Planktonic food webs: microbial hub approach

International audienceOur review consolidates published information on the functioning of the microbial heterotrophic components of pelagic food webs, and extends this into a novel approach: the `microbial hub' (HUB). Crucial to our approach is the identification and quantification of 2 groups of organisms, each with distinct effects on food-web flows and biogeochemical cycles: microbes, which are generally responsible for most of the organic carbon respiration in the euphotic zone, and metazoans, which generally account for less respiration than microbes. The key characteristics of the microbialhub approach are: all heterotrophic microbes are grouped together in the HUB, whereas larger heterotrophs are grouped into a metazoan compartment (METAZ); each food-web flow is expressed as a ratio to community respiration; summary respiration flows through, between, and from the HUB and METAZ are computed using flows from observations or models; both the HUB and METAZ receive organic carbon from several food-web sources, and redirect this carbon towards other foodweb compartments and their own respiration. By using the microbial-hub approach to analyze a wide range of food webs, different zones of the world ocean, and predicted effects of climate change on food-web flows, we conclude that heterotrophic microbes always dominate respiration in the euphotic zone, even when most particulate primary production is grazed by metazoans. Furthermore, climate warming will increase HUB respiration and channeling of primary production toward heterotrophic community respiration and decrease the corresponding METAZ flows. The microbial-hub approach is a significant evolution and extension of the microbial loop and food web, and provides a new, powerful tool for exploring pelagic community metabolism