Effect of Low Temperature on Tropical and Temperate Isolates of Marine Synechococcus.
An abundant and globally occurring marine picocyanobacterium, the genus Synechococcus is an important player in oceanic primary production and global carbon cycling. In the complex marine environment, this widespread organism has evolved to successfully colonize and inhabit different environmental niches. Their biogeographic distribution suggests that Synechococcus ecotypes exhibit thermal niche preferences. Temperature is a key environmental variable and the elucidation of the temperature stress acclimation in members of this genus can shed light on the molecular mechanisms involved in their adaptive capability. The growth of four representative Synechococcus isolates of various ecotypes from tropical and temperate regions were monitored under various temperature conditions. This revealed drastic differences in growth rates in correlation with their thermal niche preferences. The temperate strains CC9311 and BL107 displayed higher growth rates at lower temperatures while tropical strains WH8102 and WH8109 grew better at higher temperatures. In order to further elucidate their thermal niche preference, the molecular factors influencing the temperature-related growth patterns were explored through global proteomic analysis of WH8102 and BL107. Whole cell lysates of the strains grown at different temperature conditions were fractionated using 1D SDS-PAGE and analysed using label-free quantitative proteomics. Protein identifications provided 27% and 40% coverage of the whole genome for WH8102 and BL107, respectively. Quantitation of protein expression revealed 22% and 20% of the identified proteins were differentially expressed in WH8102 and BL107, respectively. The results were further investigated using qRT-PCR and PAM fluorometry. Differential expression revealed that low temperature appeared to have a significant effect on the photosynthetic machinery. The light harvesting components, phycobilisomes exhibited a reduced expression which could be the result of protein degradation due to photo-oxidative damage and/or as a mechanism to restore the energy balance disturbed as a consequence of low temperature. The lowered phycobilisome expression is found to be a common low temperature-related response between the tropical and temperate isolates. Within the photosynthetic reaction centres, differences in the expression of some core proteins were observed between the two isolates. The expression of core proteins could correlate with the efficiency of repair mechanisms involved in the replacement of photo-damaged core proteins. This differential expression sheds light on the underlying factors which potentially influence the differences in the thermal ranges of tropical and temperate isolates.