Inorganic carbon acquisition by eukaryotic algae: four current questions

Abstract

The phylogenetically and morphologically diverse eukaryotic algae are typically oxygenic photolithotrophs. They have a diversity of incompletely understood mechanisms of inorganic carbon acquisition: this article reviews four areas where investigations continue. The first topic is diffusive CO(2) entry. Most eukaryotic algae, like all cyanobacteria, have inorganic carbon concentrating mechanisms (CCMs). The ancestral condition was presumably the absence of a CCM, i.e. diffusive CO(2) entry, as found in a small minority of eukaryotic algae today; however, it is likely that, as is found in several cases, this condition is due to a loss of a CCM. There are a number of algae which are in various respects intermediate between diffusive CO(2) entry and occurrence of a CCM: further study is needed on this aspect. A second topic is the nature of cyanelles and their role in inorganic carbon assimilation. The cyanelles (plastids) of the euglyphid amoeba Paulinella have been acquired relatively recently by endosymbiosis with genetic integration of an α-cyanobacterium with a Form 1A Rubisco. The α-carboxysomes in the cyanelles are presumably involved in a CCM, but further investigation is needed.Also called cyanelles are the plastids of glaucocystophycean algae, but is it now clear that these were derived from the β-cyanobacterial ancestor of all plastids other than that of Paulinella. The resemblances of the central body of the cyanelles of glaucocystophycean algae to carboxysomes may not reflect derivation from cyanobacterial β-carboxysomes; although it is clear that these algae have CCMs but these are now well characterized. The other two topics concern CCMs in other eukaryotic algae; these CCMs arose polyphyletically and independently of the cyanobacterial CCMs. It is generally believed that eukaryotic algal, like cyanobacterial, CCMs are based on active transport of an inorganic carbon species and/or protons, and they have C(3) biochemistry. This is the case for the organism considered as the third topic, i.e. Chlamydomonas reinhardtii, the eukaryotic alga with the best understood CCM. This CCM involves HCO(3)(-) conversion to CO(2) in the thylakoid lumen so the external inorganic carbon must cross four membranes in series with a final CO(2) effux from the thylakoid. More remains to be investigated about this CCM. The final topic is that of the occurrence of C(4)-like metabolism in the CCMs of marine diatoms. Different conclusions have been reached depending on the organism investigated and the techniques used, and several aspects require further study.

Adaptive signals in algal Rubisco reveal a history of ancient atmospheric carbon dioxide

“…Furthermore, some orders are known to lack pyrenoids (Dicytotales, Sphacelariales, Laminariales and Fucales) although they do appear to be able to use HCO À 3 [5]. Even though we find no positive selection in groups which are established as lacking CCMs (table 1 and electronic supplementary material, figure S3; the Rhodophyta genera Batrachospermum, Caloglossa, Membranoptera, Nitophyllum, Phycodrys and Ptilota [58 -60], and the Chrysophyceae and Synurophyceae [60,61]), the test used for this particular analysis is very conservative. This test averages positive selection across the group and was unable to detect positive selection, even within the Haptophyta and diatoms, in which positive selection is established when individual branches were tested.…”

Interactions between pH and nutrients on benthic algae in streams and consequences for ecological status assessment and species richness patterns

“…Different factors associated with low pH (elevated H + , elevated labile Al concentration, low concentration or absence of HCO 3 -, reduced solubility of quartz (Stumm & Morgan, 1996), different benthic macroinvertebrate taxon richness and feeding types (Larranaga et al, 2010)) all impact benthic algae, and discriminating between the effects of these factors on primary producers is difficult (Sparling & Lowe, 1996). Eukaryotic algae have developed a variety of mechanisms for inorganic carbon acquisition, including utilization of HCO 3 -and different carbon concentrating mechanisms (Raven, 2010). The observed maximum in non-diatom taxon richness around pH 6.4 is very close to the point where equilibrium concentrations of CO 2 and HCO 3 -in open water systems are equal (Stumm & Morgan, 1996), suggesting the coexistence of bicarbonate and CO 2 users.…”

A Metabolomic Approach to Study Major Metabolite Changes during Acclimation to Limiting CO2 in Chlamydomonas reinhardtii

“…This possibility has been suggested for certain diatoms but is still a matter of debate (Kroth et al, 2008;Raven, 2010). Some of the evidence used to support a C4 contribution to a diatom CCM has been shown not to apply to (a marine) Chlamydomonas (Reinfelder et al, 2004), and work on C. reinhardtii also agrees with the absence of C4-like photosynthesis (Harris, 1989;Raven, 2010). The absence of C4 photosynthesis is in agreement with the location of a potential C4 photosynthesis decarboxylase, PEPCK, in mitochondria ( Fig.…”