Microbial Loop - History

History

Prior to the discovery of the microbial loop, the classic view of marine food webs was one of a linear chain from phytoplankton to nekton. Generally, marine bacteria were not thought to be significant consumers of organic matter (including carbon), although they were known to exist. However, the view of a marine pelagic food web was challenged during the 1970s and 1980s by Pomeroy and Azam, who suggested the alternative pathway of carbon flow from bacteria to protozoans to metazoans. Early work in marine ecology that investigated the role of bacteria in oceanic environments concluded their role to be very minimal. Traditional methods of counting bacteria (e.g., culturing on agar plates) only yielded small numbers of bacteria that were much smaller than their true ambient abundance in seawater. Developments in technology for counting bacteria have led to an understanding of the significant importance of marine bacteria in oceanic environments. In 1970s, the alternative technique of direct microscopic counting was developed by Francisco et al. (1973) and Hobbie et al. (1977). Bacterial cells were counted with an epifluorescence microscope, producing what is called an "acridine orange direct count" (AODC). This led to a reassessment of the large concentration of bacteria in seawater, which was found to be more than was expected (typically on the order of 1 million per milliliter). Also, development of the "bacterial productivity assay" showed that a large fraction (i.e. 50%) of net primary production (NPP) was processed by marine bacteria. In 1974, Larry Pomeroy published a seminal paper in BioScience entitled “The Ocean’s Food Web: A Changing Paradigm”, where the key role of microbes in ocean productivity was highlighted. In the early 1980s, Azam and a panel of top ocean scientists published the synthesis of their discussion in the journal Marine Ecology Progress Series entitled “The Ecological Role of Water Column Microbes in the Sea”. The term ‘microbial loop’ was introduced in this paper, which noted that the bacteria-consuming protists were in the same size class as phytoplankton and likely an important component of the diet of planktonic crustaceans. Evidence accumulated since this time has indicated that some of these bacterivorous protists (such as ciliates) are actually selectively preyed upon by these copepods. In 1986, Prochlorococcus, which is found in high abundance in oligotrophic areas of the ocean, was discovered by Chisholm, Olson, and other collaborators (although there had been several earlier records of very small cyanobacteria containing chlorophyll-b in the ocean). Stemming from this discovery, researchers observed the changing role of marine bacteria along a nutrient gradient from eutrophic to oligotrophic areas in the ocean.