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Marine diatoms play a critical role in the global carbon cycle, where they are responsible for 20% of all primary production. RubisCO, the rate limiting enzyme in carbon fixation, has a half-saturation constant many times higher than oceanic CO2 concentrations. In order to overcome this limitation, diatom species have evolved a diverse array of highly efficient carbon concentrating mechanisms. Increasing our understanding of these mechanisms provides a foundation to improve genetic engineering of these organisms for biofuel production and increased carbon sequestration, as well as a basis to potentially improve the efficiency of photosynthesis in terrestrial plants.In this study, the carbon concentrating mechanism of the diatom species Phaeodactylum tricornutum is investigated through kinetic modeling of potential pathways and analysis of differential gene expression of CCM-related genes. An existing kinetic model is rebuilt and expanded to explore proposed carbon-concentrating mechanisms and provide predictive values of carbon fluxes through the system at varying external conditions. The feasibility of each of three potential mechanisms is evaluated. Through gene expression analysis, potential major regulators of the carbon concentrating mechanism are identified.This investigation finds that the presence of bicarbonate transporters on the plasmalemma, CER, and chloroplast membranes is needed for the CCM to achieve expected carbon uptake and photosynthetic fluxes, but a bicarbonate transporter on the PPC membrane is not necessary. Additionally, the down-regulation of a chloroplast-membrane targeted bicarbonate transporter at sub-atmospheric CO2 indicates that a secondary CCM pathway takes over under low carbon stress.

In reef-building corals (order Scleractinia) and giant clams (phylum Molluca), V-type H+-ATPase (VHA) in host cells is part of a carbon concentrating mechanism (CCM) that regulates photosynthetic rates of their symbiotic algae. Here, we show that VHA plays a similar role in the sea anemone Anemonia majano, a member of the order Actinaria and sister group to the Scleractinia, which in contrast to their colonial calcifying coral relatives is a solitary, soft-bodied taxa. Western blotting and immunofluorescence revealed that VHA was abundantly present in the host-derived symbiosome membrane surrounding the photosymbionts. Pharmacological inhibition of VHA activity in individual anemones resulted in an approximately 80% decrease of photosynthetic O2 production. These results extend the presence of a host-controlled VHA-dependent CCM to non-calcifying cnidarians of the order Actiniaria, suggesting it is widespread among photosymbiosis between aquatic invertebrates and Symbiodiniaceae algae.

Symbiotic dinoflagellate algae residing inside coral tissues supply the host with the majority of their energy requirements through the translocation of photosynthetically fixed carbon. The algae, in turn, rely on the host for the supply of inorganic carbon. Carbon must be concentrated as CO2 in order for photosynthesis to proceed, and here we show that the coral host plays an active role in this process. The host-derived symbiosome membrane surrounding the algae abundantly expresses vacuolar H(+)-ATPase (VHA), which acidifies the symbiosome space down to pH ∼ 4. Inhibition of VHA results in a significant decrease in average H(+) activity in the symbiosome of up to 75% and a significant reduction in O2 production rate, a measure of photosynthetic activity. These results suggest that host VHA is part of a previously unidentified carbon concentrating mechanism for algal photosynthesis and provide mechanistic evidence that coral host cells can actively modulate the physiology of their symbionts.