Fiona J. Scott

Sublithic bacteria associated with Antarctic quartz stones

Quartz stone sublithic cyanobacterial communities are common throughout the Vestfold Hills, Eastern Antarctica (68°S 78°E) contributing biomass in areas otherwise devoid of any type of vegetation. In this study, the sublithic microbial community and underlying soil was investigated using a variety of traditional and molecular methods. Although direct epifluorescent counts of the sublithic growth (average 1.1 × 109 cells g−1 dry weight) and underlying soil (0.5 × 109 cells g−1 dry weight) were similar, sublith viable counts (2.1 × 107 cfu g−1 dry weight) were on average 3-orders of magnitude higher in the subliths. Enrichment and molecular analyses revealed the predominate cyanobacteria were non-halophilic, able to grow optimally at 15–20°C, and were related to the Phormidium subgroup with several distinct morphotypes and phylotypes present. Sublithic heterotrophic bacterial populations and those of underlying soils included mostly psychrotolerant taxa typical of Antarctic soil. However, psychrophilic and halophilic bacteria, mostly members of the alpha subdivision of the Proteobacteria and the order Cytophagales, were abundant in the sublithic growth film (20–40% of the viable count and about 50% of isolated individual taxa) but absent from underlying soils. It is suggested that quartz stone subliths might constitute a “refuge” for psychrophilic bacteria.

Grazing by the Antarctic sea-ice ciliate Pseudocohnilembus

Abstract Potential uptake and clearance rates of fluorescent microspheres (FM) from 0.25 to 4.05 μm diameter were determined for the non-loricate ciliate Pseudocohnilembus sp. from Antarctic sea ice. The percentage of ciliate cells that ingested FM after 20 min incubation decreased with increasing particle diameter. Pseudocohnilembus sp. ingested FM between 0.25 and 4.05 μm in diameter. We offered FM at concentrations less than natural concentrations for plankton plus detrital material and obtained clearance rates less than those previously reported for bactivorous ciliates. Clearance rates were 3.6–5.4 nl cell−1 h−1 for FM 0.5 and 1 μm diameter, respectively, but decreased to 1.1 nl cell−1 h−1 for 1.97 μm diameter and 1.4 nl cell−1 h−1 for 4.05-μm-diameter FM. Clearance and uptake rates of FM 0.5 and 1 μm diameter indicate that Pseudocohnilembus sp. principally grazes on bacteria-sized particles. However, it can also ingest organisms as large as nanoplankton and may graze particles as small as femtoplankton and colloids. This suggests a feeding strategy that may suit the temporal and spatial changes in food availability in the sea-ice habitat.

Seasonal variation in plankton, submicrometre particles and size-fractionated dissolved organic carbon in Antarctic coastal waters

Abstract Concentrations of plankton, suspended particles 0.74–87 μm equivalent spherical diameter and dissolved organic carbon (DOC) were measured from May to February at an Antarctic coastal site. Bacteria-sized particles 0.74–1 μm diameter, and bacterial cells and heterotrophic protists all exhibited a seasonal minimum during winter and maxima in summer. Bacteria composed <10% of the bacteria-sized particles. Release of autotrophic protists from the ice caused water column biomass of autotrophs to reach maximum concentrations in October and November, but maximum cell concentration in the water column was reached in January. Microheterotroph biomass weakly reflected the release of the ice algal community but reached maximum concentration during the water column bloom in January. Total DOC concentrations varied from 0.36 mg C l−1 in July to 3.10 mg C l−1 in October, with a yearly average of 1.51 mg C l−1. Ultrafiltration of DOC revealed that the molecular weight composition of the DOC differed greatly through the year. DOC <5 kDa molecular weight reached a maximum of 1.25 mg C l−1 in October and accounted for up to 60% of total DOC in July. Concentrations of high molecular weight DOC (>100 kDa) were highest in July and November, with the DOC (100 kDa–0.5 μm) fraction reaching a maximum of 1.22 mg C l−1 in November and composing 82% of the total DOC in January. Wet chemical oxidation and high-temperature catalytic oxidation organic carbon analyses were compared. Good correlation was observed between methods during summer but no significant correlation existed in winter, indicating that winter DOC may be refractory.