Microbial response to simulated climate change in Antarctic fellfield soil

A chronosequence of soil samples, taken over the 1992-3 austral summer, was used to evaluate the bacterial response to simulated climate change brought about by passive greenhouses (cloches) placed on fellfield soil at Signy Island, maritime Antarctic. Bacterial numbers, electron transfer chain (ETS), glucosidase and galactosidase activity were significantly elevated by the simulation, with a greater increase in numbers and activities beneath a warming and UV-blocking cloche in place for eight years prior to sampling than beneath a cloche that warmed soil under natural UV-B flux in place for five years. The simulation had no effect on soil saccharide at either site, but soil carbon was significantly greater (P < 0.05) in the cloche-cover soil beneath the warming/UV-screening cloche. In addition, moss shoots were evident beneath this cloche, leading to the conclusion that climate change will increase the rate at which microbially-dominated polygon mineral soil accumulates carbon and succeeds to moss-dominated peat soil.

he yield coefficient of a dominant bacterial isolate was calculated to determine the seasonal bacterial energy expenditure on biomass production. Cloche-cover increased bacterial energy expenditure in both sites (e.g. from 4.95 to 54.28 \(\mu\)g glucose g\(^{-1}\) soil 53 d\(^{-1}\) at polygon WW2). Microbial ATP production, calculated from measurements of ETS activity, was also significantly greater beneath warmed soil (e.g. from 9.76 to 27.1 \(\mu\)mol ATP g\(^{-1}\) soil 53 d\(^{-1}\) in polygon WW2). The potential release of hexose by polysacchridase activity during the course of the season wa also greater beneath the cloches, compared to exposed soil, at both sites (e.g. from 58.57 to 189.5 \(\mu\) mol hexose g\(^{-1}\) soil 53 d\(^{-1}\) in polygon WW2.

A second field study was conducted to assess the response of the bacterial and microalgal communities to separate warming and UV-B protection. A cloche that screened UV-B under ambient temperature resulted in a significantly greater microalgal colonisation (P<0.001), than in exposed soil, and a reduction in the biomass ratio between microalgae and bacteria. A cloche simulating global warming under a natural UV-B flux caused a significantly greater cyanobacterial colonisation, and a succession from domination by the filamentous Phormidium autumale to the aseriate Nostoc commune.

Twenty five bacterial isolates from fellfield soil at Signy Island, maritime Antarctic, were investigated to determine their ability to metabolise the two key carbon sources (polysaccharide material and low molecular weight carbohydrates) reported in polygon soil. The majority of isolates (88%) produced at least one exopolysaccharidase enzyme (of four screenedL \(\alpha\)-D-glucosidase, \(\beta\)-D-glucosidase, \(\alpha\)-D-galactosidase and \(\beta\)-D-galactosidase). Fifteen of the isolates required yeast extract for growth: the remaining ten were screened for ability to grow on 11 sugars and polyols recorded in polygon soil. A 'nutritional felixibility index' was used to compare the ability of isolates to metabolise these carbohydrates. There was no evidence of a distinct metabolic strategy for bacterial use of soluble carbohydrates against polysaccharide. All the isolates were psychrotrophic (maximum growth rate above 20°C with growth at O°C), rods with 21 gram-negative and 19 pigmented.