Active marine prokaryotic groups in two contrasting coastal systems: Blanes (NW Mediterranean) and Franklin Bay (Western Arctic)

Abstract

The identification of the phylogenetic groups of marine prokaryotes that are actively recycling the dissolved organic matter pool in the sea can greatly improve our understanding of the oceanic carbon cycle. However, for many years, the study of the activity of prokaryotic groups has been restricted to those bacteria that were easily cultured from seawater. The development of methodologies such as microautoradiography combined withfluorescence in situ hybridization (MARFISH) has been crucial to analyse the in situactivity of different groups. With this main objective, we applied this technique in two contrasting marine coastal systems: the NW Mediterranean and the coastal Western Arctic. We found some similarities in the prokaryotic assemblage structure, such as the dominance of Alphaproteobacteria (from 20% to over 50% of total cell counts), and more specifically the SAR11 group in both systems. However, Bacteroidetes reached high abundances in some periods in the Arctic (25% of cell counts during the phytoplankton bloom), while this group never exceeded 15% of cell counts in the Mediterranean. Betaproteobacteria were also more abundant in the Arctic (up to 6% of total cell counts), a system more affected by freshwater inputs, compared to Mediterranean waters, where this group was absent. Similarly, Archaea were quantitatively important during winter in the Arctic (15% of total cell counts), while in NW Mediterranean this group rarely exceeded 5% of cell counts. Surprisingly, the percentages of active cells in the uptake of three labile substrates (glucose, amino acids, and ATP) analysed by microautoradiography were similar in these two contrasting systems (generally within a range of 10-30% of active Eubacteria). The similarities of in situ activities of specific bacterial groups were also more striking than the differences. Despite the low contributions of Roseobacter to assemblage composition (range 1 to 8%), this group showed high activity in both systems, reaching values of over 70% of active cells in the uptake of all substrates. Betaproteobacteria was another low-abundance group but with high percentage of active cells in Arctic waters. On the contrary, SAR11 was more abundant year-round, but showed lower uptake activities in both systems (generally under 20% of active cells). Although more research is needed, the appearance of low-abundance but highly active groups, such as Roseobacter or Betaproteobacteria, seems to be a common feature in some coastal marine systems, and indicates a highly dynamic marine bacterioplankton assemblage with respect to the processing of organic matter