The inhibitory properties of the microbial community of the coral mucus from the Mediterranean coral Oculina patagonica were examined. Out of 156 dif

The impact of ocean acidification and carbonation on microbial community structure was assessed during a large-scale in situ costal pelagic mesocosm study, included as part of the EPOCA 2010 Arctic campaign. The mesocosm experiment included ambient conditions (fjord) and nine mesocosms with pCO₂ levels ranging from ~145 to ~1420 μatm. Samples for the present study were collected at ten time points (t–1, t1, t5, t7, t12, t14, t18, t22, t26 to t28) in seven treatments (ambient fjord (~145), 2 × ~185, ~270, ~685, ~820, ~1050 μatm) and were analysed for "small" and "large" size fraction microbial community composition using 16S RNA (ribosomal ribonucleic acid) amplicon sequencing. This high-throughput sequencing analysis produced ~20 000 000 16S rRNA V4 reads, which comprised 7000 OTUs. The main variables structuring these communities were sample origins (fjord or mesocosms) and the community size fraction (small or large size fraction). The community was significantly different between the unenclosed fjord water and enclosed mesocosms (both control and elevated CO₂ treatments) after nutrients were added to the mesocosms, suggesting that the addition of nutrients is the primary driver of the change in mesocosm community structure. The relative importance of each structuring variable depended greatly on the time at which the community was sampled in relation to the phytoplankton bloom. The sampling strategy of separating the small and large size fraction was the second most important factor for community structure. When the small and large size fraction bacteria were analysed separately at different time points, the only taxon pCO₂ was found to significantly affect were the Gammaproteobacteria after nutrient addition. Finally, pCO₂ treatment was found to be significantly correlated (non-linear) with 15 rare taxa, most of which increased in abundance with higher CO₂.

Summary: Emiliania huxleyi produces calcium carbonate (CaCO ₃ ) coccoliths and transparent exopolymer particles (TEP), sticky, acidic carbohydrates that facilitate aggregation. E. huxleyi's extensive oceanic blooms are often terminated by coccolithoviruses (EhVs) with the transport of cellular debris and associated particulate organic carbon (POC) to depth being facilitated by TEP‐bound ‘marine snow’ aggregates. The dynamics of TEP production and particle aggregation in response to EhV infection are poorly understood. Using flow cytometry, spectrophotometry and FlowCam visualization of alcian blue (AB)‐stained aggregates, we assessed TEP production and the size spectrum of aggregates for E. huxleyi possessing different degrees of calcification and cellular CaCO ₃ :POC mass ratios, when challenged with two EhVs (EhV207 and EhV99B1). FlowCam imaging also qualitatively assessed the relative amount of AB‐stainable TEP (i.e., blue:red ratio of each particle). We show significant increases in TEP during early phase EhV207‐infection (∼ 24 h) of calcifying strains and a shift towards large aggregates following EhV99B1‐infection. We also observed the formation of large aggregates with low blue:red ratios, suggesting that other exopolymer substances contribute towards aggregation. Our findings show the potential for virus infection and the associated response of their hosts to impact carbon flux dynamics and provide incentive to explore these dynamics in natural populations.

The blooming cosmopolitan coccolithophore Emiliania huxleyi and its viruses (EhVs) are a model for density-dependent virulent dynamics. EhVs commonly exhibit rapid viral reproduction and drive host death in high-density laboratory cultures and mesocosms that simulate blooms. Here we show that this system exhibits physiology-dependent temperate dynamics at environmentally relevant E. huxleyi host densities rather than virulent dynamics, with viruses switching from a long-term non-lethal temperate phase in healthy hosts to a lethal lytic stage as host cells become physiologically stressed. Using this system as a model for temperate infection dynamics, we present a template to diagnose temperate infection in other virus–host systems by integrating experimental, theoretical, and environmental approaches. Finding temperate dynamics in such an established virulent host–virus model system indicates that temperateness may be more pervasive than previously considered, and that the role of viruses in bloom formation and decline may be governed by host physiology rather than by host–virus densities.

In the frame of the European Project on Ocean Acidification (EPOCA), the response of an Arctic pelagic community (3 μm) and free-living (FL; < 3 μm > 0.2 μm) bacteria by Automated Ribosomal Intergenic Spacer Analysis (ARISA) in 6 of the mesocosms, ranging from 185 to 1050 μatm initial pCO₂, and the surrounding fjord. ARISA was able to resolve, on average, 27 bacterial band classes per sample and allowed for a detailed investigation of the explicit richness and diversity. Both, the PA and the FL bacterioplankton community exhibited a strong temporal development, which was driven mainly by temperature and phytoplankton development. In response to the breakdown of a picophytoplankton bloom, numbers of ARISA band classes in the PA community were reduced at low and medium CO₂ (~ 185–685 μatm) by about 25%, while they were more or less stable at high CO₂ (~ 820–1050 μatm). We hypothesise that enhanced viral lysis and enhanced availability of organic substrates at high CO₂ resulted in a more diverse PA bacterial community in the post-bloom phase. Despite lower cell numbers and extracellular enzyme activities in the post-bloom phase, bacterial protein production was enhanced in high CO₂ mesocosms, suggesting a positive effect of community richness on this function and on carbon cycling by bacteria.