CYANOBACTERIAL DOMINANCE OF POLAR FRESH-WATER ECOSYSTEMS - ARE HIGH-LATITUDE MAT-FORMERS ADAPTED TO LOW-TEMPERATURE

Although it is generally believed that cyanobacteria have high tempera ture optima for growth (> 20 degrees C), mat-forming cyanobacteria are dominant in many types of lakes, streams, and ponds in the Arctic and Antarctic. We studied the effect of temperature on growth (mu) and re lative pigment composition of 27 isolates of cyanobacteria (mat-formin g Oscillatoriaceae) from the Arctic, subarctic, and Antarctic to inves tigate whether they are a) adapted to the low temperature (i.e. psychr ophilic) or b) tolerant of the low temperature of the polar regions (i .e. psychotrophic). We also derived a parabolic function that describe s both the rise and the decline of cyanobacterial growth rates with in creasing temperature. The cyanobacteria were cultured at seven differe nt temperatures (5 degrees-35 degrees C at 5 degrees C intervals), wit h continuous (illumination of 225 mu mol photons . m(-2). s-(1). The p arabolic function fits the mu-temperature data with 90% confidence for 75% of the isolates. Among the 27 isolates of cyanobacteria studied t he temperature optima (T-opt) for growth ranged from 15 degrees to 35 degrees C, with an average of 19.9 degrees C. These results imply that most polar cyanobacteria are psychrotrophs, not psychrophiles. The cy anobacteria grew over a wide temperature range (typically 20 degrees C ) but growth rates were low even at T-opt (average mu(max) of 0.23 +/- 0.069 d(-1)). Extremely slow growth rates at low temperature and the high temperature for optimal growth imply that the cyanobacteria are n ot adapted genetically to cold temperatures, which characterize their ambient environment. Other competitive advantages such as tolerance to desiccation, freeze-thaw cycles, and bright, continuous solar radiati on may contribute to their dominance in polar aquatic ecosystems.