Differential visible spectral influence on carbon metabolism in heterotrophic marine flavobacteria

The visible spectrum of solar radiation is known to stimulate photoheterotrophic bacterial carbon metabolism. However, its impact on 'strictly' heterotrophic bacteria remains less explored. Here, we show that heterotrophic flavobacteria exhibit enhanced uptake and mineralization of dissolved organic carbon with increasing wavelengths of visible light, without employing any 'known' light-harvesting mechanisms. RNA sequencing identified blue light as a major constraint in the extracellular enzymatic hydrolysis of polymeric carbohydrates and acquisition of sugars, despite acting as a stimulus for inorganic carbon sequestration. In contrast, green-red and continuous full-spectrum lights activated diverse hydrolytic enzymes and sugar transporters, but obstructed inorganic carbon fixation. This 'metabolic switching' was apparent through limited nutrient uptake, suppressed light-sensitivity, oxidative stress response and promotion of inorganic carbon sequestration pathways under blue light. The visible light impact on metabolism may be of significant ecological relevance as it appears to promote cell-mediated mineralization of organic carbon in 'green-colored' chlorophyll-rich copiotrophic coastal seawater and inorganic carbon sequestration in 'blue-colored' oligotrophic open ocean. Thus, a novel regulatory role played by light on heterotrophic metabolism and a hidden potential of flavobacteria to sense and respond differentially to monochromatic lights influencing marine carbon cycling were unraveled.