Characterization of psychrophilic oscillatorians (cyanobacteria) from Antarctic meltwater ponds

Oscillatorian cyanobacteria dominate benthic microbial mat communities in m any polar freshwater ecosystems. Capable of growth at low temperatures, all benthic polar oscillatorians characterized to date are psychrotolerant (gr owth optima > 15 degrees C) as opposed to psychrophilic (growth optima less than or equal to 15 degrees C). Here, psychrophilic oscillatorians isolate d from meItwater ponds on Antarctica's Il MchMurdo Ice Shelf are described. Growth and photosynthetic rates were investigated at multiple temperatures , and compared with those of a psychrotolerant isolate from the same region . Two isolates showed a growth maximum at 8 degrees C, with rates of 0.12 a nd 0.08 doublings d(-1), respectively. Neither displayed detectable growth at 24 degrees C. The psychrotolerant isolate showed almost imperceptible gr owth at 4 degreesC and a rate of 0.9 doublings d(-1) at its optimal tempera ture of similar to 23 degreesC. In both photosynthesis versus irradiance an d photosynthesis versus temperature experiments, exponentially growing cult ures were acclimated for 14 days at 3, 8, 12, 20, and 24 degrees C under sa turating light intensity, and [C-14] photoincorporation rates were measured . Psychrophilic isolates acclimated at 8 degrees C showed greatest photosyn thetic rates; those acclimated at 3 degrees C were capable of active photos ynthesis, but photoincorporation was not detected in cells acclimated at 20 and 24 degrees C, because these isolates were not viable after 14 days at those temperatures. The psychrotolerant isolate, conversely, displayed maxi mum photosynthetic rates at 24 degrees C, though photoincorporation was act ively occurring at 3 degrees C. Within acclimation temperature treatments, short-term photosynthetic rates increased with increasing incubation temper ature for both psychrophilic and psychrotolerant isolates. These results in dicate the importance of temperature acclimation before assays when determi ning optimal physiological temperatures. All isolates displayed photosynthe tic saturation at low light levels (<128 <mu>mol.m(-2).s(-1)) but were not photoinhibited at the highest light treatment (233 mu mol.m(-2).s(-1)). Fie ld studies examining the impact of temperature on photosynthetic responses of intact benthic mats, under natural solar irradiance, showed the mat comm unities to be actively photosynthesizing from 2 to 20 degreesC, with maximu m photoincorporation at 20 degreesC, as well as capable of a rapid response to an increase in temperature. The rarity of psychrophilic cyanobacteria, relative to psychrotolerant strains, may be due to their extremely slow gro wth rates and inability to take advantage of occasional excursions to highe r temperatures. We suggest an evolutionary scenario in which psychrophilic strains, or their most recent common ancestor, lost the ability to grow at higher temperatures while maintaining a broad tolerance for fluctuations in other physical and chemical parameters that define shallow meltwater Antar ctic ecosystems.