Bacteria, and the Microbial Loop

The focus of this review has been on the history and state-of-the-art of N-cycle models. One can legitimately ask how relevant these models are given recent advances in our understanding of the factors that control primary production and particulate matter export in the ocean. Fifteen to twenty years ago, when the Fasham et al. (1990) model was stiU under development, the potential importance of iron limitation was only beginning to be appreciated. The emphasis at that time was on N limitation, the intricacies of the food web and the potential importance of bacteria and the microbial loop. We now know that iron limits phytoplankton growth over... 

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. 

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. 

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. 

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. 

There has been a considerable amount of work that has focused on the importance of chemical composition, size, and age in controlling microbial metabolism of DOM. The importance of microbes in DOM cycling was recognized and incorporated into the theory of the microbial loop, which first showed that bacteria are key in controlling the trophic linkages between DOM, POM, and inorganic nutrients in aquatic ecosystems. 

In its simplest form, the microbial loop comprises autotrophs, heterotrophs and an abiotic pool of DOM. These organisms are <100 pm and in basic models comprised bacteria, flagellates, dilates, zooplankton, phytoplankton, and cyanobacteria (Azam... 

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