Temporal variability of coastal Planctomycetes clades at Kabeltonne station, North Sea.

Members of the bacterial phylum Planctomycetes are reported in marine water samples worldwide, but quantitative information is scarce. Here we investigated the phylogenetic diversity, abundance, and distribution of Planctomycetes in surface waters off the German North Sea island Helgoland during different seasons by 16S rRNA gene analysis and catalyzed reporter deposition fluorescence in situ hybridization (CARD-FISH). Generally Planctomycetes are more abundant in samples collected in summer and autumn than in samples collected in winter and spring. Statistical analysis revealed that Planctomycetes abundance was correlated to the Centrales diatom bloom in spring 2007. The analysis of size-fractionated seawater samples and of macroaggregates showed that ~90% of the Planctomycetes reside in the >3-μm size fraction. Comparative sequence analysis of 184 almost full-length 16S rRNA genes revealed three dominant clades. The clades, named Planctomyces-related group A, uncultured Planctomycetes group B, and Pirellula-related group D, were monitored by CARD-FISH using newly developed oligonucleotide probes. All three clades showed recurrent abundance patterns during two annual sampling campaigns. Uncultured Planctomycetes group B was most abundant in autumn samples, while Planctomyces-related group A was present in high numbers only during late autumn and winter. The levels of Pirellula-related group D were more constant throughout the year, with elevated counts in summer. Our analyses suggest that the seasonal succession of the Planctomycetes is correlated with algal blooms. We hypothesize that the niche partitioning of the different clades might be caused by their algal substrates.