Factors Controlling The Microbial Loop
The efficiency of the microbial loop is determined by the density of marine bacteria within it (Taylor and Joint 1990). It has become clear that bacterial density is mainly controlled by the grazing activity of small protozoans and various taxonomic groups of flagellates. Also, viral infection causes bacterial lysis, which release cell contents back into the dissolved organic matter (DOM) pool, lowering the overall efficiency of the microbial loop. Mortality from viral infection has almost the same magnitude as that from protozoan grazing. However, compared to protozoan grazing, the effect of viral lysis can be very different because lysis is highly host-specific to each marine bacteria. Both protozoan grazing and viral infection balance the major fraction of bacterial growth. In addition, the microbial loop dominates in oligotrophic waters, rather than in eutrophic areas - there the classical plankton food chain predominates, due to the frequent fresh supply of mineral nutrients (e.g. spring bloom in temperature waters, upwelling areas). The magnitude of the efficiency of the microbial loop can be determined by measuring bacterial incorporation of radiolabeled substrates (such as tritiated thymidine or leucine).
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