Dynamic changes in carbonate chemistry in the microenvironment around single marine phytoplankton cells

Photosynthesis by marine diatoms plays a major role in the global carbon cycle, although the precise mechanisms of dissolved inorganic carbon (DIC) uptake remain unclear. A lack of direct measurements of carbonate chemistry at the cell surface has led to uncertainty over the underlying membrane transport processes and the role of external carbonic anhydrase (eCA). Here we identify rapid and substantial photosynthesis-driven increases in pH and [CO32−] primarily due to the activity of eCA at the cell surface of the large diatom Odontella sinensis using direct simultaneous microelectrode measurements of pH and CO32− along with modelling of cell surface inorganic carbonate chemistry. Our results show that eCA acts to maintain cell surface CO2 concentrations, making a major contribution to DIC supply in O. sinensis. Carbonate chemistry at the cell surface is therefore highly dynamic and strongly dependent on cell size, morphology and the carbonate chemistry of the bulk seawater

Effect of Inorganic and Organic Carbon Enrichments (DIC and DOC) on the Photosynthesis and Calcification Rates of Two Calcifying Green Algae from a Caribbean Reef Lagoon

Coral reefs worldwide are affected by increasing dissolved inorganic carbon (DIC) and organic carbon (DOC) concentrations due to ocean acidification (OA) and coastal eutrophication. These two stressors can occur simultaneously, particularly in near-shore reef environments with increasing anthropogenic pressure. However, experimental studies on how elevated DIC and DOC interact are scarce and fundamental to understanding potential synergistic effects and foreseeing future changes in coral reef function. Using an open mesocosm experiment, the present study investigated the impact of elevated DIC (pHNBS: 8.2 and 7.8; pCO2: 377 and 1076 ?atm) and DOC (added as 833 ?mol L-1 of glucose) on calcification and photosynthesis rates of two common calcifying green algae, Halimeda incrassata and Udotea flabellum, in a shallow reef environment. Our results revealed that under elevated DIC, algal photosynthesis decreased similarly for both species, but calcification was more affected in H. incrassata, which also showed carbonate dissolution rates. Elevated DOC reduced photosynthesis and calcification rates in H. incrassata, while in U. flabellum photosynthesis was unaffected and thalus calcification was severely impaired. The combined treatment showed an antagonistic effect of elevated DIC and DOC on the photosynthesis and calcification rates of H. incrassata, and an additive effect in U. flabellum. We conclude that the dominant sand dweller H. incrassata is more negatively affected by both DIC and DOC enrichments, but that their impact could be mitigated when they occur simultaneously. In contrast, U. flabellum can be less affected in coastal eutrophic waters by elevated DIC, but its contribution to reef carbonate sediment production could be further reduced. Accordingly, while the capacity of environmental eutrophication to exacerbate the impact of OA on algal-derived carbonate sand production seems to be species-specific, significant reductions can be expected under future OA scenarios, with important consequences for beach erosion and coastal sediment dynamics

Bicarbonate utilization: Function and mechanism

A review is given of the occurrence, ecological significance and physiological mechanism of bicarbonate utilization by aquatic photosynthetic organisms. Following a short description of the carbon dioxide water system, the significance of the unstirred layer and the slow diffusion rate of CO2 in water is discussed. Three experimental procedures are discussed to establish the use of bicarbonate: establishing the relationship between pH and rate of carbon assimilation; the kinetic-reaction approach, which uses the slow equilibration between CO2 and HCO3− in water; the difference in discrimination between 12C and 13C during CO2 and HCO3− assimilation. The ability to use HCO3− occurs in many different groups of aquatic photosynthetic organisms, with the notable exception of Bryophytes and Pteridophytes, and is related to growth conditions. Polarity, that is the spatial separation of HCO3− influx and resulting OH− efflux, is observed in higher plant species and Characeae that assimilate HCO3−. These polar leaves or banded Characean cells are often used to study the mechanism. Three mechanisms for HCO3− assimilation apparently exist: H+HCO3− symport, external acidification of HCO3− into CO2 and an increased rate of conversion of HCO3− into CO2 by the action of the enzyme carbonic anhydrase. The role of transfer cells or plasmalemmasomes, often observed in HCO3− assimilating leaves, is still unclear. Their role may be to create sufficient space for membrane-bound enzyme systems by increasing the membrane surface area or to confine an extracellular space with a low pH value. The ecological significance of photosynthetic HCO3− utilization seems to be that it compensates for slow supply of CO2 by diffusion and/or suppresses photorespiration by creating a high intracellular CO2 concentration

LIGHT-INDUCED POLAR PH CHANGES IN LEAVES OF ELODEA-CANADENSIS .2. EFFECTS OF FERRICYANIDE - EVIDENCE FOR MODULATION BY THE REDOX STATE OF THE CYTOPLASM

The effect of an extracellular electron acceptor, ferricyanide, on the light-induced polar leaf pH changes of the submerged angiosperm Elodea canadensis in light and in darkness was determined. The rate of transmembrane ferricyanide reduction was stimulated by increased light intensity and was inhibited by inorganic carbon, indicating that changes in the redox state of the chloroplast were reflected at the plasma membrane. The addition of ferricyanide inhibited the light-induced polar leaf pH reaction. This effect could be balanced by increasing the light intensity. In the dark, the acidification induced by ferricyanide was not influenced by diethylstilbestrol at concentrations that completely inhibited the polar leaf pH changes. This indicates that the ferricyanide-induced H+ extrusion and the H+ transport during the polar reaction were mediated by different mechanism