CHARACTERIZATION OF INORGANIC CARBON FLUXES, CARBONIC ANHYDRASE(S) AND RIBULOSE-1,5-BIPHOSPHATE CARBOXYLASE-OXYGENASE IN THE GREEN UNICELLULAR ALGA COCCOMYXA - COMPARISONS WITH LOW-CO2 CELLS OF CHLAMYDOMONAS-REINHARDTII
K. Palmqvist et al., CHARACTERIZATION OF INORGANIC CARBON FLUXES, CARBONIC ANHYDRASE(S) AND RIBULOSE-1,5-BIPHOSPHATE CARBOXYLASE-OXYGENASE IN THE GREEN UNICELLULAR ALGA COCCOMYXA - COMPARISONS WITH LOW-CO2 CELLS OF CHLAMYDOMONAS-REINHARDTII, Planta, 197(2), 1995, pp. 352-361
Processes involved in the uptake and fixation of dissolved inorganic c
arbon (DIG) were characterised for Coccomyxa, the green algal primary
photobiont of the lichen Peltigera aphthosa and compared with the free
-living alga Chlamydomonas reinhardtii Dangeard (WT cc 125(+)). A mass
-spectrometer disequilibrium technique was used to quantify fluxes of
both HCO3- and CO2 in the two algae, while activities of carbonic anhy
drases (CAs) were examined in intact cells by measuring O-18 exchange
from doubly labelled CO2 ((COO)-C-13-O-18-O-18) to water and by using
CA inhibitors. The CO2-fixation kinetics of intact Coccomyxa cells wer
e also compared with the carboxylation efficiency of its isolated and
purified primary carboxylating enzyme, ribulose-1,5-bisphosphate carbo
xylase-oxygenase (Rubisco). The two algae were found to be significant
ly different in their modes of acquiring CO2 for photosynthesis. Chlam
ydomonas was able to actively transport both HCO3- and CO2 from the ex
ternal medium, while Coccomyxa clearly favoured CO2 as a substrate. Bo
th algae were found to possess external as well as internal CAs, but t
he relative amounts of these as well as their overall significance for
the functioning of photosynthesis differed. In Coccomyxa, the interna
l CA activity was significantly higher than in Chlamydomonas and also
predominated over the external activity. In Chlamydomonas, both transp
ort and fixation of DIC were severely inhibited by ethoxyzolamide, an
inhibitor of external and internal CAs as well as the DIG-transporting
system, while this inhibitor only caused a minor inhibition of photos
ynthesis in Coccomyxa. These results thus give strong support for earl
ier indirect observations of the absence of a CO2-concentrating mechan
ism in Coccomyxa. In addition, Coccomyxa was found to possess a Rubisc
o with a higher carboxylation efficiency than Chlamydomonas, having a
Km(CO2) of 12 +/- 3 mu M CO2 and a CO2/O-2 specificity factor (S-c/o)
of 83 +/- 2, and it may hence be concluded that the absence of the CO2
-concentrating mechanism is positively correlated with a more efficien
t Rubisco in this alga.