2019 Volume 28 Issue 1 Pages 1-18
The microbial food chain (microbial loop), which links dissolved organic carbon through bacteria to protists and viruses, is an important driver of carbon and nitrogen cycling in the oceans. However, there are many unknown aspects of the variability in and controlling mechanisms of microbial loops in the meso- and bathypelagic zones, and the embedment of microbial processes in oceanic biogeochemical models remains rudimentary. During the 1990ʼs, the author examined the geographic distribution of bacteria in deep oceans, which led to the finding that bacterial production in the deep layer was coupled to sinking particle fluxes. In the following years, this study was further developed not only through large-scale, meridional transect studies conducted in the North and South Pacific Oceans and the Southern Ocean but also through time-series observations conducted at fixed oceanic stations. The new data have revealed that, in some cases, there are time lags between carbon transport from the surface and bacterial production response at depth. It has also become clear that microbial processes in the meso- and bathypelagic layers are more dynamic than previously thought. This paper provides an overview of the history of microbial loop studies in the meso- and bathypelagic zones and introduces case studies on the experimental analyses of carbon cycle systems that are driven by viruses and other microbes. Future research should focus on clarifying the mechanisms underlying the formation and destruction of large aggregates (marine snow), which are major mediators of the vertical carbon transport that connects the surface and the deep ocean.
