Reviews and syntheses: Bacterial bioluminescence – ecology and impact in the biological carbon pump

Abstract. Around thirty species of marine bacteria can emit light, a critical characteristic in the oceanic environment where the major part is deprived of sunlight. In this article, we first review current knowledge on bioluminescent bacteria symbiosis in light organs. Then, focusing on gut-associated bacteria, we highlight that recent works, based on omics methods, confirm previous claims about the prominence of bioluminescent bacterial species in fish guts. Such host-symbiont relationships are relatively well established and represent important knowledge in the bioluminescence field. However, the consequences of bioluminescent bacteria continuously released from light organ and through the digestive tracts to the seawater have been barely taken into account at the ecological and biogeochemical level. For too long neglected, we propose to consider the role of bioluminescent bacteria, and to reconsider the biological carbon pump taking into account the bioluminescence effect (bioluminescence shunt hypothesis). Indeed, it has been shown that marine snow and fecal pellets are often luminous due to microbial colonization, which makes them a visual target. These luminous particles seem preferentially consumed by organisms of higher trophic levels in comparison to non-luminous ones. As a consequence, the sinking rate of consumed particles could be either increased (due to repackaging) or reduced (due to sloppy feeding or coprophagy/coprorhexy) which can imply a major impact on global biological carbon fluxes. Finally, we propose a strategy, at a worldwide scale, relying on recently developed instrumentation and methodological tools to quantify the impact of bioluminescent bacteria in the biological carbon pump.