Microbial Loop

The inefficiency of microbial heterotrophy does have a side benefit as it enhances nutrient remineralization rates. This serves to increase the availability of inorganic nitrogen and phosphorus for the photoautotrophs. The multiple roles of bacteria in the marine food web were shown in Figure 23.2, with the component of the food web controlled by the algal herbivores depicted on the left side and the microbial loop on the right. The viral shunt acts on both pathways. 

An understanding of the retention of sulfur in lake sediments is important for paleolimnological reconstructions, for an understanding of the alkalinity balance of lakes, for determination of rates and pathways of diagenesis, and for an understanding of the dynamics of the microbial loop in sediments. Profiles of rates and forms of S accumulation in sediments may preserve records of past conditions of climate, lake chemistry, or atmospheric de-... 

Model of plankton blooms date back to the classical work of Riley et al. (19microbial organisms was incorporated in models of the pelagic plankton by Pomeroy (1979), Pace et al. (1984) and Fasham (1985). The above works have all treated plankton communities on a seasonal scale. In the model described below, we deal with a simple model of phytoplankton and micro-organisms concerned with the "microbial loop" (Azam et al., 1983) on a daily time scale appropriate to the turnover time of these organisms. The model is based on biomass data collected by Holligan et al. 

Bratbak, G. F., F. Thingstad, and M. Heldal. 1994. Viruses and the microbial loop. Microbial Ecology 28 209-221. 

Jumars, P. A., D. H. Penry, J. A. Baross, M. J. Perry, and B. W. Frost. 1989. Closing the microbial loop Dissolved carbon pathway to heterotrophic bacteria from incomplete digestion and absorption in animals. Deep Sea Research 4 483-495. 

Murray, A. G. 1995. Phytoplankton exudation Exploitation of the microbial loop as a defense against algal viruses. Journal of Plankton Research 17 1079-1094. 

FIGURE 1 Schematic representation of the microbial loop. (Adapted from Fuhrman, 1999). 

Thingstad, T. F., A. Hagstrom, and F. Rassoulzadegan. 1997. Accumulation of degradable DOC in surface waters Is it caused by a malfunctioning microbial loop Limnology and Oceanography 42 398-404. 

The microbial loop concept has been the prevailing paradigm for marine microbial food webs for two decades and has stimulated work on DOM sources and composition, rates of biomass production, transfer efficiencies, and respiratory losses (Benner, 1998 del Giorgio and Cole, 2000 Ducklow, 2000 Williams, 2000). The only major modification has arisen from new information on the abundance and ecology of viruses (Wilhelm and Suttle, 1999 Fuhrman, 1999, 2000). 

Hart, D. R., L. Stone, and T. Berman. 2000. Seasonal dynamics of the Lake Kinneret food web The importance of the microbial loop. Limnology and Oceanography 45 350-361. 

Meyer, J. L. 1994. The microbial loop in flowing waters. Microbial Ecology 28 195-199. 

Tranvik L. J. 1992. Allochthonous dissolved organic matter as an energy source for pelagic bacteria and the concept of the microbial loop. FFydrobiologia 229 107-114. 

FIGURE 1 Detrital structure and primary fluxes of dissolved organic carbon in lake ecosystems with the food-web structure of predominantly pelagic components (box) that reflects the importance of the microbiota, the microbial loop, and physical processes by which most other organic matter inputs are metabolized heterotrophically or degraded by physical processes. DOM denotes dissolved organic matter, with labile (LDOM) and recalcitrant (RDOM) components POM denotes particulate organic matter. 

Osmotrophs are organotrophic organisms that acquire nutrients from their environment one molecule at a time. Osmotrophs include most members of the bacteria and archaea domains, as well as fungi and several protistan groups. Except in association with coarse organic particles where eukaryotic fungi and oomycetes are prominent, prokaryotes dominate heterotrophic metabolism, so are the focus of the text that follows. We use the term bacteria loosely, recognizing that heterotrophic archaea may well be major consitituents of the microbial loop of many systems. 

At the ecosystem scale, the significance of microbial community organization is that it determines the magnitude and efficiency of carbon transfer to other portions of the food web. High density communities like biofilms and floes can be directly grazed by metazoans, which divert microbial and detrital carbon out of the microbial loop (Wotton, 1994). In dilute planktonic systems, much of the carbon reaches metazoans through a microbial food-web after two to three trophic transfers, only a small fraction of microbial production remains. 

There are four major transformation pathways leading from the DOM pool into the microbial loop direct uptake and photolysis-, ectoenzyme-, and sorption-mediated uptake (Fig. 1). Each of these pathways or processes is regulated by a combination of intrinsic and extrinsic factors. Intrinsic factors are elements of the pathway itself and include DOM characteristics, enzyme kinetics, and microbial diversity. For instance, the uptake characteristics of the resident microbial community will affect which monomers are assimilated from the pool of DOM. Conversely, the composition of the DOM pool is likely to affect which microbial consortia are present and active at any given time. 

Clarholm, M. (1994).The microbial loop in soil. In Beyond the Biomass, Ritz, K., Dighton, J., and Giller, K. E., eds., Wiley-Sayce, Chichester, pp. 221-230. 

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