Quantitative mapping of bacterioplankton populations in seawater: field tests across an upwelling plume in Monterey Bay

Few methods are available for quantifying specific prokaryotic taxa in mari ne plankton samples. In this study, we report a novel sampling and analysis strategy that circumvents some of the difficulties associated with current methods. This new approach allows for increased spatial, temporal and phyl ogenetic resolution over what has been achievable in routine bacterioplankt on surveys. Picoplankton from small volume samples (30 mi) were collected o n polysulfone filters and DNA was extracted with a commercially available D NA purification kit. The contribution of different bacterioplankton members at the group and subgroup levels was quantified by 5' nuclease assays. Per centages of small subunit (SSU) rDNAs from SAR11, SAR86, Roseobacter, Cytop haga and Synechococcus clades in DNA extracted from small samples were comp ared with SSU rDNA in DNA samples extracted from 6 to 91 seawater. Only sma ll differences were observed between the methods. The approach was also tes ted by estimating gene copy numbers in a seawater sample spiked with varyin g numbers of cells from a cultivated marine Roseobacter strain. Finally we measured SSU rDNAs from the same groups of marine bacterioplankton in sampl es from a rapid survey of an upwelling plume in Monterey Bay, California, U SA. A strong negative correlation between the percentage of Cytophagales an d recently upwelled water, and an overlap between higher SAR86 percentages and a chlorophyll a concentration peak was found. The results confirm that rapid mapping of specific bacterioplankton groups is achievable using small samples and 5' nuclease assays.