Systematics:
Carbon in Aquatic Plants
Why do we care?
• Food Web Dynamics
• Ancient [CO2]aq and pCO2 concentrations
• Cell Mechanisms (diffusion/assimilation)
in different marine environments
Why are there variations in 13C of
aquatic plants?
Water Temperature
pCO2 and [CO2]aq
Growth Rate
Cell Size and Geometry
Type of Organism
Active vs. Diffusive Inorganic C uptake
CCM (CO2 concentrating
mechanisms)
13C variances with Temperature and Latitude
Lower 13C values
found in cold,
southerly latitude
Antarctic waters
Less variability
shown in Arctic
waters
Stable carbon isotopes in marine organic matter
vary significantly over geologic time.
Cretaceous sediments are thought to have
existed in a time with elevated CO2 levels.
First study to show relationship between
phytoplankton 13C and CO2 concentrations.
Temperature vs. Latitude and Temperature vs. pCO2
pCO2 has highest
variability at coldest
temperatures;
however high pCO2
found at all temps
Colder at
higher
latitudes
13C vs. [CO2]aq
[CO2]aq =  x pCO2
[CO2]aq is dissolved
CO2 concentration;
 is solubility constant
(a function of temp)
Greater fractionation
at higher [CO2]aq and
colder temps
Cretaceous [CO2 ]aq
[CO2]aq =  x pCO2
Today [CO2]aq ~ 20 M @ T = -2 to +2°C
To calculate Cretaceous atmospheric CO2 concentrations:
1) Low productivity Cretaceous ocean
2) 32°C Cretaceous ocean
3) Modern Antarctic ≈ Cretaceous Atlantic 13C (low)
4) Similar 13C means similar [CO2]aq
Plug and chug!
Low latitude Cretaceous ocean >800 pmv
2 - 13 x higher than prior estimates
Why are there variations in 13C of
aquatic plants?
Water Temperature
pCO2 and [CO2]aq
Growth Rate
Type of Organism
Cell Size and Geometry
Active vs Diffusive Inorganic C uptake
CCM (CO2 concentrating
mechanisms)
Cultured diatom to test
1) growth rate
2) CO2 variability.
Measure p (aka isotopic
discrimination factor)
Phaeodactylum tricornutum
p = 1000(e-p)/(1000+p)
p = 1000(-1)
CO2 (g)
CO2 (aq)
Dissolution
(Henry’s law,
T dependent)
CO2 (aq) + H2O
Rubisco + -carboxylase carboxylations
εp = 25-28‰ when growth rate  0
Equilibrium
εHCO3/CO2 = +9‰ @ 25°C
H2CO3
H+ + HCO3-
Growth Rate vs. Fractionation
Low CO2 =
Faster growth
rates =
Lower p
Remember: Rubisco
+ -carboxylase
carboxylations
εp = 25-28‰ when
growth rate  0
Predicted growth rate
based off [CO2]aq to be
0.58 d-1.
That is almost identical
to mean values in the
Eq. Pacific (0.585 d-1).
Mid-range p values
suggest that plankton
are not actively
transporting carbon
(unless <10mol CO2)
“Cell size effects may change slope of p vs /[CO2]aq
sufficiently to invalidate growth rates determined from p
and [CO2]aq, but these cases are likely to be the
exception rather than the rule.”
Cell Volume of diatom
in this study = 100m3
Average plankton has
diameter = 1 m
Hmmm..is Cell Size really not an issue?
Why are there variations in 13C of
aquatic plants?
Water Temperature
pCO2 and [CO2]aq
Growth Rate
Type of Organism
Cell Size and Geometry
Active vs Diffusive Inorganic C uptake
CCM (CO2 concentrating
mechanisms)
Cell Size effects on p under variable growth rates
Max (25‰)
fractionation
associated with
Rubisco and carboxylases at
low grow rate or
high pCO2
2.4 SA/V
4.4 SA/V
1.1 SA/V
0.2 SA/V
Cell size (and
shape) influence
p, with great
impacts on large
and/or round
cells.
What’s up with
Synechococcus?
Cell Size effects on p under variable growth rates
For eukaryotes, can
scale V/SA and all
fall on a single
relationship.
2.4 SA/V
QuickTime™ and a
decompressor
are needed to see this picture. 1.1 SA/V
0.2 SA/V
Conclude cells assimilate carbon by diffusive and ACTIVE
uptake or conversion of bicarbonate to CO2
To understand C isotope fractionation in marine phytoplankton must know:
1) f
2) Growth rate
3) [CO2]aq
4) Cellular carbon-to-surface area ratio (or volume-to-surface ratio)
εp is greater for small, slow-growing, high surface/volume
Such algae have low δ13C values
εp is smaller for large, fast growing, low surface/volume
Such algae have high δ13C values
Onshore-Offshore isotope Gradients:
For those who love the food webs, this explains the difference in δ13C
values from coastal to offshore waters.
Plankton in upwelling zones grow faster and tend to be bigger. Plankton in
offshore regions are smaller and grow slower. The differences can be 2
to 3‰, with lower values offshore. This happens despite the fact that
upwelling is bringing up 13C-depleted water.
Why are there variations in 13C of
aquatic plants?
Water Temperature
pCO2 and [CO2]aq
Growth Rate
Type of Organism
Cell Size and Geometry
Active vs Diffusive Inorganic C uptake
CCM (CO2 concentrating
mechanisms)
C3 vs. C4 photosynthesis: C4 in the ocean
Diatoms growing in low
CO2 conditions have
enriched 13C values possibly undergo C4
assimilation.
Increase in PEP with low CO2 or
Low Zn (≈low carbonic anhydrase)
C4 compound malate:
70% after 15 sec
and 25% after 2 hr
in low Zn conditions
sugars
Malate is being
decarboxylated and
released CO2 is fixed by
Rubisco to form sugars and
phosphoglyceric acid
(PGA)
malate
PGA
Diffusion or Active Uptake in C4 plankton?
Active HCO3 uptake (PEP and CA activity)
rather than passively diffusing dissolved
CO2(aq) results in higher 13C values (-10‰)
These values found in diatoms
during the Mesozoic…before C4
found in terrestrial land plants
Active HCO3 uptake in this coastal, upwelling region
Monterey Bay lower p than
global, Peru diatoms even
lower.
Attributed to CO2 concentrating
mechanisms.
This mechanism is not always
restricted to diatoms.
Moving on from phytoplankton to coastal
macroalgae and seagrasses
MAJOR review paper (super wordy yet not very synthetic)
13C differences on large data set
565 species assessed!
Low 13C values (<-30‰) mainly subtidal red macroalgae
High 13C values (>-10‰) mainly green macroalgae and
seagrasses
Low 13C values (<-30‰) mainly subtidal red macroalgae
(HIGHER p)
• rely on diffusive CO2 supply to Rubisco
• conversion of photosynthate to lipids; more negative
13C inputs (terr); low photon flux densities
• lack of pyrenoids result in no CO2 concentrating
mechanisms
• C4-like metabolism
High 13C values (>-10‰) mainly green macroalgae and
seagrasses (LOWER p)
• uptake of HCO3 combined with a CO2 concentrating
mechanism
• very little leakage
What does all this mean?
• Aquatic plants have complex fractionation
and carbon uptake mechanisms.
• Many factors have been discovered to
influence 13C and p values and more are to
come in the future.
• Be careful when making trophic level
assumptions and predicting ancient CO2
levels.