Establishment of microbiota in larval culture of Pacific oyster, Crassostrea gigas

Abstract

This study has two main objectives: (1) to implement a recycling aquaculture system (RS) for the larvae of the oyster Crassostrea gigas, and (2) to characterise the bacterial communities established in different compartments of this system. An RS with 25% fresh seawater addition per hour and another with no addition (0%) were compared with a flow-through system (FT). Larval survival was equivalent in RS and FT, but growth rate was 17% slower in RS than in FT. The physical chemical parameters remained stable, except for pH that decreased to 7.75 and salinity that increased to 37.5‰ in the RS 0%. In both systems, the cultivable bacteria were present in similar numbers in seawater (around 105 ml− 1) and in larvae (103 larva− 1) on day 15. Bacterial assemblages, characterised by 454 pyrosequencing of the V1-V3 region of 16S rRNA, were highly similar (50-65%) for compartments, regardless of rearing system and sampling time, but the compartments were clearly different from one another. At the beginning of rearing, larval microbiota was mostly composed of Proteobacteria (~ 90%), with 47% Rhodobacteraceae (α-Proteobacteria). γ-Proteobacteria, including Pseudoalteromonas, Alteromonas and a few vibrios, declined in the rearing period (25% on day 7 to 9% on day 15). At the end of rearing, colonisation by two members of the Burkholderiales (β-Proteobacteria), 45% on average on day 15, had decreased overall diversity. Seawater microbiota was more stable with in all batches as one unclassified bacterium present in all batches (27 ± 7%), 42 OTUs of α-Proteobacteria (19 ± 7%) and 26 of γ-Proteobacteria (14%). Change was due notably to a species of Cryomorphaceae (Flavobacteria) that reached 15 ± 7% on day 15. Predatory bacteria, Bdellovivrio spp. and Bacteriovorax spp. were present (3-12%) and could participate in the regulation of bacterial populations. Bacterial assemblages in RS bioreactors remained stable and were mainly composed of Rhodobacteraceae, Rhizobiales and Planctomycetes. Only a few nitrite oxidisers were detected and no ammonia oxidisers, although nitrification was efficient in RS without addition of new water. The larval microbiota was made up of bacteria growing in seawater, but some such as the Burkholderiales could have come from the broodstock. Several bacteria predominant in seawater were first harboured in the algal culture. Finally, despite sanitary measures in the hatchery (UV treatment and frequent cleaning), the diversity of Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site. microbiota remained high, and it did not contain pathogenic bacteria. The presence of this microbiota might even be indispensable to ensure normal ontogenesis, as suggested by the holobiont concept. Statement of relevance RAS and control of microbiota should improve bivalve larval culture Highlights ► Oyster larval survival was equivalent between recycling and flow-through rearing systems....

Determination of stocking density limits forCrassostrea gigaslarvae reared in flow-through and recirculating aquaculture systems and interaction between larval density and biofilm formation

“…Both systems correctly functioned until 150 larvae mL À1 . A previous experiment using the same FTS with Crassostrea gigas larvae at 50 and 100 larvae mL À1 showed similar results (da Costa et al, 2015), while a comparison between FTS and RAS at 50 larvae mL À1 (Asmani et al, 2016) revealed a growth deficit in RAS. Equivalent performances were previously obtained with an FTS in a 150-L tank (Rico-Villa et al, 2008, 2009.…”

“…In addition, a few opportunists such as V. aestuarianus and Vibrio neptunius were present at very low levels in both systems. The filamentous bacteria found in this study have not yet been reported in the formation of biofilm in aquaculture systems, particularly in bioreactors (Schneider et al, 2007;Blancheton et al, 2013;Ruan et al, 2015;Asmani et al, 2016). These were however encountered when OM was abundant, as in sludge, wastewater, and sediment (Lu et al, 2013;Gonzalez-Gil et al, 2015;Raulf et al, 2015).…”

“…However, the decline in performance, primarily of the rate of metamorphosis observed in RAS at 50 larvae mL À1 in comparison with FTS as well as in FTS with increasing density, could limit the use of these conditions unless this problem occurred only by accident. In other experiments, no difference was found in metamorphosis rates at day 30 between both systems (Asmani et al, 2016;unpublished results). The analyzed biofilm could be the cause of these low metamorphosis rates.…”

“…The experiment was performed using two different rearing systems, FTS and RAS, at the IFREMER experimental station in Argenton (Brittany, France) and three larval concentrations: 50, 150, and 300 larvae mL À1 . The stocking density of 50 larvae mL À1 was used as a control, since it has already been validated in previous experiments (da Costa et al, 2015;Asmani et al, 2016). Overall, four treatment experiments denoted FT150, FT300, RS150 and RS300, and two control experiments, denoted FT50 and RS50, were carried out.…”

“…In these conditions, the role of biofilms needs to be reconsidered. In a recent study, Asmani et al (2016) examined bacterial communities associated with different compartments of FTS and RAS including seawater, larvae, and bioreactor but not the biofilm on the tank wall.…”