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2012/2013 Heterotrophic nanoflagellate (HNF) grazing is one of the major source of prokaryotic mortality in marine ecosystems, acting as a strong selection pressure on communities. Protozoans may thus affect prokaryotic abundance and alter the diversity and the taxonomic composition of the prey community, as individual prokaryotes can develop distinct grazing-resistant mechanisms. Moreover, the microbial loop is well known to regulate carbon fluxes in surface marine environments but few studies have quantified the impact of HNF predation on prokaryotes in the dark ocean. The present work was aimed to: (1) quantify the impact of HNF predation on the deep prokaryotes biomass; (2) investigate if and how prey diversity varies in response to different predation pressure; (3) define taxonomic community composition in studied areas and identify most affected prokaryotic phylotypes by HNF grazing (4) evaluate the effects of small HNF (3 µm, variazione significative dell’abbondanza sono state riscontrate in una maggiore percentuale di filotipi procariotici. Flagellati di maggiori dimensioni possono quindi mitigare l’impatto dei piccoli predatori sulle prede, con una maggior influenza nei sistemi fotici. Alcuni taxa batterici sono stati consumati prevalentemente dal grandi HNF (3-10 µm), mentre altri sono stati selezionati dai piccoli flagellati (

Ostreopsis ovata Fukuyo is a benthic dinoflagellate widespread from tropical to subtropical and warm temperate coastal areas world-wide. Since the species produces palytoxin-like substances that can accumulate in seafood, the apparent expansion of its range in recent years represents an increasing risk for human health. This leads to the necessity of monitoring protocols that enable the rapid detection of the presence of this microalga in environmental samples and sea-food. We developed an identification protocol based on real-time PCR (qPCR) to detect O. cf. ovata presence in different matrices. The protocol was proved to be able to reveal microalgal traces in both soft tissues and intervalvar liquid of mussels exposed to O. cf. ovata in natural and experimental conditions as well as in seawater samples. O. cf. ovata could also be detected in mussel tissues after the end of the bloom, when it was no longer detectable in sea water. We were able to detect O. cf. ovata in copepods fed on unialgal cultures as well. Cell density estimates based on standard curves resulted to be comparable to direct microscopical counts. The method is therefore suitable to ascertain the origin of palytoxin-like substances in toxic seafood. In addition, our results confirm that mussels and other predators can actually ingest O. cf. ovata cells and act as a vector for toxin transfer through both benthic and planktonic food webs.

The coastal northern Adriatic Sea receives pulsed inputs of riverine nutrients, causing phytoplankton blooms and seasonally sustained dissolved organic carbon (DOC) accumulation—hypothesized to cause episodes of massive mucilage. The underlying mechanisms regulating P and C cycles and their coupling are unclear. Extensive biogeochemical parameters, processes and community composition were measured in a 64-day mesocosms deployed off Piran, Slovenia. We followed the temporal trends of C and P fluxes in P-enriched (P +) and unenriched (P −) mesocosms. An intense diatom bloom developed then crashed; however, substantial primary production was maintained throughout, supported by tightly coupled P regeneration by bacteria and phytoplankton. Results provide novel insights on post-bloom C and P dynamics and mechanisms. 1) Post-bloom DOC accumulation to 186 μM remained elevated despite high bacterial carbon demand. Presumably, a large part of DOC accumulated due to the bacterial ectohydrolytic processing of primary productivity that adventitiously generated slow-to-degrade DOC; 2) bacteria heavily colonized post-bloom diatom aggregates, rendering them microscale hotspots of P regeneration due to locally intense bacterial ectohydrolase activities; 3) P i turnover was rapid thus suggesting high P flux through the DOP pool (dissolved organic phosphorus) turnover; 4) Alpha - and Gamma-proteobacteria dominated the bacterial communities despite great differences of C and P pools and fluxes in both mesocosms. However, minor taxa showed dramatic changes in community compositions. Major OTUs were presumably generalists adapted to diverse productivity regimes.We suggest that variation in bacterial ectohydrolase activities on aggregates, regulating the rates of POM → DOM transition as well as dissolved polymer hydrolysis, could become a bottleneck in P regeneration. This could be another regulatory step, in addition to APase, in the microbial regulation of P cycle and the coupling between C and P cycles.