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The effects of grazing pressure and inorganic nutrient availability on the direct carbon transfer from freshly produced phytoplankton exudates to heterotrophic bacteria biomass production were studied in Mediterranean coastal waters. The short-term incorporation of ¹³C (H¹³CO₃) in phytoplankton and bacterial lipid biomarkers was measured as well as the total bacterial carbon production (BP), viral lysis and the microbial community structure under three experimental conditions: (1) High inorganic Nutrient and High Grazing (HN + HG), (2) High inorganic Nutrient and Low Grazing (HN + LG) and (3) under natural in situ conditions with Low inorganic Nutrient and High Grazing (LN + HG) during spring. Under phytoplankton bloom conditions (HN + LG), the bacterial use of freshly produced phytoplankton exudates as a source of carbon, estimated from ¹³C enrichment of bacterial lipids, contributed more than half of the total bacterial production. However, under conditions of high grazing pressure on phytoplankton with or without the addition of inorganic nutrients (HN + HG and LN + HG), the ¹³C enrichment of bacterial lipids was low compared with the high total bacterial production. BP therefore seems to depend mainly on freshly produced phytoplankton exudates during the early phase of phytoplankton bloom period. However, BP seems mainly relying on recycled carbon from viral lysis and predators under high grazing pressure.

The abundances of 17 cytometric groups encompassing four groups of bacteria, three groups of cyanobacteria, six groups of eukaryotic picophytoplankton (<2–3 µm), and four groups of small eukaryotic nanophytoplankton (between 3 and 6 µm) were studied in the Thau Lagoon across different seasonal conditions. Among them, the growth (µ) and mortality rates due to grazing (g) of 12 groups of bacteria, cyanobacteria, and eukaryotic pico- and nanophytoplankton were simultaneously studied in the Thau Lagoon via four dilution experiments across different seasonal conditions. The abundances of heterotrophic flagellates and ciliates and their potential predators were studied and linked to prey mortality. Bacteria were more active than phytoplankton and displayed the highest growth and grazing mortality rates. Most studied groups had g:µ ratios > 1, suggesting that predators efficiently grazed and transferred these preys. Surprisingly, the observed variations in predator abundance across seasons did not necessarily correspond with changes in grazing pressure. The significant positive relationship found between water temperature and bacterial grazing mortality rates and the reverse trend observed for larger eukaryotic prey suggested that warmer water increases small prokaryote transfer to higher trophic levels but disadvantages larger eukaryotic prey transfer. [ABSTRACT FROM AUTHOR]

Vibrio species have been associated with recurrent mass mortalities of juvenile oysters Crassostrea gigas threatening oyster farming worldwide. However, knowledge of the ecology of pathogens in affected oyster farming areas remains scarce. Specifically, there are no data regarding (i) the environmental reservoirs of Vibrio populations pathogenic to oysters, (ii) the environmental factors favoring their transmission, and (iii) the influence of oyster farming on the persistence of those pathogens. This knowledge gap limits our capacity to predict and mitigate disease occurrence. To address these issues, we monitored Vibrio species potentially pathogenic to C. gigas in 2013 and 2014 in the Thau Lagoon, a major oyster farming region in the coastal French Mediterranean. Sampling stations were chosen inside and outside oyster farms. Abundance and composition of phyto-, microzoo-, and mesozooplankton communities were measured monthly. The spatial and temporal dynamics of plankton and Vibrio species were compared, and positive correlations between plankton species and vibrios were verified by qPCR on isolated specimens of plankton. Vibrio crassostreae was present in the water column over both years, whereas Vibrio tasmaniensis was mostly found in 2013 and Vibrio aestuarianus was never detected. Moreover, V. tasmaniensis and V. crassostreae were found both as free-living or plankton-attached vibrios 1 month after spring mortalities of the oyster juveniles. Overall, V. crassostreae was associated with temperature and plankton composition, whereas V. tasmaniensis correlated with plankton composition only. The abundance of Vibrio species in the water column was similar inside and outside oyster farms, suggesting important spatial dispersion of pathogens in surrounding areas. Remarkably, a major increase in V. tasmaniensis and V. crassostreae was measured in the sediment of oyster farms during cold months. Thus, a winter reservoir of pathogenic vibrios could contribute to their ecology in this Mediterranean shellfish farming ecosystem. [ABSTRACT FROM AUTHOR]

The Thau lagoon is a Mediterranean coastal lagoon used for shellfish farming. It is periodically affected by anoxia events that trigger oyster mortality. To investigate the effects of an anoxia event focussed on nutrient dynamics and the responses of the microbial planktonic communities a 13-day in situ experiment was performed in September 2020. Transparent mesocosms (270 L) were placed at a depth of 4 m, inserted in the sediment, and kept closed throughout the experiment. The experiment comprised three treatments: i) Natural environment (N), i.e. in the natural water outside the mesocosms containing a rope of 30 oysters (Crassostrea gigas), ii) Control mesocosm (C) filled with natural water with no oysters, and iii) Oyster mesocosm (O) filled with natural water containing a rope of 30 oysters. Oyster respiration in the oyster mesocosm depleted oxygen after 54 h. All the oysters from O mesocosm were dead after nine days and decomposition of their flesh combined with releases from the water-sediment interface increased dissolved inorganic nitrogen (dominated by ammonium), phosphates, and ∑H 2 S up to 390, 17 and 295 μmol·L−1, respectively. Phytoplankton biomass consequently increased by 20 (11.8 μg chl al −1) and abundance by 4.5 (186 × 106 cells·L1) dominated largely by green algae <5 μm. During the oyster mortality period (day 6 to day 9) high abundances of heterotrophic flagellates and large ciliate specimens were observed. This shift in the community towards small phytoplankton favours the microbial loop and is detrimental to shellfish farming. In a context of global warming in which the risk of anoxia is higher, the results of the present investigation demonstrate that anoxia triggers shellfish mortality and that the change in the plankton community disrupts the normal functioning of the ecosystem, causing serious financial losses. In this context, it is crucial to predict possible hypoxia and anoxia events using high frequency measurements of dissolved oxygen, by avoiding using shallow zones for oyster production and by reducing shellfish stocks, or by mechanically lifting the oysters out of the water during the night to reduce oxygen respiration in the ecosystem. • In presence of oysters, anoxia was reached after 54 h in the mesocosm. • A picophytoplankton bloom was observed during anoxia and oyster decomposition. • Anoxia and oyster mortalities induced a community shift towards small plankton. [ABSTRACT FROM AUTHOR]