PHOTOSYNTHETIC INORGANIC CARBON ACQUISITION IN AN ACID‐TOLERANT, FREE‐LIVING SPECIES OF COCCOMYXA (CHLOROPHYTA)¹

Abstract

The processes of CO2 acquisition were characterized for the acid-tolerant, free-living chlorophyte alga, CPCC 508. rDNA data indicate an affiliation to the genus Coccomyxa, but distinct from other known members of the genus. The alga grows over a wide range of pH from 3.0 to 9.0. External carbonic anhydrase (CA) was detected in cells grown above pH 5, with the activity increasing marginally from pH 7 to 9, but most of the CA activity was internal. The capacity for HCO3 (-) uptake of cells treated with the CA inhibitor acetazolamide (AZA), was investigated by comparing the calculated rate of uncatalyzed CO2 formation with the rate of photosynthesis. Active bicarbonate transport occurred in cells grown in media above pH 7.0. Monitoring CO2 uptake and O2 evolution by membrane-inlet mass spectrometry demonstrated that air-grown cells reduced the CO2 concentration in the medium to an equilibrium concentration of 15 μM, but AZA-treated cells caused a drop in extracellular CO2 concentration to a compensation concentration of 27 μM at pH 8.0. CO2 -pulsing experiments with cells in the light indicated that the cells do not actively take up CO2 . An internal pool of unfixed inorganic carbon was not detected at the CO2 compensation concentration, probably because of the lack of active CO2 uptake, but was detectable at times before compensation point was reached. These results indicate that this free-living Coccomyxa possesses a CO2 -concentrating mechanism (CCM) due to an active bicarbonate-uptake system, unlike the Coccomyxa sp. occurring in symbiotic association with lichens.

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

“…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. Verma et al (2009) proposed the presence of an external carbonic anhydrase (CA) in Coccomyxa, which operates at pH above 7, with a carbon transport facilitating role rather than a concentrating function. These findings might also explain partly the moderate productivity values obtained for C. onubensis cultured at acidic pH (Fig.…”

Effects of acidic conditions on the physiology of a green alga (Micractiniumsp.) before and after a 5-year interaction withTetrahymena thermophilain an experimental microcosm

“…The physiology of both free-living and symbiotic Chlorella-like algae is affected by the acidic conditions of their respective habitats (McAuley, 1992;Dorling et al, 1997;Verma et al, 2009). However, less is known about what kind of physiological traits can evolve in algae involved in long-term intimate interactions with protist host organisms, specifically, in response to acidic conditions.…”

Inorganic carbon acquisition by eukaryotic algae: four current questions

“…A free-living species of the trebouxophycean Coccomyxa had a CCM when grown at higher pH values than the lake water, in contrast to the symbiotic species of Coccomyxa examined previously which lacked a CCM. Verma et al (2009) independently reported that a freeliving strain of Cocomyxa expressed a CCM when grown at high pH. Diaz and Maberly (2009) noted that some of the algae they examined showed characteristics that could not readily be assigned to diffusive entry of CO 2 or the expression of a CCM.…”