INORGANIC CARBON REPLETION DISRUPTS PHOTOSYNTHETIC ACCLIMATION TO LOW TEMPERATURE IN THE CYANOBACTERIUM SYNECHOCOCCUS ELONGATUS 1 Robert A. Burns 2 Department of Biology and Coastal Wetlands Institute, Mount Allison University, 63BYork Street, Sackville, NB, E41 1G7, Canada, and Department of Biology, University of New Brunswick, P.O. Bag Service 45111, Fredericton, NB, E3B 6E1, Canada C. Danielle MacDonald, Patrick J. McGinn Department of Biology and Coastal Wetlands Institute, Mount Allison University, 63B York Street, Sackville, NB, E41 1G7, Canada and Douglas. A. Campbell 3 Department of Biology and Coastal Wetlands Institute, Mount Allison University, 63BYork Street, Sackville, NB, E41 1G7, Canada, and Department of Biology, University of New Brunswick, P.O. Bag Service 45111, Fredericton, NB, E3B 6E1, Canada Acclimation of cyanobacteria to ambient fluctua- tions in inorganic carbon (Ci) and temperature requires reorganization of the major protein complexes involved in photosynthesis. We grew cul- tures of the picoplanktonic cyanobacterium Syn- echococcus elongatus Naegeli across most of its range of tolerable temperatures from 23 to 351 C at both low (o0.1 mM) and high Ci (approximately 4 mM). Over that range of temperatures, the chl- based doubling time did not differ between low and high Ci grown cells but did increase with decreas- ing temperature. Cells grown at 231 C high Ci showed an elongated morphology, which was not present in 231 C low Ci cells nor at 351 C high and low Ci. Furthermore, 231 C high Ci cells showed premature senescence and death compared with all other treatments. Phycocyanin per cell was greater in high Ci grown cells at all temperatures but showed a characteristic decrease with decreasing temperature. Functional PSII determination showed that 231 C high Ci cells had 1.5  10 5 PSII . cell –1 compared with only 6.9  10 4 PSII . cell –1 for 231 C low Ci. The 351 C high and low Ci cells had 7.7  10 4 and 6.4  10 4 PSII . cell –1 , re- spectively. These data were supported by immuno- blot determinations of PsbA content . cell –1 . As a result of their high PSII . cell –1 , 231 C high Ci cells generated more reductant from PSII than could be accommodated by downstream assimilative metab- olism, resulting in early senescence and death of 231 C high Ci cells, probably as a result of the gen- eration of reactive byproducts of electron transport. Key index words: CCM; electron transport; gluta- mine synthetase; PSII; PSI; RUBISCO Abbreviations: AtpB, ATP synthetase; CCM, carbon- concentrating mechanism; Ci, inorganic carbon; ETR, electron transport rate; GlnA, glutamine syn- thetase; PsaC, PSI protein; PsbA, PSII protein; RbcL, RUBISCO large unit protein Cyanobacteria are the largest, most functionally di- verse, and widely distributed group of photosynthetic prokaryotes (Stanier and Cohen-Bazire 1977) occupy- ing marine, freshwater, and terrestrial environments from arctic to tropical climates. Furthermore, in each of these habitats, widely fluctuating environmental pa- rameters, including light level and quality (Schubert et al. 1995), temperature, and mineral nutrient availa- bility, interact to influence growth, molecular resource allocation, and photosynthesis through complex acc- limatory strategies. Ambient growth temperature is a fundamental physical parameter that fluctuates substantially in na- ture. Research on acclimatory responses to low tem- perature in cyanobacteria has focused on the enzyme- mediated desaturation of cellular membrane fatty acids (Murata and Wada 1995, Gombos and Murata 1998, Sakamoto and Bryant 2002). In an early study Halldal (1958) examined the interactive effects of temperature and light on growth and pigmentation of several cyanobacterial strains. In his experiments, the phyco- cyanin-to-chl ratio of cells grown at lower temperature was lower compared with cells grown at higher tem- peratures. Later work (Fujita 1997) rationalized these findings, primarily in terms of changes in the stoic- hiometries of phycobilisomes, PSII, and PSI. Huner and coworkers integrated these findings in their excitation pressure concept. They found that dif- fusive movement of plastiquinone limits electron trans- 1 Received 9 June 2004. Accepted 29 November 2004. 2 Current address: LI-COR Biosciences, Lincoln, NB, USA. 3 Author for correspondence: e-mail dcampbell@mta.ca. 322 J. Phycol. 41, 322–334 (2005) r 2005 Phycological Society of America DOI: 10.1111/j.1529-8817.2005.04101.x