Multiple stressors on an Antarctic microplankton assemblage:: water soluble crude oil and enhanced UVBR level at Ushuaia (Argentina)

Changes in phytoplankton pigment content, in vivo fluorescence as well as in abundance and cell characteristics of phyto- and bacterioplankton were investigated on a field-collected microplankton assemblage from Ushuaia Bay (Southern Argentina). Effects of different experimental treatments were examined: natural and enhanced ultraviolet-B radiation (UVBR: 280-320 nm) exposures, and water soluble fraction (WSF) of crude oil contamination under both UVBR exposures. After a 5-day exposure to experimental treatments in microcosms, significant UVBR-induced deleterious effects were observed with afternoon depression of the photochemical yield followed by night-time recovery. A significant increase in photoprotective pigments (PPCs) was also observed. Due to their smaller size, picophytoplankton cells appeared to be more impacted than nanophytoplankton cells as revealed by their increasing mean cell size and decreasing growth rate implying a perturbation of the cell cycle. On the other hand, the differential response between the two bacterial sub-populations identified (i.e., as sub-populations 1 and 2 according to their cellular characteristics) suggests a higher vulnerability for only one of these sub-populations to UVBR stress. WSF alone was also shown to induce deleterious effects on phytoplankton assemblage. Nevertheless, bacteria were positively affected, and particularly bacterial sub-population 2. The combination of WSF and enhanced UVBR exposure resulted in an exacerbation of these individual effects, demonstrating a synergistic effect of both stresses. Moreover, Cryptomonas sp. were observed to develop only under dual stresses in response to their capacity to switch between phototrophic and mixotrophic states following stressed conditions. In situ studies with natural communities provided a unique tool for determining the short-term biological response of microplankton assemblages exposed to multiple stressors.