Ecology of a novel Synechococcus clade occurring in dense populations in saline Antarctic lakes

The seasonal distribution and abundance of Synechococcus-like morphotypes was investigated in meromictic lakes and coastal areas of the Vestfold Hills, Antarctica. Populations were monitored by flow cytometry utilising phycoerythrin content and small cell size to distinguish the cells from other phytoplankton. In Ace Lake, the Synechococcus bloom commenced in September at the water temperature minimum and peaked in late November. Populations (up to 8 x 10(6) cells ml(-1)) were maximally stratified at a depth of 11 m, corresponding to waters which were supersaturated with oxygen, high in phosphate and which received > 5 pmol photons m(-2) s(-1) of light. At this depth, salinity (30 g kg(-1)) was constant throughout the year and temperature ranged from 4.5 degrees C in October to 10.5 degrees C in February. In late November, high numbers of Synechococcus cells also occurred in the moderate salinity water bodies Lake Abraxas and Pendant Lake (salinity 16.5 to 31.0 g kg(-1)), with populations highly stratified in Lake Abraxas (up to 1.5 x 10(7) cells ml(-1), temperature 8.0 degrees C, salinity 20.3 g kg(-1)) but less so in the colder waters of Pendant Lake (max. 1.5 x 10(7) cells ml(-1), temperature 0.1 to 1.1 degrees C, salinity 31.0 g kg(-1)). Synechococcus populations did not occur in brackish, coastal marine or hypersaline water bodies in the Vestfold Hills. Populations appear to be controlled primarily by temperature and to a lesser extent by light availability and grazing. Characterization of non-axenic cultures indicated that the Antarctic lake Synechococcus populations were similar to other polar picocyanobacteria in terms of cardinal growth temperatures (minimum, optimum, maximum: T-min -17.0 degrees C, T-opt, 19.8 degrees C, T-max 29.5 degrees C) and slow growth. Related only peripherally to Synechococcus sp. Cluster 5.2 (Marine Cluster B), the Antarctic strains represent a unique and highly adapted clade in the stable water columns of some saline Antarctic lakes.