Comparison of sea surface microlayer and subsurface water bacterial communities in the Baltic Sea

Abstract

Sea surface microlayer (SML) and underlying subsurface water (SSW) bacterial communities from the Baltic Sea were studied using culturing, flow cytometry and terminal restriction fragment length polymorphism (T-RFLP). Dissolved organic matter (DOM) of water samples was determined using high performance liquid size-exclusion chromatography and by measuring dissolved organic carbon. Cadmium, copper, lead and mercury were also measured from the water samples. Two methods were compared for SML sampling: metal screen (MS) and polycarbonate membranes (PCM). All sampling sites were located in the Archipelago Sea off the southwest coast of Finland. The hypothesis was that because of its unique features, such as accumulation of nutrients and pollutants and being unshaded from UV-radiation, the SML would support a different bacterial community to that of SSW. T-RFLP profiles showed differences between the bacterial community compositions of SML and SSW at most sampling sites. In addition, temporal and sampling-site-dependent differences were observed. There were no significant differences in DOM in SML and SSW samples. In most cases, the number of bacteria was slightly higher in the SML than in SSW. No significant cytotoxicity was observed in any of the water samples. Since no temporal trends were obvious, our results suggest that, although bacterial communities in the SML Archipelago Sea differ to some extent from the ones living in SSW, SML is a dynamic environment where bacterial communities possibly develop originally from bacterioplankton and change over time.