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Free keywords:
community composition; diversity; patchiness; FISH; flow cytometry; marine bacteria; North Sea
Abstract:
A case study was performed to evaluate the potential of a combination of flow cytometry, cell sorting and fluorescence in situ hybridisation (FISH) for mesoscale monitoring of dominant bacterioplankton groups. In June 1999, the spatial distribution of phylogenetically characterised bacterioplankton groups in an area of 150 x 350 km in the northern North Sea was studied in conjunction with blooms of phytoplankton including coccolithophores. The bacterial cells were enumerated using flow cytometry and 3 groups were defined on the basis of cellular light scatter, nucleic acid and protein content. The phylogenetic composition of cells sorted from those 3 groups was analysed using FISH with a restricted set of rRNA targeted oligonucleotide probes. Cells with high nucleic acid and high protein content were mainly affiliated to the alpha- proteobacterial genus Roseobacter. Members of the Cytophaga- Flavobacterium cluster consistently accounted for the majority of cells in the high nucleic acid and low protein group; and members of the gamma-proteobacterial SAR86 cluster were always present in significant amounts among the cells with low nucleic acid and low protein content. The composition of bacteria within the groups was remarkably conservative in 13 randomly selected samples, while the concentration of the groups varied considerably on the 10 to 100 km scale. The bacterial groups formed patches of high abundance; these were spatially separated and could remain traceable for 2 to 3 d. The distribution of bacterioplankton groups did not correlate with distribution of either chlorophyll a (chl a), or phytoplankton groups (small coccolithophores and picoeukaryotic algae) or physical parameters such as temperature and salinity. It seems unlikely therefore that currently used remotely sensed parameters may be used as proxies of bacterioplankton group concentration at basin scales. However, this case study proves that a shipboard survey conducted over 4 to 6 d is effective for identifying and monitoring patches of certain bacterioplankton groups.
