Growth of the Antarctic sea ice diatom Navicula cf. normaloides Cholnoky at different temperatures and salinities
The Antarctic continent is separated and isolated by the Antarctic Circumpolar Current from more northern water masses since millions of years, and hence characterised by low and stable temperatures throughout the whole year. Due to this long isolation a large number of endemic marine algae and animals have developed in this southern polar region, which probably might respond negatively to the predicted climate change as already observed around the Antarctic Peninsula. For algae the degree of endemism in the Antarctic flora had been well analysed for macroalgae, while for microalgae such as phytoplankton, sea ice algae or microphytobenthos only few data exist until now. From an ecophysiological standpoint the evaluation and characterisation of the temperature requirements for growth represents a widely used approach to identify algae endemic to Antarctica because of their adaptation to low temperatures. Since such data on polar diatoms are rare, an Antarctic isolate of the genus Navicula was incubated under a range of temperatures (1, 5, 15, 20, 25 degC) and the growth response followed. In addition, since salinity represents also a variable parameter along the ice edges of Antarctica, growth was measured under various salinities between 5 and 50 S A Navicula cf. normaloides Cholnoky grew under all salinities between 5 and 50 S A, but with different rates indicating a broad tolerance range. This diatom grew between 1degC and 15 degC with an optimum at 5 degC, but at 20 degC and 25 degC growth was completely inhibited. Hence this Navicula species can be described as a stenothermal/psychrotolerant species, because it has a broader temperature tolerance and prefers higher temperatures compared to other micro and macroalgae endemic to Antarctica. Navicula cf. normaloides Cholnoky exhibited an unexpectedly broad temperature tolerance, and hence might certainly not be considered as an endemic species, although it can well cope with cold to temperate conditions.