. 9.zo"
o
g
--
9. l -
9.0-~
8.9 [
0
70
hours
Fig. 5. Time course of: (A) incorporation rate (not incorporation) of [3HJthymidine; and (B)
bacterial numbers in incubated contents of trap F2. The mUnlde was i n c u ~
for 48 h on ship
irn~y
following recovery, after a 24-h deployment. In this experintent, alaRge (ep. 1 !) ,ample
was incubated, and 10nd aliquots were withdrawn periodically for AODC or [*H]thymidine
incorporation assays.
Growth rates and generation (doubtinlOtimes calculated from chattBm in cell nmnbers were
generally slow, ranging from 0 to 0.33 d -1 and 2.1 to >77 days, respectively (Table 5). In
general the highest rates were found when growth was monitored over shorter time scales,
and also in the freshly captured materials from the floating traps. These samples had the
highest organic matter contents (Table 2), and were derived from a sedimenting Skeletonema
eostatum bloom following a storm (MALONEet al., I983). Growth was very slow when
Table 5.
Sample
SpeeO~c growth rates of bacteria in sediment trap contents, estimated from
dmnges in cell number
Incubation temperature
(*C)
Rate
(d-O
Generation time
(d)
F1
F2
F3
3
3
3
0.33
0.09
0.34
M la
Mlb
Mlc
5
5
5
0.009*
0*
0.015"
77
46
M2,a
M2b
M2c
5
5
5
0.012"
0.009*
0.013"
58
77
53
M4
15
* During the first 24 days (Fig. 3).
0.21
2.1
7.7
2.0
3.3
Bacterialgrowthon POC in sedimenttraps
455
integrated over longer periods (the M 1, M2 samples). These rates could be more representative of bacterial activity in traps deployed for longer periods. However the contmts of trap
M4 retained the capacity to support measurable growth over hourly time scales ¢ven after a
99-day deployment and 6 months of storage (Fig. 8B, M4). The prolonged growtbsustaining
quality of sediment trap contents is discussed with respect to macromolecular synthesis in the
next section.
Incorporation o f nucleic acid precursors and macromolecular synthesis
Bacterial activity related to cell division was also studied by monitoring the inc3rporation
of the nucleic acid precursors [3H]thymidine (3H-TdR) and [3H]adenine into macrcmolecular
fractions of sediment trap contents. In the incubation experiment with samples fror~ the floating traps, the rate of total [3H]TdR incorporation into cold, 5% trichloroacetic add (TCA)
extracts increased by a factor of 10 in 2.5 days (Fig. 5A), at a rate of 0.90 d "~. Thi2 was well
in excess of the growth rate computed from observing changes in cell numbers (0.0~ d -t , Fig.
5B and Table 5). These rates should be equal if [3H]TdR incorporation into DN,A and cell
division are closely coupled (KmcHMAN et al., 1982). There are several differexR re.sons for
this discrepancy. It is possible that in these samples, as in most seawater samples (IUHRMAN
and AZAM, 1982), [3H]TdR was incorporated mostly into DNA, but that this prccess was
not coupled to cell division over the time scale of this experiment (KIRCHMAN et ~L, 1982;
DUCgLOW and HILL, 1984). Alternatively the [3H]TdR may have been incc'porated
primarily into RNA or protein, with only small amounts going into DNA. If this ~,ere the
case, the increase in the rate of [3H]TdR incorporation into DNA alone may h~ve been
closer to the growth rate. We studied these processes in greater detail by determaing the
rates of specific incorporation of [3H]TdR and [3H]adenine into DNA, RNA, am protein
separately in the contents of trap M4.
Although the sample was more than six months old at the time of the experimnt, the
incorporation of both [3H]TdR and [3H]adenine began immediately, and boh were
approximately linear over a 60 to 90-rain incubation period (Figs 6A and 7A. Total
incorporation rates of both compounds were high (Table 6). None of the incorporatim rates
showed a tendency to saturate, even at 50 nM levels of added labelled substrate (Figs6B and
7B). This behavior suggests that even after six months of storage at low temperature, ~actetia
were not dormant, and that the sediment trap material retained the capacity to suppot acti~
Table 6. Average specbqcity of labelling by exogeneous [3H]thymidine and PH]adenine of macromoleadarfractions of sediment trap material Incubation times were 60 mtn
Rate of labeUingof fraction _+s.d.
pmol 1-~ h-t (% of total)
[~HI-Labelledsubstrate
Thymidine*
Adenine*
RNA
DNA
Protein
Total
13.1 + 1.2
(63)
3.3 + 0.7
(16)
4.3 + 1.3
(21)
20.7
(IOO)
n=6
n=7
n=8
561 + 133
(83)
n=6
82 _+20
(12)
n=9
34 + 9
(5)
n=10
* Both substrates addedto 5 nM finalconcentration.
677
(lOO)
456
H . W . DUCKLOWe t aL
25-
20-
RNA
T
/
y
:O--
~ IO-q I
o.
o. I0-
5-
5-
!
DNA
0-
I
0
30
60
,o
90
minutes
Fig.6.
~o
I
~o ~o ~o
subtrote conc., InM):
(A) Time course Of specific incorporation of [3Hlthymidine into ~ N A and RNA, for con-
tent of trap M4 followin86 monthsof storage at 5"C. (B) Effect of initial [ H~yn',idine ~centrationon incorporation into DNA and RNA for contents of M4. In th--,~,ee ~ m e n t s ,
added to each sample at zero time, but after 6 months of storage.
A
isotope was
B
RNA
P
900-
//
//
600-
~C0-
//
T.
////
i
).500!000-
/ II
°E
o..
/
300-
RNA
5500-
//
750-
500I000-
DNA
DNA
500-
x~
I
I
[
30
I
I
60
90
minutes
FiB 7.
0
I
I
I
0
substrate conc., (nMI
(A and B) as in Fig. 5 for trap sample M4, but employing [3H]adenine as substrate. In Fig.
6A, the 60-rain sample was lost.
bacterial synthesis of cellular constitutents. However these results also suggest that growth,
estimated from D N A synthesis, was slow compared to R N A or protein synthesis.
The ncorporation rate of each precursor into D N A was only 12 to 16% of its total
incorporation into cold 5% TCA insoluble cell fractions (Table 6). In the upper water column
usually 60 to 8096 of [3H]TdR in such extracts is in D N A , with only 20 to 40% in R N A and
protein (FUHRMANand AZAM, 1982). In this experiment R N A synthesis accounted for 63 to
83% of tae total incorporation, and more [3 H] TdR was incorporated into protein than D N A
in a 60-nin period (Table 6). These patterns of [3H]TdR incorporation are not consistent
with our current understanding of pathways for the incorporation of exogenous TdR
(KoRNSE~, 1980). HANSON and LOWERY(1983) also observed that in bacteria from below
1000 m u! to 8096 of [s H]TdR incorporated was in protein. We do not know the reason for
these cont-asting incorporation patterns. In another incubation experiment (trap M4) similar
457
Bacterial growth on POC in sediment traps
to those shown in Fig. 5, the incorporation rate of [3H]thymidine into DNA was essentially
constant over a 10-h incubation period (Fig. 8A), while the incorporation rate of [ 3H]thymidine
into RNA rose over the fu'st 4 h, then levelled off. These results indicate that in the samples
we have studied, the total incorporation of nucleic acid precursors into DNA, RNA and
protein are not closely coupled to cell growth (as evidenced by different slopes for the time
course of radioactivity and cell numbers) because most of the label is incorporated into RNA
and protein and not into DNA. The main significance of this observation for the present
study is that even though bacterial growth per se may be low, bacterial metabolism as a
whole may be significant. With this point in mind, we proceed to estimate the decomposition
of POC in our samples from bacterial growth rates.
Bacterial production and POC decomposition
We have calculated attached bacterial production and decomposition rates from the
growth rates listed in Table 4, the abundances of attached cells (Table 2), and mean cell
masses of 10 fgC cell-~ (equations 1 to 3). The resulting values (Table 7) are conservative for
several reasons, which are discussed below. Attached bacterial production ranged from 0.1
to 6.3 ttgC 1-1 d-~ (mean 1.2 + 0.9, n = 7), which is similar to values for attached bacteria in
the Hudson River plume at 3°C (DucKLOWand KIRCHMAN,1983). The turnover times of
POC in the traps due to in situ bacterial decomposition alone were long, averaging
1800 + 600 days (Table 7). The significance of this finding is discussed below.
DISCUSSION
Sediment traps capture a population of particles which are presumed to be relatively rare
in the water column and may not be efficiently sampled by water bottles (SHELDONet al.,
1972; SASAKIand NISHIZAWA,1981). Therefore, we do not have direct methods for sampling
these large, rapidly settling particles and measuring their decomposition rates. At present we
need to extrapolate such estimates from measurements made on collections of particles
collected in and recovered from sediment traps (DUCKLOWet al., 1982; KARLand K~AUER,
1984). The accuracy of these extrapolated estimates will be greatest when derived from traps
Table 7. Bacterial production estimates and decomposition turnover times for sediment trap POC
Attached
bacterial prod.
(p.g C i-~ d -~)
Decomposition
rate
(IJ,g C 1-l d -~)
POC
concentration
~ g C !-l)
POC
turnover time
(days)~
FI
F2
F3
6,3
0.3
0.7
57
2.7
6.3
5106
2923
4571
90
1082
726
Mla
Mlc
0.4
~1
3.8
0.9
22284
1889
5829
2100
M2a
M2b
0,4
0,2
3.6
1.8
10708
1598
2974
888
M3
1.5"
15908
1223
Sample
13
* Assumed growth rate of 0.01 d -~.
? Calculated from equation (2) assuming bacterial conversion efficiency of 1096.
458
H.W. DUCKLOWet al.
deployed for short periods. Since the rates of decay of particulate organic matter decline as
particles age (FENCHI~Land JORGENSEN, 1977) estimates from long deployments are likely to
be underestimates of the 'true' decomposition rates. Measurements made on sediment trap
collections also yield data on the related problem of particle decomposition rates inside the
traps following capture. The length of the trap deployment also is a factor influencing these
estimates. As particles age and decay, an increasingly refractory suite of organic compounds
remains in the traps, and decay rates decline (GARDNERet al., 1983). The effect of deployment time, and the interaction between vertical flux, or collection rates and varying decay
rates has been modelled previously (GARDNERel al., 1983) and will not be treated in detail
here. However, the relationship between the decay of particles in the water column, and their
decay inside traps will be discussed below.
In our study, traps were deployed for either 1 or ca. 100 days, and trap depths ranged
from 10 to 1000 m in water columns 20 to 1200 m deep. Later, we will assess decomposition
of sediment trap particle collections in two contrasting regimes: short deployments in shallow
water, and long deployments below the euphoric mixed layer.
Our studies concern particles originating and collected over the continental shelf and
upper to mid-slope region, not from the deep ocean. Particles, verticalfluxes, and associated
biological activities in open ocean areas differ qualitatively and quantitatively from shelfslope regions (EPPLEY, 1980; WALSa, 1983). For instance, in midwater depths in the open
ocean, chemoautotrophic production associated with particles near the oxygen minimum
may elevate microbial activity in traps without directly affecting decomposition of particles
(KARL and KNAUER, 1984). Our conclusions will not necessarily apply to similar oceanic
regimes.
To our knowledge, there has only been one other study in which bacterial colonization of
sediment trap particles was measured. ITURRIAGA(1979) found that at 10 to 18-m depths in
the Baltic Sea, there were 0.2 to 0.6 x 109 cells per gram of POM in traps deployed monthly
over the course o f a year. Our values are much higher, ranging from 300 to 3000 x 109 cells
g-i POC (Table 4). We can further assess bacterial colonization of large particles by comparing our data to bacterial abundances in surface sediments, which are likely to be derived in
part from deposition of the vertical flux. Some values for total bacterial abundances per gram
of sediment are presented in Table 8, for comparison with the frst column of data in Table 4.
Even the highest values in rich salt marsh sediments (50 to 70 x 109 cells g-') are only
equivalent to the lower values we measured (20 to 200 x 109 cells g-'). Since the organic
content of such sediment is likely to be around 1 to 10% (RusLEE and DORNSEIF, 1978), these
levels translate to 500 to 7000 × 109 cells g-' POC, which is similar to our corresponding
values. NowrsKV (1983) carefully sampled the interfacial sediments most likely to be similar
to falling particles, in an environment similar to some of our study areas. He found only 1 to
10 × 109 cells g-t, about an order of magnitude lower than our estimates. While such
measurements are not directly comparable, they indicate that very high numbers of bacteria
are associated with the contents o f sediment traps.
High numbers of bacteria associated with material in sediment traps suggest a great potential for rapid decay and a potentially major role for bacteria in particle decay. But our findings also suggest that this potential may not be realized, or perhaps that in most cases, our
observations missed the period of rapid bacterial decomposition (Table 7). On average, only
about 50% of the particles were colonized, by a mean of ca. 1 to 10 cells per particle (Table
3, Fig. 2). The bacterial abundance data in Tables 3 to 5 suggest that bacterial carbon is
probably a small fraction of the total POC in the traps. Using a mean value of 10-'4 gC
Bacterial growth on POC in sediment traps
Table 8.
Bacterial content of marine sediments. Unless otherwise noted, values are for surface (0 to 1 cm) of
sediments, and per gram total sediment
10~ cells g-i
0.8-22
69.7
60.1--66.3
38-54
3.3-8.8
4.2-6.0
2-100
0.3--10
0.4-0.5
1.2-2.3
0.4--2.7
1.2-1.7
1.1--9.7
0.2-0.6
0.16---0.66
16-310
459
Sediment type
Spartina marsh, North Carolina
Spartina marsh, North Carolina
Low and high marshes, Rhode River, MD
Sippewissett marsh, MA
Subtidal mud, Newport River estuary, NC
Intertidal mud flats, Nova Scotia
Subtidal sediments, Denmark
Intertidal sand flat, Nova Scotia
Shallow subtidal sediments,
Shark Bay, Australia
Stromatolite mats, Shark Bay, Australia
Sandy beaches, Baltic Sea
Seagrass sediments,
Queensland, Australia
Sediment-water interface,
Halifax Harbor. Nova Scotia, 22 m deep
Sediment trap contents in Baltic Sea
at 10and 18m, per g POM
Suspended POC, Hudson River plume
of New York Bight
Reference
RUBLEE(1982a)
RUBLEEand DORNSEIF (1978)
RUBLEE(1982b)
RUSLEE (1982b)
RUSLEE (1982b)
RUSLEE (1982b)
C AMMEN(1982)
DALE (1974)
MORIARTY (1983)
MORIARTY (1983)
MEYER-REIL et al. (1978)
MORIARTY and
POLLARD (1981 )
NOVlTSKY (1983)
ITURRIAGA(1979)
DUCKLOW and
KIRCHMAN (1983)
cell-m, data in Table 4 indicate that attached bacterial carbon is a maximum of only 3.3% of
the POC (trap M2b, Table 4). Therefore, unless the bacteria are growing very rapidly, their
impact on the POC in the traps could only be minor.
The growth rates we observed were relatively low (Table 5), though we emphasize that
when rates are measured over longer periods (e.g., traps M 1, M2) they are lower than shorter
term estimates. Nonetheless, even the faster estimates are low compared to growth rates
measured for planktonic bacteria in the water column. This result may be because bacteria in
the water column are growing on fresher, more labile material than bacteria in traps. These
rates usually average 0.5 to 2 d-~ (DucKLOW, 1983). Since the bacteria were a small fraction
of the total POC in the traps, and grew slowly, the decomposition due to bacteria was also
slow (Table 7).
Our decomposition rate estimates are subject to several uncertainties. Chief among these
are the accuracy of the growth rates, and the conversion efficiency of the bacteria. For the
estimates in Table 7, we took the rate data at face value, and assumed conversion efficiencies
of 10% (i.e., 0.1 g of biomass carbon produced per gram POC consumed). Although
planktonic bacteria utilizing dissolved compounds like amino acids and mono- or
oligosaccharides convert DOC into biomass at efficiencies of 50 to 90% (WmUAMS, 1981),
attached bacteria converting POC into biomass are probably less efficient. This is because
they must synthesize and release extracellular enzymes to degrade their substrates which are
more refractory than low molecular weight DOC. NEWELLet al. (1981) found that bacteria
growing on phytoplankton debris had conversion efficiencies averaging 10%. POM~gOY et al.
0 9 8 4 ) found that bacteria converted pelagic fecal material at 10 to 20% efficiency. It is
likely that phytoplankton debris and fecal materials make up an important fraction of the
materials in our samples. There is not a convenient direct method for estimating this
parameter in our samples. However, we observed that bacterial populations in samples M4
were synthesizing RNA and protein, but not D N A rapidly (Table 6 and Figs 6 to 8). Cells
460
H.W. DUCKLOWet al.
R~A
1214
t _-/ /x~------°'--'-~"o RNA
IO-c,--jr
-6
i ~~~=~_~-.-~
o
i
i
1
DNA
I
IO.4- B
T
o_
=_o
T
10.5-
1
i
hours
Fig. 8. Time courses of: (A) specific incorporation rates of [3H]thymidine into D N A and RNA;
and (B) bacterial numbers, for incubated contents o f trap M4. This sample was incubated for 10 h
following storage at 5°C for ca. 6 months. Each rate measurement was derived from a 30-ram
assay. This experimentis similar to that described for Fig. 5.
were active but not growing very fast. It is unlikely that bacteria in our samples were more
efficient than 1096, though they could have been less efficient. As we suggested above, some
of the growth rates we measured are likely to be underestimates, since predation in the
samples, low temperatures, and the time scales of the trap deployments and measurements
would all interact to lower the observed rates of increase in cell numbers. Finally, even free
bacteria probably contribute to particle decay in traps. For these reasons, the decomposition
turnover times presented in Table 7 are likely to be conservative. The estimates of turnover
times can be scaled down by assuming that growth rates are higher, conversion efficiencies
are lower, and that free bacteria also contribute to particle decay. For exam#c, by assuming
that in M l a , the free and attached cells all degraded POC and grew at 0.05d -t , the
decomposition turnover time is reduced to 620 days. This is still a tong time, and helps to
explain why slope sediments off the MAB have high organic carbon content (WALSH, 1983).
Other reports suggest that bacteria do grow in sediment traps and show that preservatives
inhibit bacterial growth. LEE et al. (1983) analyzed samples from a time-series sediment trap
deployed for 3 months for muramic acid, a compound unique to the cell walls of gramnegative bacteria. Muramic acid was detected in the early samples which were closed and
incubated in the trap the longest, but not in later samples which were in thetrap for a shorter
time. Because of the presence of bacteria in the older samples they assumed that sodium
azide added to the trap at the time of deployment had diffused away. They did not detect
muramic acid in other experiments where deployment time was short and no preservatives
were used (LEE and CRONIN, 1982), in a 2-week deployment with chloroform used as a preservative (LEE etaL, 1983), nor in a 61-day deployment in which sodium azide was continuously leached into the sample (LEE etal., 1983). In all these experiments, bacteria
undoubtedly entered the traps in the vertical flux, but their biomass was probably too low for
detection by the chemical method used.
Bacterial growthon POC in sedimenttraps
461
There exist several data sets for comparison with our estimates of bacterial POC
decomposition. ITUR~AGA (1979) and LORENZENet al. (1983) noted decay rates of 0.03 to
0.08 d-I for materials in traps deployed at shallow depths for short periods (i.e. turnover
times of 12 to 30 days). GARDNERet al. (1983) found that mixed zooplankton held in deep
traps for 125 days had a turnover time of 235 days (0.004 d-m). They also formulated a
model of trap collection and decay rates, and using data from LEEet al. (1983) suggested that
POC turnover times in deep traps deployed for longer periods should be on the order of 100
to 1000 days, within the range shown in Table 8.
Several lines of evidence indicate that particles in the surface layer of the ocean are more
rapidly decomposed. From observed changes in the composition of particulate amino acids
in a series of traps, LEE and CRONIN(1982) estimated that 80% of the primary production
was decomposed above 14 m in coastal waters off Peru. EEPPLEYand PETERSON(1979)
suggest that in most of the ocean 80% of production is recycled in the surface layer. If one
assumes that most primary production results in large particles (Diatoms, fecal pellets, or
scavenged aggregates) on time scales of a few days, then these findings demand that
decomposition rates must be close to 0.5 to 1.0 d-~ in the surface layer. This is because large
particles sink at rates of at least 10 m d -~, and probably as fast as 50 to 100 m d-~ (DEUSER
etal., 1981; LORENZENetal., 1983). Therefore, 80% of the large particle flux must be
recycled in the few days it takes to leave the 20 to 100 m deep surface layer. EPPLEYet al.
(1983) estimated from POC stocks and regenerated production rates (NH~-supported
primary production sustained from recycled PON) that recycling times in the Southern
California Bight were 1 0 _ 5 days.
Comparison of these two sets of estimates suggest two time scales for POC decomposition: 1 to 10 days for POC in the surface layer, and 100 to 1000 days for the remaining POC
which sinks out of the surface layer. We expect that the fraction of POC not recycled in the
first few days is less labile than the 80% which is recycled, so a reduction in decay rates as
particles fall into deeper, colder water is not surprising. LORENZENet al. (1983) have observed
lower carbon loss rates in deeper sediment traps. However, FUHRMANand AZAM(1983) have
observed that attached bacteria collected below the mixed layer have temperature optima
characteristic of surface layer planktonic (free) bacteria. They assumed that these attached
cells were derived from the surface layer on falling particles. Thus we are not yet certain of
the time scales for the transition from high to low growth and decomposition rates. Our
observations on decomposition rates are more characteristic of more slowly decomposing
POC from deep or cold water.
Our estimates for decomposition turnover times average 1500 + 350 days, with the two
extreme values removed (Table 7). We believe these may be overestimates of the true turnover times, but it is not possible to judge how conservative our values are. If we assume that
these values are realistic, we must conclude that bacteria acting alone are not the principal
agents of particle decomposition within sediment traps. The logarithmic mean of the
GARDNER et al. 0983) range is 300 days, so we suggest that bacteria alone contribute a
minimum of about 20% of the total decomposition of POC in sediment traps. It seems likely
that the main fate of POC is to be ingested rather than decomposed.
This is not an unreasonable conclusion. Since only about l0 bacterial cells colonized the
average particle (Table 3), and bacteria were only about 1% of the POC in the traps, it is
unrealistic to imagine that rather small bacteria (ca. 10fgC cell-~ ) which only sparsely
colonize large particles, are major agents of particle decomposition. In comparison, bacteria
account for about 2 to 4% of the POC in marine sediments (CAMMEtq, 1982; Rt:BI~EE, 1982b).
462
H.W. DUCKLOWet al.
Our measurements did not take into account the decomposing activities of fungi,
protozoans, and micrometazoans. Larger suspended particles like organic aggregates
(CARON et al., 1982) and marine snow from sediment traps (SILVERand ALLDREDGE, 1981)
have dense and diverse collections of these organisms. The respiratory activity of phagotrophic proto- and microzooplankton can directly mineralize ingested particles, and grazing
reduces particle sizes, thereby increasing surface areas for bacterial attachment (JOHANNES,
1965). Finally, we note that sediment traps sometimes have accumulations of gel-like
material coating trap walls, which contains high densities of bacteria and densely colonized
particles (Table 3). Our mooring experiences indicate that the abundance of slime decreases
rapidly with depth on PVC, metal and glass surfaces, and seldom occurs below ca. lO00 m.
Build-up of slime was greatly reduced in sediment traps by the use of antibiotics imbedded
in a silica gel attached inside of traps (DYMONV et al., 1981). Based on uptake rates of
14C-labelled glutamic acid, however, the biological activity in sediment in poisoned and
unpoisoned traps was similar. This slime may render sediment traps analogous to biofdmcoated stones in the trickling filters used in sewage treatment (LARIVX~RE, 1972). If such a
function is common in traps, the role of bacteria may be greater than we have suggested here.
It is clear that few unequivocal generalizations concerning POC decomposition are
warranted at present. But most of the data suggest that bacterial decomposition of large
particles and suspended POC in the upper and lower water column is stow, while total
decomposition rates may be more rapid. To resolve the processes of POC decomposition in
the water column, bacteriologists, protozoologists, and geochemists need to conduct sampling and experimental techniques with close attention to the diversity of particle types and
time and space scales on which the processes proceed. In this study we have extended several
earlier observational approaches to sediment trap collections from several locations, and
have confLrmed earlier bacteriological observations that bacterial involvement in POC
transformation appears to be minor on long time scales. This conclusion should be examined
with caution until further data become available.
Acknowledgements--Sediment trap deploymentswere supported by DOE Contracts DE-AC02,76-EV02185Eto
LDGO ~ DE-AC02-76-CH00016 to BrooldmvenNational Laboratory and by BLM Contract AA851-CTO,59
to LDGO. We are grateful to P. Biscaye, T. Malone, G. Rowe; J. Walsh and C. McCulloch for scientific
assistance, and to the officers and crews of R.V. Knorr and=R.V. Cape Henlopen for logisticalSupport. We also
thank P. Bist~ye, J. Marra and D. Kirohman for reading and commenting on the m a n ~
Hugh Ducklow and
Suzanne HiU were supported by DOE Contract DE,AC02.76-EV02185E,F for this study and W. Gardner Was
supportedbyBLMcontractAA851-CTO-59. LDGO contributionNo. 3720. UMCEES contribution No. 1567.
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