In the marine ecosystem the foregoing elementary account of the organic food cycle must be extended to take account of the significance of dissolved organic matter (DOM) in seawater. As mentioned earlier (see Section 4.3.3) an appreciable proportion of the products of photosynthesis become released from plant cells and soon appear in the water as DOM. Although some of this component of primary production may be reabsorbed by phytoplankton, much of it is rapidly taken up by planktonic bacteria. The importance of these bacteria in the organic food cycle of the sea has only recently been realized. The development of new nucleopore filters that can retain the smallest bacteria of 0.5 ¡m or less has shown that such free-living bacterioplankton are much more abundant in the water column than was previously thought (Hobbie and Williams, 1984).
These bacteria are thought to utilize the greater part of DOM as a nutrient source. By virtue of their small size and correspondingly large surface-to-volume ratio, the bacteria are well adapted to absorb nutrients at low concentrations. Some estimates suggest such direct uptake may account for up to 50 per cent of the total annual production of dissolved organic carbon (Andrews and Williams,
Figure 5.2 A simplified schematic illustration of the microbial loop (bacteria and protozoans) and how it fits in with the basic pelagic grazing
Figure 5.2 A simplified schematic illustration of the microbial loop (bacteria and protozoans) and how it fits in with the basic pelagic grazing food chain (phytoplankton to piscivorous fish). DOC is dissolved organic carbon (or dissolved organic matter). (From Lalli and Parsons (1997) by kind permission of Butterworth-Heinemann.)
1971). This uptake of DOM results in increased bacterial production which in turn is grazed by small pelagic protista, notably the smallest heterotrophic zooflagellates. These provide food for microzooplanktonts, probably chiefly ciliates and dinoflagellates, which are then preyed upon by other zooplanktonts. In this way a proportion of the energy from primary production released into the water as DOM eventually becomes returned to the main food-web several stages along a subsidiary food-chain known as the 'microbial loop' (Pomeroy, 1974).
The relationships are complex and not yet well understood. For example, some pigmented protista can ingest bacteria, thus obtaining energy both by photosynthesis and from bacterial protoplasm. Furthermore at each stage of this microbial food-chain, animal metabolism returns to the water some DOM as well as inorganic nitrogen and phosphorus compounds, thereby restoring to some extent the supply of major nutrients for photosynthetic organisms. This simplified account illustrates the intricacy of the 'microbial loop', which in recent years has become increasingly recognized as a significant part of marine food-webs.
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