MEPS 753:55-72 (2025)  -  DOI: https://doi.org/10.3354/meps14769

ABSTRACT: Oyster biofilms have significant potential as reactors for excreted nitrogen (N) via assimilative, dissimilative and oxidative pathways. However, the interplay between molluscs, associated microbes and microalgae within biofilms is not well characterized. This work aimed to quantify dark and light oxygen (O₂) fluxes and N transformations in biofilm-colonised and biofilm-free oyster holobionts (i.e. oysters and their associated microbial communities) and to contrast N processes in oyster holobionts with those underneath the sediments. Oysters and sediments were collected from Goro Lagoon in northern Italy. Measurements were carried out in mesocosms and cores, tracing the fate of ¹⁵N-labelled ammonium (NH₄⁺) and nitrate (NO₃^-). We hypothesized the dominance of assimilative and dissimilative N pathways under light and dark conditions, respectively, and large stimulation of NO₃^- reduction in sediments underneath oysters due to biodeposits. Oyster holobionts, regardless of biofilm presence or absence on their outer shells and the presence or absence of illumination, were net O₂ sinks and NH₄⁺ and NO₃^- sources to the water column. Biofilm photosynthesis was insufficient to contrast respiratory O₂ demand but promoted nitrification of excreted NH₄⁺. Nitrification and denitrification were also recorded in biofilm-free oysters, suggesting active nitrifying and denitrifying microbial communities inside molluscs. Denitrification efficiency measured in biofilm-colonised and biofilm-free oyster holobionts was <10%, suggesting that oysters in the Goro Lagoon are weak N sinks and promote its recycling. Sediments receiving biodeposits recycled NH₄⁺ at rates comparable to control sediments; however, the former displayed higher denitrification rates. N processes mediated by oyster holobionts largely exceeded those occurring in sediments.

KEY WORDS: Magallana (Crassostrea) gigas · Biofilm · Nitrogen recycling · Nitrification · Excretion · Denitrification