Primary producers
Primary consumers (herbivores)
Secondary consumers (carnivores)
Tertiary consumers
The way biomass is distributed among trophic levels in the food web provides
clues to the efficiency of energy transfer through the ecosystem .
Note: this is a static depiction-it does not provide information on how fast
biomass turns over within each trophic level.
Commonly used determinations
of bacterial biomass
(mg C L-1)
• Cell abundance and cell volume (by
microscopy and more recently by flow
cytometry)
“Typical” bacterial cell densities
in aquatic ecosystems
Habitat
Estuaries
Cell density
(cells ml-1)
>5 x 106
Coastal (near shore)
1-5 x 106
Open Ocean
0.5-1 x 106
Deep Sea
<0.01 x 106
Remember that cell abundance ≠
carbon biomass
Size matters
Fagerbakke et al. (1996)
Determinations of cell volume
•
Need to measure 300-400
cells per sample for
statistical precision.
•
Assumes spherical cells
•
No standards; calibrated
with microspheres that likely
have different fluorescence
and shape characteristics
than bacterial cells
•
Image analyses is highly
dependent on edge
determination
Bacterial cell volumes by depth in several ocean
ecosystems
Depth (m)
Polar Front
Ross Sea
EQ-PAC (March)
EQ-PAC (September)
Location
Cell
Volume
(µm3)
Study
0
Escherichia
coli
0.9-2.4
Watson et
al. (1977)
200
Vibrio
natriegens
0.9-3.5
Fagerbakke
et al. (1996)
400
Sargasso
Sea
0.03-0.06
Carlson et
al. (1996)
600
North Sea
0.1-0.4
Fagerbakke
et al. (1996)
800
Ross Sea,
Antarctica
0.04-0.15
Ducklow et
al. (2001)
Sargasso
Sea
0.03-0.1
Gundersen
et al. (2002)
1000
2 3 4 5 6 7 8
0.0 0.0 0.0 0.0 0.0 0.0 0.0
Cell Volume
(µm3 cell-1)
Cell abundance (cells ml-1)
x Cell volume (µm3 cell-1) =
Biovolume (µm3 ml-1)
Why are oceanic bacteria so small?
• Inactive or dormant – low growth rates?
• High S/V advantageous for growth in
limiting nutrient media?
• Predation defense?
Consider a spherical microbe:
SA=
4πr2
r = 0.50 µm
V= 4/3 πr3
r = 1.0
SA = 3.1 µm2
V = 0.52 µm3
SA : V = 15.7
SA = 12.6 µm2
V = 4.2 µm3
SA : V = 3.0
The smaller the cell, the larger the SA : V
Greater SA : V increases number of transport sites
(per unit biomass), and may allow smaller cells to out compete
larger cells under limiting nutrient conditions.
To get from abundance and volume to
biomass requires conversion factors
Location
Density
(fg C
µm-3)
C content
(fg C cell-1)
Method
Study
E. coli
126-132
109-323
CHN
analyses
Watson et
al. (1977)
E. coli, P.
putida, B.
subtilius
160-930
CHN
analyses
Bratbak et
al. (1985)
Natural
plankton
280
20
CHN
Lee and
Fuhrman
(1987)
Station
ALOHA
3.5-8.8
Biomass
constraint
Christian
and Karl
(1994)
Southern
Ocean
12
Direct
Measure
by TOC
Fukuda et
al. (1998)
Ross Sea,
Antarctica
77-165
7-13
C mass
balance
Carlson et
al. (1999)
Sargasso
Sea
148
4-9
X-Ray
micro.
Gunderson
et al. (2002)
Most studies
converge
~10 fg C cell-1
45oN
25oN
5oS
Buck et al. (1996)
Generalized relationships between
bacterial population size and chlorophyll
Slope = 0.78 - 0.84
Slope = 0.52
Bird and Kalff (1984)
Cole et al. (1988)
Relationships between bacterial abundance and Chl a across diverse aquatic
ecosystems.
1) At low concentrations of Chl a, bacterial abundance becomes increasingly
important.
2) Along gradients in productivity, increases in Chl are proportionality greater than
bacterial abundance.
Bacterial and phytoplankton biomass in
the upper ocean in various ecosystems
Location
Bacterial Biomass
(mg C m-2)
Phytoplankton
Biomass
(mg C m-2)
Bact:Phyto
Sargasso Sea
659
573
1.2
North Atlantic
500
4500
0.11
Subarctic North
Pacific
571
447
1.2
Station ALOHA
750
447
1.7
Arabian Sea
724
1248
0.58
Average
641
1443
0.96
Stand dev.
105
1740
0.62
CV (%)
16%
120%
64%
Bacterial biomass constitutes a large pool of living carbon in marine
ecosystems. Note greater variation between ecosystems in phytoplankton
biomass relative to bacterial biomass.
Bacterial biomass calculated assuming 10 fg C cell-1; phytoplankton biomass
calculated assuming C:Chl of 50:1.
Plankton biomass relationships
• Over wide range of aquatic ecosystems, bacterial
biomass appears correlated with phytoplankton
biomass.
• Bacterial biomass is generally less variable than
phytoplankton biomass across large gradients in
productivity.
• With increasing oligotrophy, bacterial biomass
becomes a proportionality larger fraction of total
plankton biomass.
An inverted food web in low
productivity ecosystems?
Primary producers
Primary consumers (Bacteria)
Secondary consumers (microzooplankton)
Tertiary consumers (mesozooplankton)
Total carbon consumed by bacteria =
(biomass x turnover rate) ⁄ growth efficiency
We need information on turnover and efficiency
To determine the importance of microbes to
ocean food webs we need to:
1. Quantify population size and mass
• Biomass, abundance, cell sizes
2. Quantify growth rates and production
•
Biomass production, respiration,
cell division, and turnover
3. Understand factors limiting microbial growth
•
Metabolic flexibility, physiology
What is it we want to know?
• Biomass (biogenic carbon, trophic
linkages)
• Carbon fluxes (production, respiration,
DOC utilization rates)
• Rates of growth (cell physiology,
nutritional status)
The Microbial Loop :
A central theme in marine microplankton ecology
Classic Food web
Phytoplankton
Inorganic
Nutrients
A simplified
depiction of the
microbial loop
Heterotrophic bacteria
Herbivores
Higher trophic levels
(zooplankton, fish, etc.)
Dissolved
organic
matter
Protozoa
Growth in a closed system
Closed system; variable
growth rate – cells are
inoculated into media and
grow until resources are
depleted
(logistic growth model).
Cell abundance
(cells ml-1)
300
250
200
Exponential
growth phase
150
100
50
ln cell abundance
Calculating growth rates from a population
dividing exponentially
Nt
(cells or
biomass)
0
1
3
1
2
2
2
4
3
8
4
16
5
32
6
64
7
128
8
256
µ= 0.693 d-1
d = 1 day
5
4
1
0
0
0 1 2 3 4 5 6 7 8
Time (hours or days)
•
T
(hours or
days)
6
From an exponentially growing
population the specific growth
rate (µ) can be derived from:
dN/dt = µN
Nt = Noeµt
or alternatively:
µ = (ln Nt-ln No) / t
µ has units of time-1
0
1
2
3
4
5
6
7
Time (hours or days)
8
Doubling time (d) is the time required
for the population to increase by 100%;
it is related to µ by:
Nt = Noeµd
Nt/No = eµd = 2
d = ln 2/µ
d has units of days or hours.
Growth in a chemostat
Seawater
Media
Air Inlet
Sampling port
Open system: constant
supply of limiting
nutrients;
growth rate is
determined by the rate
that a limiting nutrient is
added or removed from
the system. Typically
use an exponential
model to model growth
dynamics.
Is the ocean more
like a batch culture
or a chemostat?
Overflow
Growth chamber
Measuring the rate of growth by
natural assemblages of plankton
is complicated…
• Most direct method would be to
measure changes in biomass
over time.
Why is that difficult for naturally
occuring plankton?
In natural seawater sample….
•Mixed assemblage of microbes with
variable growth rates.
•Growth is often balanced by loss
(predation or disease), this no net
change in biomass with time.
Biomass Production
• Production is defined as the rate that new biomass is
synthesized.
• Production is mathematically related to biomass and
growth as:
P = µB
µ = specific growth rate (time-1)
B = biomass (mg C L-1)
• **Note that µ = P/B
• Thus, P has units of mg C L-1 d-1
• Two main forms of biomass production:
– Primary production: is the rate of biomass
synthesis via reduction of CO2; in the ocean mostly
controlled by the growth and biomass of
photosynthetic organisms.
– Secondary production: formation of biomass via
assimilation of organic matter; controlled by growth
and biomass of chemoheterotrophs (heterotrophic
bacteria, zooplankton, etc.)
Photosynthesis
• Absorption of light energy by pigments or
photoproteins (light antenna). Energy excites e- in the
antenna and this energy is passed to the
photosynthetic reaction centers via the flow of
electrons. This process creates reducing power
(NADPH) and chemical energy (ATP).
• Energy and reducing power gained from light
harvesting are used to reduce CO2 to organic matter
(dark reactions).
Photosynthesis
CO2
ATP,
NADPH
H2O
O2
Sunlight
CO2 + 2H2O
CH2O + O2 + H2O + heat
3 mol ATP and 2 mol NADPH are consumed for every 1 mol CO2 fixed.
Inorganic carbon pools in
the sea:
CO2 : carbon dioxide
H2CO3 : carbonic acid
HCO3- : bicarbonate
CO32- : carbonate
CO2 ↔ H2CO3 ↔ HCO3- ↔ CO320.5%
89%
10.5%
Proportion of carbon species in seawater
2 Key enzymes for
phytoplankton photosynthesis
• RUBISCO (1,5-bisphosphate
carboxylase/oxygenase)-key enzyme in the
Calvin-Benson cycle, incorporates CO2 into 3phosphoglycerate. Most abundant protein on
Earth.
• Carbonic anhydrase: converts bicarbonate to
CO2, and vice versa. Most marine phytoplankton
transport bicarbonate and carbonic anhydrase
dehydrates to CO2 intracellularly near
RUBISCO.
Net and Gross Photosynthesis
Sunlight
CO2 + 2H2O
CH2O + O2 + H2O + heat
•
Net photosynthesis (PN): net organic carbon production in the light.
Rate of photosynthesis excluding material lost to respiration in the
light (RL).
PN = PG - RL
•
Gross photosynthesis (PG): The total amount of light energy
converted into biochemical energy.
PG = PN + RL
•
Relatively straightforward to measure net or gross photosynthesis in
pure cultures, but the oceans contain many organisms that
contribute to respiration (e.g., bacteria, zooplankton).
Primary Production
• Gross Primary Production: The rate of organic
carbon production via the reduction of CO2; in the ocean
predominately due to photosynthesis.
• Net Primary Production: Gross primary production,
less photoautotrophic respiration (RA), integrated over
time and depth.
NPP = PN – RA
• Net Community Production: Gross primary
production, less all autotrophic and heterotrophic losses
due to respiration (RA+H).
NCP = PG – RA+H
• Units for all measures of production: mg C m-2 d-1
Trapezoidal integration
0
Depth (m)
50
100
150
Depth (m)
Production
(µg C L-1 d-1)
5
6.5
25
6.4
45
5.0
75
3.0
5-75 m Int.
363 mg C m-2 d-1
200
0
1
14
2
3
4
5
6
-1
7
-1
C-assimialtion rate (µg C L d )
Area of trapezoid = Height * avg. base
[(25 m -5 m) * (6.5 mg C m-3 d-1 + 6.4 mg C m-3 d-1)/2] = 129 mg C m-2 d-1
[(45 m -25 m) * (6.4 mg C m-3 d-1 + 5.0 mg C m-3 d-1)/2] = 114 mg C m-2 d-1
[(75 m -45 m) * (5.0 mg C m-3 d-1 + 3.0 mg C m-3 d-1)/2] = 120 mg C m-2 d-1
Sum 5-75 m = 363 mg C m-2 d-1
Primary productivity in the sea
Net Photosynthesis
Compensation depth (Pcell = Rcell)
100
150
200
250
Respiration
Depth (m)
50
∫
Z
0
1/p dP/dt >0
Critical depth (Pwater = Rwater)
∫
Z
0
1/p dP/dt <0
The Spring Bloom
Sverdrup (1953)
Critical depth
Mixed layer
Winter mixing
introduces nutrients
to the upper ocean;
seasonal increases in
irradiance results in
deepening of the
critical depth and
shoaling of the mixed
layer. The result: net
accumulation of
biomass.
Phytoplankton biomass can only accumulate
(production exceeds respiration) when the depth of the
mixed layer shoals above the critical depth.