Oyster Farming, Temperature, and Plankton Influence the Dynamics of Pathogenic Vibrios in the Thau Lagoon

Abstract

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.

Keywords: Vibrio; bivalve mollusks; mortality outbreak; phytoplankton; shellfish farming; zooplankton.

FIGURE 1

Location and map of the Thau Lagoon. The sampling sites (red dots) are located inside (A9 and Rephy) and outside (A3 and A5) the oyster farms. The position of oyster-culture rafts within farms is indicated by gray blocks. The position of seawater channels is indicated by an ^∗.

FIGURE 2

Monthly abundance of free-living vibrios in the water column. Histograms show apparent vibrio concentrations based on qPCR amplification (16S rRNA, ompU, and R5-2) of DNA extracted from the 0.2–5 μm fraction of the water column. Samples were collected outside the oyster culture area (A3, gray bars and A5, white bars) and inside an oyster farm (A9, hatched white bars and Rephy, hatched black bars). Mean seawater temperature (°C) is represented by a red line. n.d., not determined.

FIGURE 3

Monthly abundance of plankton-associated vibrios in the water column. Histograms show apparent vibrio concentrations based on qPCR amplification (16S rRNA, ompU, and R5-2) of DNA extracted from the >20 μm fraction of the water column. Samples were collected outside the oyster culture area (A3, gray bars and A5, white bars) and inside an oyster farm (A9, hatched white bars and Rephy, hatched black bars). Mean seawater temperature (°C) is represented by a red line. n.d., not determined.

FIGURE 4

Monthly abundance of total mesozooplankton and phytoplankton. (A,B) 2013 and (C,D) 2014 monthly values of plankton abundance outside (A5, white bars) and inside oyster farms (Rephy, black bars). Total mesozooplankton counts (individuals m^-3) are shown in (A,C). Total phytoplankton counts (cells L^-1) are shown in (B,D). n.d., not determined.

FIGURE 5

Relationships between total vibrios abundance and environmental parameters. (A) Principal component analysis of environmental data from 2013 to 2014: Temperature, Salinity, Total Zooplankton, Total Phytoplankton, Plankton-associated vibrios (>20 μm fraction), Free-living vibrios (0.2–5 μm fraction). For 2014, ciliates data (microzooplankton) are also presented divided into two types of organisms: Aloricates and Tintinnids. (B) Results of a general linear model showing the relative importance of environmental variables on the plankton associated vibrios abundance (16S rRNA in the >20 μm fraction) and the free-living vibrios (0.2–5 μm fraction). Results in boldface indicate a significant (p < 0.05) effect of the variable.

FIGURE 6

Seasonal abundance of vibrios in the sediment. Mean concentration of Vibrio tasmaniensis and Vibrio crassostreae (MPN CFU/g) in the surface sediment collected outside the oyster culture area (A5, white bar) and inside an oyster farm (Rephy, black bars). Mean subsurface seawater temperature (°C) is represented by a red line.