Systematics:

Carbon in Aquatic Plants

• Food Web Dynamics

• Ancient [CO2]aq and pCO2 concentrations

• Cell Mechanisms (diffusion/assimilation)

in different marine environments

Why do we care?

Why are there variations in 13C of aquatic plants?

Growth Rate

Active vs. Diffusive Inorganic C uptake

Type of Organism

CCM (CO2 concentrating mechanisms)

pCO2 and [CO2]aq

Water Temperature

Cell Size and Geometry

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

Colder at higher latitudes

pCO2 has highest variability at coldest temperatures; however high pCO2 found at all temps

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

[CO2]aq = x pCO2

Cretaceous [CO2 ]aq

To calculate Cretaceous atmospheric CO2 concentrations: 1) Low productivity Cretaceous ocean2) 32°C Cretaceous ocean3) Modern Antarctic ≈ Cretaceous Atlantic 13C (low)4) Similar 13C means similar [CO2]aq

Today [CO2]aq ~ 20 M @ T = -2 to +2°C

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?

Growth Rate

Active vs Diffusive Inorganic C uptake

Type of Organism

CCM (CO2 concentrating mechanisms)

pCO2 and [CO2]aq

Water Temperature

Cell Size and Geometry

Phaeodactylum tricornutum

Cultured diatom to test1) growth rate2) CO2 variability.

Measure p (aka isotopic discrimination factor)

p = 1000(e-p)/(1000+p)p = 1000(-1)

CO2 (aq) + H2O

CO2 (g)

CO2 (aq)Dissolution(Henry’s law,T dependent)

H2CO3 H+ + HCO3-

EquilibriumεHCO3/CO2 = +9‰ @ 25°C

Rubisco + -carboxylase carboxylationsεp = 25-28‰ when growth rate 0

Growth Rate vs. Fractionation

Low CO2 =Faster growth rates = Lower p

Remember: Rubisco + -carboxylase carboxylationsεp = 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 <10mol CO2)

Cell Volume of diatom in this study = 100m3

Average plankton has diameter = 1 m

“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.”

Hmmm..is Cell Size really not an issue?

Why are there variations in 13C of aquatic plants?

Growth Rate

Active vs Diffusive Inorganic C uptake

Type of Organism

CCM (CO2 concentrating mechanisms)

pCO2 and [CO2]aq

Water Temperature

Cell Size and Geometry

Cell Size effects on p under variable growth rates

Max (25‰) fractionation associated with Rubisco and -carboxylases at low grow rate or high pCO2

0.2 SA/V

1.1 SA/V

2.4 SA/V

4.4 SA/V

What’s up with Synechococcus?

Cell size (and shape) influence p, with great impacts on large and/or round cells.

Cell Size effects on p under variable growth rates

Conclude cells assimilate carbon by diffusive and ACTIVE uptake or conversion of bicarbonate to CO2

0.2 SA/V

1.1 SA/V

2.4 SA/V

QuickTime™ and a decompressor

are needed to see this picture.

For eukaryotes, can scale V/SA and all fall on a single relationship.

To understand C isotope fractionation in marine phytoplankton must know: 1) f

2) Growth rate3) [CO2]aq

4) Cellular carbon-to-surface area ratio (or volume-to-surface ratio)

εp is greater for small, slow-growing, high surface/volumeSuch algae have low δ13C values

εp is smaller for large, fast growing, low surface/volumeSuch 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?

Growth Rate

Active vs Diffusive Inorganic C uptake

Type of Organism

CCM (CO2 concentrating mechanisms)

pCO2 and [CO2]aq

Water Temperature

Cell Size and Geometry

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 orLow Zn (≈low carbonic anhydrase)

C4 compound malate:70% after 15 secand 25% after 2 hrin low Zn conditions

Malate is being decarboxylated and released CO2 is fixed by Rubisco to form sugars and phosphoglyceric acid (PGA)

malate

sugars

PGA

Active HCO3 uptake (PEP and CA activity) rather than passively diffusing dissolved

CO2(aq) results in higher 13C values (-10‰)

Diffusion or Active Uptake in C4 plankton?

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)

High 13C values (>-10‰) mainly green macroalgae and seagrasses (LOWER 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

• 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.