Influence of OSHV-1 oyster mortality episode on dissolved inorganic fluxes: An ex situ experiment at the individual scale

Abstract

Ostreid herpesvirus 1 (OsHV-1 μvar) infection has caused significant mortalities in juvenile oysters (Crassostrea gigas). In contrast to the practices of other animal production industries, sick and dead oysters are not separated from live ones and are left to decay in the surrounding environment, with unknown consequences on fluxes of dissolved materials. A laboratory approach was used in this study to test the influence of oyster mortality episode on dissolved inorganic fluxes at the oyster interface, dissociating (i) the effect of viral infection on metabolism of juvenile oysters and (ii) the effect of flesh decomposition on oxygen consumption and nutrient releases at the individual scale. Nine batches of juvenile oysters (Individual Total wet weight 1 g) were infected via injection of OsHV-1 enriched inoculums at different viral loads (108 and 109 OsHV-1 DNA copies per oyster) to explore infection thresholds. Oysters injected with filtered seawater were used as controls (C). Oysters were maintained under standard conditions to avoid stress linked to hypoxia, starvation, or ammonia excess. Before, after the injection and during the mortality episode, i.e. at days 1, 3, 7, 10 and 14, nine oysters per treatment were incubated in individual metabolic chambers to quantify oxygen, ammonium and phosphate fluxes at the seawater-oyster interface. Nine empty chambers served as a reference. Injections of the two viral loads of OsHV-1 induced similar mortality rates (38%), beginning at day 3 and lasting until day 14. The observed mortality kinetics were slower than those reported in previous experimental pathology studies, but comparable to those observed in the field (Thau lagoon, France). This study highlights that oxygen and nutrient fluxes significantly varied during mortality episode. Indeed (i) OsHV-1 infection firstly modifies oyster metabolism, with significant decreases in oxygen consumption and ammonium excretion, and (ii) dead oysters lead to a strong increase of ammonium (6 fold) and phosphate (41 fold) fluxes and a decrease in the N/P ratio due to mineralisation of their flesh. The latter may modify the structure of the planktonic community in the field during mortality episode. This study is a first step of the MORTAFLUX program. The second step was to in situ confirm this abnormal nutrient loading during a mortality episode and show its impact on bacterio-, phyto-and protozoo-plankton. Highlights ► This study is the first one to show consequences of oyster mortality episode in matter cycle using a lab approach ► Dissolved inorganic fluxes at the water-shell interface of Pacific oyster significantly varied during mortality episode ► OsHV-1 infection decreases oxygen consumption and ammonium excretion of oyster juveniles. ► Mineralisation of the flesh of dead oysters leads to a profound increase in ammonium and phosphate fluxes and a decrease in the N/P ratio with potential incidence on planktonic communities.

Changes in planktonic microbial components in interaction with juvenile oysters during a mortality episode in the Thau lagoon (France)

“…Indeed mineralization of dead oyster flesh was shown to induce huge increases of PO 4 (41 fold) and NH 4 (6 fold) fluxes at the water-dead oyster interface and a significant decrease of NH 4 :PO 4 ratio in lab conditions ). In the same way, DIN:DIP significantly decreased in the Thau lagoon during the "moribund" week (W18), reaching 2.6, which is lower than recorded during decomposition of oyster flesh in laboratory conditions (11.3: Richard et al 2017). hypothesized that the disequilibria in ammonia and phosphorus releases in relation to flesh decomposition may favour bacterial proliferation.…”

“…Dead oysters are kept in the rearing environment until their flesh totally disappears. Using a laboratory approach, Richard et al (2017) showed that mortality of oyster juveniles leads to a significant increase of ammonium and phosphate fluxes and a decrease in the N:P ratio in relation to the decomposition and mineralization of oyster flesh with a possible impact on the planktonic community structure.…”

“…Oyster flesh were pooled in Eppendorf and stored in ethanol at -80°C for OsHV-1 analysis. Absolute quantification of OsHV-1 genome copies was estimated for each oyster sample as described inRichard et al (2017). Genomic DNA was purified from pool of oyster tissues using the Wizard® SV Genomic DNA Purification System (Promega).…”

Body size and temperature effects on standard metabolic rate for determining metabolic scope for activity of the polychaete Hediste (Nereis) diversicolor

“…SMR was proxied by oxygen consumption rate and measurements were made using chambers (125-mL glass bottles H = 102 mm, Ø= 51 mm) with rubber-sealed polypropylene caps (Fisher Brand 2911475) fitted with optodes so a fiber optic oxygen transmitter (Fibox 4 trace, PreSens—Precision Sensing GmbH, Regensburg, Germany) could obtain a phase measurement which was later converted to oxygen concentration ( Richard et al, 2017 ). The optodes were previously calibrated with an oxygen-saturated solution and an oxygen-free solution (containing Mn(OH) 2 ).…”

Toward reference intervals for shellfish: An illustrative case of feeding and respiratory activities in the Pacific cupped oyster, Crassostrea gigas

“…Temperature, 5‐7 salinity, 7 and the quantity or quality of suspended matter 2,6,8 have shown to be the main environmental factors influencing C gigas metabolism. Some authors showed that CR and OCR also fluctuate during pathogenic infections in oysters 9‐12 . These rates could be measured simultaneously at high flow rates using a non‐invasive approach allowing for the preservation of an animal's integrity 13 .…”