Artificial Light at Night Affects Microbiota and Growth in the Oyster Crassostrea gigas: Correlations with the Daily Rhythm Robustness

Widespread in coastal environments, artificial light at night (ALAN) is suspected to disrupt organisms’ biological rhythms by altering natural light cycles and thus constitutes a growing threat to these ecosystems. This study evaluates the effects of ALAN exposure at low and realistic intensity (~1 lx) on a coastal keystone species, the oyster Crassostrea gigas. The results reveal that ALAN significantly impairs the expression of core circadian clock genes (CgClock and CgBmal1) as well as the valve opening behavior, affecting rhythmic characteristics such as its robustness and daily profile. At the same time, ALAN leads to a decrease in daily shell growth and to a disruption of the gill microbiota, associated with an obliterated day/night difference in microbial alpha diversity. A direct correlation between a decrease in daily rhythm robustness, limitation of shell growth, and some microbial strands is shown, suggesting that biological rhythm disruption caused by ALAN might have harmful physiological consequences in oysters.