Structural and Biochemical Features of Carbon Acquisition in Algae

Abstract

Inorganic carbon assimilation in algae depends ultimately on the activity of ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco) and the Calvin-Benson-Bassham Cycle (Photosynthetic Carbon Reduction Cycle; PCRC). The kinetic characteristics of the different forms of Rubisco found in algae and cyanobacteria are such that under present day levels of CO₂ and oxygen, inefficiencies in carbon assimilation, involving, inter alia, photorespiration would occur. Given this, achievement of significant rates of net photosynthesis necessitates the operation of CO₂ concentrating mechanisms (CCMs) to elevate CO₂ concentrations, and increase CO₂:O₂ ratios at the active site of Rubisco. The biochemical, C₄ photosynthesis, CCM appears to be limited to the marine ulvophycean alga Udotea flabellum and one species of a marine diatom (Thalassiosira weisflogii) as well as in some seagrasses and freshwater macrophytes. With few exceptions all other algae and the cyanobacteria possess a biophysical CCM, based on active transport of inorganic carbon. Despite many years of research into CCMs, there is still a good deal of misunderstanding about what comprises reliable and robust evidence for CCM activity. By definition, there needs to be a demonstrable net positive gradient of CO_{2 in} > CO_{2 out}. In addition, cells with active CCMs have K_0.5CO₂ values for DIC-dependent photosynthesis less than that for their Rubiscos. Other features such as isotope discrimination can also be useful, with cells using CO₂ diffusion alone showing ∆¹³C values ~ −30‰ and discrimination values becoming less negative as CCM activity increases. However, the presence of gene sequences for, or even expressed activity of, enzymes such as PEP carboxylase, are not good indicators of operation of C₄ photosynthesis. It is a common misconception that possession of external carbonic anhydrase can alone result in CO₂ accumulation. CCMs involve active transport of bicarbonate and/or CO₂. In cyanobacteria this occurs at the thylakoid membrane or plasmalemma and in eukaryotes at the plasmalemma or inner plastid envelope, or both. CCMs in cyanobacteria also involve polyhedral protein-walled bodies termed carboxysomes it is here that the bulk of Rubisco is found and CO₂ is accumulated due to the activity of carbonic anhydrase. The analogous structure found in some eukaryotes is the pyrenoid (Chap. 9). All pyrenoid-containing algae have CCMs but not all algae with CCMs have pyrenoids. Though work on eukaryotic algae has, so far, lagged behind that in cyanobacteria new advances in molecular biology have led to identification of many of the proteins, and elucidation of the mechanisms, involved in inorganic carbon acquisition.

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