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Abstract Marine diatoms are key primary producers across diverse habitats in the global ocean. Diatoms rely on a biophysical carbon concentrating mechanism (CCM) to supply high concentrations of CO2around their carboxylating enzyme, RuBisCO. The necessity and energetic cost of the CCM are likely to be highly sensitive to temperature, as temperature impacts CO2concentration, diffusivity, and the kinetics of CCM components. Here, we used membrane inlet mass spectrometry (MIMS) and modeling to capture temperature regulation of the CCM in the diatomPhaeodactylum tricornutum (Pt). We found that enhanced carbon fixation rates byPtat elevated temperatures were accompanied by increased CCM activity capable of maintaining RuBisCO close to CO2saturation but that the mechanism varied. At 10 and 18 °C, diffusion of CO2into the cell, driven byPt’s ‘chloroplast pump’ was the major inorganic carbon source. However, at 18 °C, upregulation of the chloroplast pump enhanced (while retaining the proportion of) both diffusive CO2and active HCO3−uptake into the cytosol, and significantly increased chloroplast HCO3−concentrations. In contrast, at 25 °C, compared to 18 °C, the chloroplast pump had only a slight increase in activity. While diffusive uptake of CO2into the cell remained constant, active HCO3−uptake across the cell membrane increased resulting inPtdepending equally on both CO2and HCO3−as inorganic carbon sources. Despite changes in the CCM, the overall rate of active carbon transport remained double that of carbon fixation across all temperatures tested. The implication of the energetic cost of thePtCCM in response to increasing temperatures was discussed.
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Influence of Temperature and CO 2 On Plasma‐membrane Permeability to CO 2 and HCO 3 − in the Marine Haptophytes Emiliania huxleyi and Calcidiscus leptoporus (Prymnesiophyceae)
Membrane permeabilities to CO2and HCO3−constrain the function of CO2concentrating mechanisms that algae use to supply inorganic carbon for photosynthesis. In diatoms and green algae, plasma membranes are moderately to highly permeable to CO2but effectively impermeable to HCO3−. Here, CO2and HCO3−membrane permeabilities were measured using an18O‐exchange technique on two species of haptophyte algae,Emiliania huxleyiandCalcidiscus leptoporus, which showed that the plasma membranes of these species are also highly permeable to CO2(0.006–0.02 cm · s−1) but minimally permeable to HCO3−. Increased temperature and CO2generally increased CO2membrane permeabilities in both species, possibly due to changes in lipid composition or CO2channel proteins. Changes in CO2membrane permeabilities showed no association with the density of calcium carbonate coccoliths surrounding the cell, which could potentially impede passage of compounds. Haptophyte plasma‐membrane permeabilities to CO2were somewhat lower than those of diatoms but generally higher than membrane permeabilities of green algae. One caveat of these measurements is that the model used to interpret18O‐exchange data assumes that carbonic anhydrase, which catalyzes18O‐exchange, is homogeneously distributed in the cell. The implications of this assumption were tested using a two‐compartment model with an inhomogeneous distribution of carbonic anhydrase to simulate18O‐exchange data and then inferring plasma‐membrane CO2permeabilities from the simulated data. This analysis showed that the inferred plasma‐membrane CO2permeabilities are minimal estimates but should be quite accurate under most conditions.
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- Award ID(s):
- 1744760
- PAR ID:
- 10361770
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Journal of Phycology
- Volume:
- 56
- Issue:
- 5
- ISSN:
- 0022-3646
- Page Range / eLocation ID:
- p. 1283-1294
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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