Importance in Marine Ecosystems
The microbial loop is of particular importance in increasing the efficiency of the marine food web via the utilization of dissolved organic matter (DOM), which is typically unavailable to most marine organisms. In this sense, the process aids in recycling of organic matter and nutrients and mediates the transfer of energy above the thermocline. More than 30% of dissolved organic carbon (DOC) incorporated into bacteria is respired and released as carbon dioxide. The other main effect of the microbial loop in the water column is that it accelerates mineralization through regenerating production in nutrient-limited environments (e.g. oligotrophic waters). In general, the entire microbial loop is to some extent typically five to ten times the mass of all multicellular marine organisms in the marine ecosystem. Marine bacteria are the base of the food web in most oceanic environments, and they improve the trophic efficiency of both marine food webs and important aquatic processes (such as the productivity of fisheries and the amount of carbon exported to the ocean floor). Therefore, the microbial loop, together with primary production, controls the productivity of marine systems, together with primary production, in the ocean.
Many planktonic bacteria are motile, using a flagellum to propagate, and show chemotaxis to locate, move toward, and attach to a point source of dissolved organic matter (DOM) where fast growing cells digest all or part of the particle. Accumulation within just a few minutes at such patches is directly observable. Therefore, the water column can be considered to some extent as a spatially organized place on a small scale rather than a completely mixed system. This patch formation affects the biologically-mediated transfer of matter and energy in the microbial loop.
More currently, the microbial loop is considered to be more extended (Kerner, Hohenberg, Ertl, Reckermannk, Spitzy 2003). Chemical compounds in typical bacteria (such as DNA, lipids, sugars, etc.) and similar values of C:N ratios per particle are found in the microparticles formed abiotically. It can be qualified to microparticles as a potentially attractive food source to bacterivorous planktons. If this is the case, the microbial loop should be considered to be extended by the pathway of direct transfer of dissolved organic matter (DOM) via abiotic microparticle formation to higher trophic levels. This has great ecological importance in two ways. First, it occurs without carbon loss, and makes organic matter more efficiently available to phagotrophic organisms, rather than only heterotrophic bacteria. Furthermore, abiotic transformation in the extended microbial loop depends only on temperature and the capacity of DOM to aggregate, while biotic transformation is dependent of its biological availability (Kerner, Hohenberg, Ertl, Reckermannk, Spitzy 2003).
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