Tom Fenchel 2008
This review talks of the development of our knowledge of nutrient cycling in the water column since the paper in 1983, that the author also wrote, that coined the term 'microbial loop'.
The paper acknowledges the previous findings and then goes through some of the major developments such as improvements in techniques for counting bacteria. This combined with the use of 14 C-labelled substrates showed microbes were very active and play a major role in the transformation of matter and energy in the plankton.
It turns out that the microbial loop dominates primary produced carbon cycling and the classical plankton food chain only dominates when there is a rich supply of nutrients in up-welling areas, for example. The competition for dissolved mineral nutrients favours small organisms. New techniques have also found that unicellular photosynthetic prokaryotes such as ubiquitous Synechococcus and Prochlorophytes are major primary producers. Other novel and maybe important energy inputs into the microbial loop include Rosebactor and relatives that are aerobic photoheterotrophs that can generate ATP from light. Mixotrophy represent another complexity that was later added to the microbial loop.
However the most important new functional group is constituted by viruses. Mortality caused by viruses is of a similar magnitude as protozoan grazing; however, the fact that virus – in contrast to protozoan grazing- is highly host-specific means that the effect is different. Viruses drive successional changes of the bacterial biota and perhaps viruses sustain higher bacterial species diversity because the most numerous types of bacteria are the most susceptible.
Despite insight deriving from methods of molecular genetics it will be important to develop methods for growing organisms in the laboratory in order to unravel their
functional properties. It is not known to what extent genetic distances actually correlate with phenotypic differentiation and much genetic variation is likely to be selectively neutral. Also, isolated DNA strands deriving from lysed cells may float around in the environment. Therefore DNA -extraction and subsequent multiplying and sequencing RNA-genes may provide a rather misleading picture of the microbial diversity in terms of the number of organisms that actually play a role at a particular place and time. This problem gets worse with PCR and low copy number techniques. Detection of the presence of gene families involved in metabolic traits may provide more information. Methods such as FISH may then establish the in situ quantitative role of the different strains.
The paper then discusses the spatial heterogeneity of plankton. Evidence indicates that a large fraction of the microbial activity in the water column takes place on suspended particles such as marine snow that may also serve as food for zooplankton such as copepods. Modelling studies have suggested that bacterial chemotaxis may roughly double the rate of mineralisation in seawater relative to the situation in which bacteria are immobile. The water column cannot therefore be considered as a completely mixed system in which encounters between interacting organisms are entirely random. Rather, plankton is to some extent spatially organised on a small scale and this affects biologically-mediated transfer of matter and energy.
In conclusion the paper discusses the impact of the name ‘microbial loop’.
Before Azam et al 1983, Keys et al. (1935) showed that dissolved organic matter in seawater is mineralised exclusively by bacteria and that paper also discussed whether protozoa control bacterial population size and so anticipated the microbial loop by half a century. Just the couple of words: “microbial loop” described that plankton food chains are more complex and that microbes play a much greater role in them than had been acknowledged than how marine plankton food chains were
presented in textbooks of the day.
The paper is very relevant to the module.
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