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Archaeal activity dynamics in marine snow vs. ambient water in coastal European Sea

Final Report Summary - ARCADIA (Archaeal activity dynamics in marine snow vs. ambient water in coastal European Sea)

Project context and objectives

Bacterial assemblages from subsurface (100 m depth), meso- (200-1000 m depth) and bathypelagic (1000-4000 m depth) zones along a North Atlantic Ocean transect from 60 °N to 5 °S were characterised using pyrotag sequencing of the V6 region of the 16S rRNA gene. In a dataset of more than 830 000 pyrotags, 10 780 OTUs (operational taxonomic units) were identified. Statistics revealed a clear clustering of the bacterial communities according to the water masses. The bathypelagic zone showed higher taxonomic evenness than the overlying waters, albeit bacterial diversity was remarkably variable. Both abundant and low-abundance OTUs were responsible for the distinct bacterial communities characterising the major deep-water masses. These results reveal that deep-water masses act as bio-oceanographic islands for bacterioplankton leading to water mass-specific bacterial communities in the deep waters of the Atlantic.

Among the specific OTUs of the bathypelagic realm, cells belonging to the SAR324 cluster were shown to assimilate dissolved inorganic carbon (DIC) and to be associated with particles as revealed by microautoradiography linked to catalysed reporter deposition fluorescence in situ hybridisation (MAR-CARD-FISH). The SAR324 comprised 6-17 % of all prokaryotes throughout the meso- and bathypelagic. The DIC-positive SAR324 cells comprised 20.4 ± 13.3 % of total DIC-positive picoplankton cells, suggesting that SAR324 might be a significant contributor to the dark ocean's chemoautotrophy. This study also demonstrated that several uncultured Proteobacteria lineages which are indigenous and abundant in the dark, oxygenated ocean are likely mixotrophs and have the potential for autotrophic CO2 fixation, coupled to the oxidation of reduced sulphur compounds. These previously unrecognised metabolic types of dark ocean bacteria may play an important role in global biogeochemical cycles, and their activities may in part reconcile current discrepancies in the dark ocean's carbon budget.

The rank-frequency distribution of the 1000 most abundant OTUs indicates that only a few OTUs are very abundant with a long tail of low-abundance OTUs. We aimed at testing the seed-bank hypothesis in marine microbial communities. The rare OTUs (frequency < 0.01 %) becoming abundant in at least one sample represented 1.4% of the total number of OTUs but accounted for 22 % to the total frequency of OTUs. Mantel tests and canonical correspondence analysis revealed that 50 % of the patterns in community distribution of these "rare becoming abundant" OTUs were explained by the water masses properties. Statistics were used to determine the extent of stochastic processes (dispersal, immigration) shaping the microbial community structure.

To give insights into the dynamics in abundance and activity of marine aggregate-associated Archaea, sampling was carried out in Northern Adriatic from December 2009 to August 2010. Particles and aggregates were selectively collected by means of syringes operated by SCUBA divers. Total prokaryotes exhibited higher heterotrophic biomass production and higher abundance on particles than in the ambient water. CARD-FISH results showed that particle-associated Archaea accounted for up to 60 % of total prokaryotes and were significantly more abundant than free-living Archaea towards the summer period. Surprisingly, archaeal amoA genes (encoding the key enzyme ammonia monooxygenase in the oxidation of ammonia to nitrite) could not be detected in marine aggregates while bacterial and archaeal amoA genes were readily detected in ambient water during the winter period. Cultivation attempts with prokaryotes including Crenarchaea from marine aggregates showed promising preliminary results and are still in progress. The prokaryotic dynamics on deep-ocean particles was also investigated in terms of abundance, activity and diversity during a cruise in the Atlantic in October 2010. These analyses are still in progress at the University of Vienna, Austria.