Strategies of thermal adaptation by high-latitude cyanobacteria

Although mat-forming cyanobacteria dominate many freshwater ecosystems in the Arctic and Antarctic, their optimal temperature for growth (T-opt) is usually much higher than the temperature range of their native habitat. The present study compared the temperature dependence of growth, pigment composition and absorbance, photosynthesis and photosynthate partitioning for two strains of cyanobacteria with contrasting T-opt values; Phormidium subfuscum, isolated from McMurdo Ice Shelf, Antarctica, and Phormidium tenue, collected from the Kuparuk River in the tundra region of northern Alaska. Phormidium subfuscum grew between 5 and 20 degrees C with a T-opt of 15 degrees C whereas P. tenue showed detectable growth from 10 to 40 degrees C and a T-opt of 30 degrees C. Light utilization efficiency, photosynthetic capacity and the irradiance at the onset of light saturation increased with increasing temperature up to T-opt in both strains. The cellular concentrations of chlorophyll a (Chl a) and carotenoid (CAR) and the in vivo absorbance maxima for Chi a, CAR, C-phycocyanin and allophycocyanin changed little for P. subfuscum but all these variables increased across the temperature range up to T-opt for P. tenue. Neither P. subfuscum nor P. tenue showed changes in relative carbon allocation with varying temperature, suggesting that gross biochemical alterations are not a characteristic of temperature acclimation in these cyanobacteria. We conclude that the eurythermal cyanobacterium P. tenue optimizes growth over a wide range of temperatures by adjusting its light-capturing as well as carbon fixation characteristics, whereas stenothermal P. subfuscum relies on changes in carbon fixation without concomitant shifts in pigment content.