Carbon acquisition by diatoms

Abstract

Diatoms are responsible for up to 40% of primary productivity in the ocean, and complete genome sequences are available for two species. However, there are very significant gaps in our understanding of how diatoms take up and assimilate inorganic C. Diatom plastids originate from secondary endosymbiosis with a red alga and their Form ID Rubisco (ribulose-1,5-bisphosphate carboxylase-oxygenase) from horizontal gene transfer, which means that embryophyte paradigms can only give general guidance as to their C acquisition mechanisms. Although diatom Rubiscos have relatively high CO(2) affinity and CO(2)/O(2) selectivity, the low diffusion coefficient for CO(2) in water has the potential to restrict the rate of photosynthesis. Diatoms growing in their natural aquatic habitats operate inorganic C concentrating mechanisms (CCMs), which provide a steady-state CO(2) concentration around Rubisco higher than that in the medium. How these CCMs work is still a matter of debate. However, it is known that both CO(2) and HCO (3) (-) are taken up, and an obvious but as yet unproven possibility is that active transport of these species across the plasmalemma and/or the four-membrane plastid envelope is the basis of the CCM. In one marine diatom there is evidence of C(4)-like biochemistry which could act as, or be part of, a CCM. Alternative mechanisms which have not been eliminated include the production of CO(2) from HCO (3) (-) at low pH maintained by a H(+) pump, in a compartment close to that containing Rubisco.

Southern Ocean phytoplankton physiology in a changing climate

“…Even though carbon fixation rates of SO phytoplankton species with highly efficient CCMs are already close to saturation under presentday CO 2 concentrations Kranz et al, 2015;Young et al, 2015), lower energy expenditures, as well as optimised resource allocation resulting from CCM down-regulation, may enable enhanced carbon fixation rates and/or growth at lowered pH levels. Under these conditions, temperate diatom species, being characterised to operate highly efficient CCMs (Burkhardt et al, 2001;Roberts et al, 2007;Trimborn et al, 2008Trimborn et al, , 2009, were estimated to save about 20% of the CCM-related energy expenditure and 3-6% of the energy expended on carbon fixation (Hopkinson et al, 2011). Among tested SO phytoplankton species, CO 2 -dependent responses on growth strongly vary.…”

Algal evolution in relation to atmospheric CO 2 : carboxylases, carbon-concentrating mechanisms and carbon oxidation cycles

“…Most CCMs also involve one or more carbonic anhydrase enzymes: an exception is a C 4 mechanism which involves, as indicated above, HCO À 3 entry to the cytosol, (C 3 þ C 1 ) carboxylation using HCO À 3 , and (C 4 -C 1 ) decarboxylation in the plastid stroma with CO 2 as the inorganic carbon product [13,92,95,104,110]. All other well-investigated CCMs seem to involve 'normal' carbonic anhydrases, i.e.…”

The Import and Export Business in Plastids: Transport Processes across the Inner Envelope Membrane

“…Many of these organisms are of enormous medical importance while unicellular algae like the diatoms represent a major part of the phytoplankton and are responsible for a large part of the primary productivity in the ocean (Roberts et al, 2007).…”