PHYSIOLOGIA PLANTARUM 90; 537-547 1994 Capynglu tQ Phs^iohtm piamamm I9V4 ISSN ix)fiyu7 PrifiU'tl in Di'nitturk - usi nf;ht.\ rr.\('!^vd Carbonic anhydrase activity and inorganic carbon fluxes in iow- and high-Ci ceiis of Chiamydomonas reinhardtii and Scenedesmus obliquus Kristin Palmqvist, Jian-Wei Yu and Murray R. Badger Palmqvist, K,, Yu, 1-W, and Badger, M. R, 1994, Carbonic anhydrase activity and inorganic carbon fluxes in low- and high-Cj eells of Chiamydomonas reinhardtii and Scenedesmus ohliquus. - Physiol, Plant. 90; 537-547, Carbonic anhydrase iQ.\) activity associated with higb- and low-dissolved inorganic carbon (C.) grown cells was examined in whole cells by measuring '"O exchange from doubly labeled CO; ("C'*O"'O). Both aigal species showed the presence of extracellu- lar (peripJasmic) as well as intracellular CA actiiily, which were both greatly increased in low-C, cells. The periplasmic CA activity was at least 40-fold higher in low- compared to high-C, cells in both C, reinhardtii and 5, obliquus. while low-C, cells of S. ohiiquus showed the highest activity of intemai CA, The CA inhibitor ethoxyzola- mide .showed a ,strong inhibition of the C, uptake process in both C, reinhardtii and S. ohiiquus as in eyanobacteria, which may indicate that the nature of the primary uptake process is similar in bolh green aigae and cyanobacteria. By using a mass spectrometric disequilibrium technique it w as possible to separate the C, fluxes of net HCO^-uptake and net CO;-uptake during steady-state photosynthesis in high- and low-C, grow'n cells of Chiamydomonas reinhardtii (WT, 2137-H and Scenedesmus ohiiquus (WT, D3), It was found that both high- and low-C; cells of the two algae ean utilize both CO; and HCOT for photosynthesis, although low-C, celis have a higher affinity for the uptake of both Ci species. Induction at iow-C; causes an increase in tbe affinity of both species for HCO; and CO;; changes in net CO;-uptake were, however, significantly greater. Key words - CO; concentrating mechanism, microalgae, photosynthesis. K. Faimqvist. Dept of Piant Ph\sioli>^y. Univ. of Umea, S-90IH7 Umed. Swedeti: J.-W. Yu and M. R. Badger feorresponding author). Plant Environmental Biology Group. Research School of Bioioi^ical Sciences, Australian National Univ.. P.O. Box 475. Canberra. ACT 2601. Australia. a,s a substrate for the pump has, however, been lively debated and several different and somewhat conflict- The two tnicroalgae Chlatnydotnonas reinhardtii and ing models have been postulated for tbe microalgae Scenedesmus oMiquus can grow autotrophically at very (Findenegg 1979, Marcus et al, 1984, Moroney el al, low inorganic carbon (Q = CO; -i- HCOj) concentrations 1985, Badger 1987, Williams and Turpin 1987, Palmqvist due to the ability to actively transport and accumulate C,, el al, 1988, Thielmann et al, 1990, Coleman 1991, Ku- This CO;-concentrating mechanism (CCM) functions to chitsu et al, 1991, Sultemeyer et al, 1991b), increase tbe CO; concentration within tbe cell and as a Transfer of both C. reinhardtii and S. obUquus from result the pbotosynthetic affinity for CO; increases, pbo- high- to low-C, conditions induces an increased activity torespiration and the sensitivity to O; decreases, and the of the CCM (Badger 1987, Coleman 1991, Badger and CO; compensation point is lowered (cf. Badger 1987). Price i992). As part of thi,s induction process, there i,s a The actual site of C; pumping and accumulation, tbe dramatic increase in periplasmic carbonic anhydrase C| species accumulated as well as the C, species that acts (CA) (Findenegg 1976, 1979, Moroney et al, 1985, Received 12 July, 1993: revised 26 October, 1993 Physiot Plain. %, 1994 537