Résumé
Nutrient cycling is a key biogeochemical process underlying the
functioning of marine ecosystems. Yet, the contribution of fishes is still
poorly understood. This is problematic considering the current
modifications of fish assemblages experienced in certain regions such as
the Mediterranean Sea, and their potential consequences in terms of
ecosystem functioning. In this study, we used a mesocosm experiment to
test the effect of nutrient recycling by an invasive marine herbivorous
fish (Siganus rivulatus) on planktonic and benthic microbial communities.
The response of these communities was assessed using a variety of
analytical approaches such as measures of nutrient concentration, flow
cytometry, and metabarcoding of the 16S rRNA gene. Our results show that
several microbial compartments of marine ecosystems respond to the
nutrients released by fish through excretion and egestion. The nutrients
contained in the macroalgae consumed by S. rivulatus were excreted in
large amounts as dissolved nutrients, which resulted in higher
concentrations of N-based nutrients in the water (NH4, NO2/NO3). This
excess of N in the system was associated with higher abundances of
planktonic microbes (phyto- and bacterioplankton), modifications of the
structure of planktonic bacterial communities, and the tissue composition
of the remaining macroalgae. Non-assimilated nutrients were released in
the form of feces under the shelters where the fish spent most of their
time and defecated during the night, leading to local increases in
diversity and significant shifts in the structure of sediment bacterial
communities. Overall, our results suggest that the impact of S. rivulatus
on planktonic microbes was related to the indirect bottom-up effect
induced by excreted nutrients while its effect on benthic microbes was due
to a direct release of microbes from its gut microbiome. This study
represents one of the first assessment of the effect of nutrient recycling
by fishes on the microbial communities from several compartments of marine
ecosystems and one of the first evidence of the invisible effect of
invasive species on the microbial components of marine ecosystems.