Mechanism of CO₂ acquisition in an acid‐tolerant Chlamydomonas

Abstract

The acid-tolerant green alga Chlamydomonas (UTCC 121) grows in media ranging in pH from 2.5 to 7.0. Determination of the overall internal pH of the cells, using 14 C-benzoic acid (BA) or [2-14 C]-5,5-dimethyloxazolidine-2,4-dione (DMO), showed that the cells maintain a neutral pH (6.6 to 7.2) over an external pH range of 3.0-7.0. The cells express an external carbonic anhydrase (CA) when grown in media above pH 5.5, and CA increases to a maximum at pH 7.0. Removal of external CA by trypsin digestion or by acetazolamide (AZA) inhibition indicated that CA was essential for photosynthesis at pH 7.0 and that the cells had no capacity for direct bicarbonate uptake. Monitoring of CO2 uptake and O2 evolution by mass spectrometry during photosynthesis did not provide any evidence of active CO2 uptake. The CO2 compensation concentration of the cells ranged from 9.4 mM at pH 4.5 to 16.2 mM at pH 7.0. An examination of the kinetics of ribulose 1·5-bisphosphate carboxylase/oxygenase (Rubisco), in homogenates of cells grown at pH 7.0, showed that the Km (CO2) was 16.3 mM. These data indicate that the pH between the cell interior and the external medium was large enough at acid pH to allow the accumulation of inorganic carbon (Ci) by the diffusive uptake of CO2, and the expression of external CA at neutral pH values would maintain an equilibrium CO2 concentration at the cell surface. This species does not possess a CO2-concentrating mechanism because the whole cell affinity for Ci appears to be determined by the low Km (CO2) Rubisco of the alga.

Phylogenetic characterization and morphological and physiological aspects of a novel acidotolerant and halotolerant microalga Coccomyxa onubensis sp. nov. (Chlorophyta, Trebouxiophyceae)

“…The low concentration of inorganic carbon present in acidic waters would limit the photosynthesis of algae. However, to cope with the limited carbon bioavailability, the microalgae of acidic habitats often have high-affinity mechanisms for CO 2 uptake or highaffinity Rubisco enzymes (Balkos and Colman 2007;Spijkerman 2008). In this respect, based on the genetic analysis of Coccomyxa subellipsoidea strain, Blanc et al (2012) suggested the existence of a functional CO 2 -concentrating mechanism.…”

A Novel Miniature Culture System to Screen CO2-Sequestering Microalgae

“…On the other hand, for the response to pH, microalgae can be divided into acid-tolerant and non-acid-tolerant species [28]. Acid-tolerant microalgae can accommodate low pH to a certain extent, such as that caused by high CO 2 concentration, as seen in many green algae species [29][30][31]. For such species, as they might not have very large differences on response to CO 2 availability, in addition to figuring out the optimal CO 2 level for cell growth, it is probably more important to distinguish their CO 2 fixation capacity by examining X t at various screening conditions.…”

Inorganic carbon acquisition by eukaryotic algae: four current questions

“…While Euglena mutabilis showed no evidence of a CCM under any condition and, therefore, always depended on diffusive exchange of CO 2 , the acid-tolerant Euglena gracilis expressed a CCM based on active influx of HCO 3 -when grown at alkaline pH values. Balkos and Colman (2007) investigated an acidtolerant strain of the chlorophycean Chlamydomonas, and found no evidence of a CCM under any of the conditions tested. The CO 2 affinity of the Rubisco from this alga could account for the CO 2 affinity of whole cells, making plausible assumptions about the catalytic capacity of the enzyme per cell and the diffusion path used by CO 2 entering the cells.…”