The RCA (Roseobacter clade affiliated) cluster, with an internal 16S rRNA gene sequence similarity of 498%, is the largest cluster of the marine Roseobacter clade and most abundant in temperate to (sub)polar oceans, constituting up to 35% of total bacterioplankton. The genome analysis of the first described species of the RCA cluster, Planktomarina temperata RCA23, revealed that this phylogenetic lineage is deeply branching within the Roseobacter clade. It shares not 465.7% of homologous genes with any other organism of this clade. The genome is the smallest of all closed genomes of the Roseobacter clade, exhibits various features of genome streamlining and encompasses genes for aerobic anoxygenic photosynthesis (AAP) and CO oxidation. In order to assess the biogeochemical significance of the RCA cluster we investigated a phytoplankton spring bloom in the North Sea. This cluster constituted 5.1% of the total, but 10-31% (mean 18.5%) of the active bacterioplankton. A metatranscriptomic analysis showed that the genome of P. temperata RCA23 was transcribed to 94% in the bloom with some variations during day and night. The genome of P. temperata RCA23 was also retrieved to 84% from metagenomic data sets from a Norwegian fjord and to 82% from stations of the Global Ocean Sampling expedition in the northwestern Atlantic. In this region, up to 6.5% of the total reads mapped on the genome of P. temperata RCA23. This abundant taxon appears to be a major player in ocean biogeochemistry.

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The Pacific Ocean constitutes about half of the global oceans and thus microbial processes in this ocean have a large impact on global elemental cycles. Despite several intensely studied regions large areas are still greatly understudied regarding microbial activities, organic matter cycling and biogeography. Refined information about these features is most important to better understand the significance of this ocean for global biogeochemical and elemental cycles. Therefore we investigated a suite of microbial and geochemical variables along a transect from the subantarctic to the subarctic Pacific in the upper 200 m of the water column. The aim was to quantify rates of organic matter processing, identify potential controlling factors and prokaryotic key players. The assessed variables included abundance of heterotrophic prokaryotes and cyanobacteria, heterotrophic prokaryotic production (HPP), turnover rate constants of amino acids, glucose, and acetate, leucine aminopeptidase and β-glucosidase activities, and the composition of the bacterial community by fluorescence in situ hybridization (FISH). The additional quantification of nitrate, dissolved amino acids and carbohydrates, chlorophyll a, particulate organic carbon and nitrogen (POC, PON) provided a rich environmental context. The oligotrophic gyres exhibited the lowest prokaryotic abundances, rates of HPP and substrate turnover. Low nucleic acid prokaryotes dominated in these gyres, whereas in temperate and subpolar regions further north and south, high nucleic acid prokaryotes dominated. Turnover rate constants of glucose and acetate, as well as leucine aminopeptidase activity, increased from (sub)tropical toward the subpolar regions. In contrast, HPP and bulk growth rates were highest near the equatorial upwelling and lowest in the central gyres and subpolar regions. The SAR11 clade, the Roseobacter group and Flavobacteria constituted the majority of the prokaryotic communities. Vertical profiles of the biogeochemical and microbial variables markedly differed among the different regions and showed close covariations of the microbial variables and chlorophyll a, POC and PON. The results show that hydrographic, microbial, and biogeochemical properties exhibited distinct patterns reflecting the biogeographic provinces along the transect. The microbial variables assessed contribute to a better and refined understanding of the scales of microbial organic matter processing in large areas of the epipelagic Pacific beyond its well-studied regions.

Heterotrophic bacterioplankton communities play an important role in organic matter processing in the oceans worldwide. In order to investigate the significance of distinct phylogenetic bacterial groups it is not only important to assess their quantitative abundance but also their growth dynamics in relation to the entire bacterioplankton. Therefore bacterial abundance, biomass production and the composition of the entire and cell-proliferating bacterioplankton community were assessed in North Sea surface waters between the German Bight and 58°N in early summer by applying catalyzed reporter deposition (CARD-FISH) and bromodeoxyuridine fluorescence in situ hybridization (BrdU-FISH). Bacteroidetes and the Roseobacter group dominated the cell-proliferating fraction with 10–55 and 8–31% of total BrdU-positive cells, respectively. While Bacteroidetes also showed high abundances in the total bacterial fraction, roseobacters constituted only 1–9% of all cells. Despite abundances of up to 55% of total bacterial cells, the SAR11 clade constituted <6% of BrdU-positive cells. Gammaproteobacteria accounted for 2–16% of the total and 2–13% of the cell-proliferating cells. Within the two most active groups, BrdU-positive cells made up 28% of Bacteroidetes as an overall mean and 36% of roseobacters. Estimated mean growth rates of Bacteroidetes and the Roseobacter group were 1.2 and 1.5 day-1, respectively, and much higher than bulk growth rates of the bacterioplankton whereas those of the SAR11 clade and Gammaproteobacteria were 0.04 and 0.21 day-1, respectively, and much lower than bulk growth rates. Only numbers of total and cell-proliferating roseobacters but not those of Bacteroidetes and the other groups were significantly correlated to chlorophyll fluorescence and bacterioplankton biomass production. The Roseobacter group, besides Bacteroidetes, appeared to be a major player in processing phytoplankton derived organic matter despite its low partitioning in the total bacterioplankton community.

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