1. Systematics: Carbon in Aquatic Plants

2. Food Web Dynamics Ancient [CO 2 ] aq and pCO 2 concentrations Cell Mechanisms (diffusion/assimilation) in different marine environments Why do we care?

3. Why are there variations in  13 C of aquatic plants? Growth Rate Active vs. Diffusive Inorganic C uptake Type of Organism CCM (CO 2 concentrating mechanisms) pCO 2 and [CO 2 ] aq Water Temperature Cell Size and Geometry

4.  13 C variances with Temperature and Latitude Lower  13 C values found in cold, southerly latitude Antarctic waters Less variability shown in Arctic waters

5. Stable carbon isotopes in marine organic matter vary significantly over geologic time. Cretaceous sediments are thought to have existed in a time with elevated CO 2 levels. First study to show relationship between phytoplankton  13 C and CO 2 concentrations.

6. Temperature vs. Latitude and Temperature vs. pCO 2 Colder at higher latitudes pCO 2 has highest variability at coldest temperatures; however high pCO 2 found at all temps

7.  13 C vs. [CO 2 ] aq [CO 2 ] aq =  x pCO 2 [CO 2 ] aq is dissolved CO 2 concentration;  is solubility constant (a function of temp) Greater fractionation at higher [CO 2 ] aq and colder temps

8. [CO 2 ] aq =  x pCO 2 Cretaceous [CO 2 ] aq To calculate Cretaceous atmospheric CO 2 concentrations: 1) Low productivity Cretaceous ocean 2) 32°C Cretaceous ocean 3) Modern Antarctic ≈ Cretaceous Atlantic  13 C (low) 4) Similar  13 C means similar [CO 2 ] aq Today [CO 2 ] aq ~ 20  M @ T = -2 to +2°C Plug and chug! Low latitude Cretaceous ocean >800 pmv 2 - 13 x higher than prior estimates

9. Why are there variations in  13 C of aquatic plants? Growth Rate Active vs Diffusive Inorganic C uptake Type of Organism CCM (CO 2 concentrating mechanisms) pCO 2 and [CO 2 ] aq Water Temperature Cell Size and Geometry

10. Phaeodactylum tricornutum Cultured diatom to test 1)growth rate 2)CO 2 variability. Measure  p (aka  isotopic discrimination factor)  p = 1000(  e -  p )/(1000+  p )  p = 1000(  -1)

11. CO 2 (aq) + H 2 O CO 2 (g) CO 2 (aq) Dissolution (Henry’s law, T dependent) H 2 CO 3 H + + HCO 3 - Equilibrium ε HCO 3/ CO 2 = +9‰ @ 25°C Rubisco +  -carboxylase carboxylations ε p = 25-28‰ when growth rate  0

12. Growth Rate vs. Fractionation Low CO 2 = Faster growth rates = Lower  p Remember: Rubisco +  -carboxylase carboxylations ε p = 25-28‰ when growth rate  0

13. Predicted growth rate based off [CO 2 ] 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 CO 2 )

14. Cell Volume of diatom in this study = 100  m 3 Average plankton has diameter = 1  m “Cell size effects may change slope of  p vs  /[CO 2 ] aq sufficiently to invalidate growth rates determined from  p and [CO 2 ] aq, but these cases are likely to be the exception rather than the rule.”

15. Hmmm..is Cell Size really not an issue?

16. Why are there variations in  13 C of aquatic plants? Growth Rate Active vs Diffusive Inorganic C uptake Type of Organism CCM (CO 2 concentrating mechanisms) pCO 2 and [CO 2 ] aq Water Temperature Cell Size and Geometry

17. Cell Size effects on  p under variable growth rates Max (25‰) fractionation associated with Rubisco and  - carboxylases at low grow rate or high pCO 2 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.

18. Cell Size effects on  p under variable growth rates Conclude cells assimilate carbon by diffusive and ACTIVE uptake or conversion of bicarbonate to CO 2 0.2 SA/V 1.1 SA/V 2.4 SA/V For eukaryotes, can scale V/SA and all fall on a single relationship.

19. To understand C isotope fractionation in marine phytoplankton must know:  f 2)Growth rate 3)[CO 2 ] aq 4)Cellular carbon-to-surface area ratio (or volume-to-surface ratio) ε p is greater for small, slow-growing, high surface/volume Such algae have low δ 13 C values ε p is smaller for large, fast growing, low surface/volume Such algae have high δ 13 C values Onshore-Offshore isotope Gradients: For those who love the food webs, this explains the difference in δ 13 C 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 13 C-depleted water.

20. Why are there variations in  13 C of aquatic plants? Growth Rate Active vs Diffusive Inorganic C uptake Type of Organism CCM (CO 2 concentrating mechanisms) pCO 2 and [CO 2 ] aq Water Temperature Cell Size and Geometry

21. C3 vs. C4 photosynthesis: C4 in the ocean Diatoms growing in low CO 2 conditions have enriched 13 C values - possibly undergo C4 assimilation.

22. Increase in PEP with low CO 2 or Low Zn (≈low carbonic anhydrase)

23. C4 compound malate: 70% after 15 sec and 25% after 2 hr in low Zn conditions Malate is being decarboxylated and released CO 2 is fixed by Rubisco to form sugars and phosphoglyceric acid (PGA) malate sugars PGA

24. Active HCO 3 uptake (PEP and CA activity) rather than passively diffusing dissolved CO 2(aq) results in higher  13 C values (-10‰) Diffusion or Active Uptake in C4 plankton? These values found in diatoms during the Mesozoic…before C4 found in terrestrial land plants

25. Active HCO 3 uptake in this coastal, upwelling region Monterey Bay lower  p than global, Peru diatoms even lower. Attributed to CO 2 concentrating mechanisms. This mechanism is not always restricted to diatoms.

26. Moving on from phytoplankton to coastal macroalgae and seagrasses MAJOR review paper (super wordy yet not very synthetic)

27.  13 C differences on large data set 565 species assessed! Low  13 C values (<-30‰) mainly subtidal red macroalgae High  13 C values (>-10‰) mainly green macroalgae and seagrasses

28. Low  13 C values (<-30‰) mainly subtidal red macroalgae (HIGHER  p ) High  13 C values (>-10‰) mainly green macroalgae and seagrasses (LOWER  p ) rely on diffusive CO 2 supply to Rubisco conversion of photosynthate to lipids; more negative 13 C inputs (terr); low photon flux densities lack of pyrenoids result in no CO 2 concentrating mechanisms C4-like metabolism uptake of HCO 3 combined with a CO 2 concentrating mechanism very little leakage

29. What does all this mean? Aquatic plants have complex fractionation and carbon uptake mechanisms. Many factors have been discovered to influence  13 C and  p values and more are to come in the future. Be careful when making trophic level assumptions and predicting ancient CO 2 levels.