Inorganic carbon repletion disrupts photosynthetic acclimation to low temperature in the cyanobacterium Synechococcus elongatus

Acclimation of cyanobacteria to ambient fluctuations in inorganic carbon (Ci) and temperature requires reorganization of the major protein complexes involved in photosynthesis. We grew cultures of the picoplanktonic cyanobacterium Synechococcus elongatus Naegeli across most of its range of tolerable temperatures from 23 to 35 degrees C at both low (< 0.1 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 decreasing temperature. Cells grown at 23 degrees C high Ci showed an elongated morphology, which was not present in 23 degrees C low Ci cells nor at 35 degrees C high and low Ci. Furthermore, 23 degrees 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 23 degrees C high Ci cells had 1.5 x 10(5) PSII.cell(-1) compared with only 6.9 x 10(4) PSII.cell(-1) for 23 degrees C low Ci. The 35 degrees C high and low Ci cells had 7.7 x 10(4) and 6.4 x 10(4) PSII.cell(-1), respectively. These data were supported by immunoblot determinations of PsbA content.cell(-1). As a result of their high PSII.cell(-1), 23 degrees C high Ci cells generated more reductant from PSII than could be accommodated by downstream assimilative metabolism, resulting in early senescence and death of 23 degrees C high Ci cells, probably as a result of the generation of reactive byproducts of electron transport.