Bacterial activity in the sea-surface microlayer: in situ investigations in the Baltic Sea and the influence of sampling devices

The sea-surface microlayer (SML) is considered to be an 'extreme' environment. How- ever, it is still unclear how bacteria that inhabit the SML (bacterioneuston) react to conditions within this interface. This deficiency is partly caused by the difficulty in obtaining representative samples. Our aim was to examine different sampling devices and characterize bacterioneuston activity in the Baltic Sea. Initial in situ studies revealed a decreased incorporation of 3 H-thymidine ( 3 H-TdR) by up to 90% in both glass-plate and metal-screen samples compared to the underlying bulk water. How- ever, a series of tank experiments showed selective inhibition of bacterial productivity with either of these sampling devices, although bacterial cell counts and community composition were unaltered. The inhibition introduced by the glass plate could not be nullified by different cleaning treatments, but by the wiping technique used to scrape off the sample. Even with this modified, unbiased glass- plate technique, 3 H-TdR incorporation of the bacterioneuston was still reduced by 50 to 80% com- pared to that in the underlying water, whereas the abundance of 5-cyano-2,3-ditolyl tetrazolium chloride (CTC)-positive cells was not affected. Our in situ study thus revealed that in the Baltic Sea the presence of a pronounced bacterioneuston community different from that in the underlying water is unlikely. Reduced bacterial activities within the SML support the concept of a demanding habitat. Additionally, this study emphasizes the need to carefully evaluate the sampling devices used when measuring bacterial parameters. Furthermore, it supports the view that caution is required in com- parisons of results from different studies.