The microbial decomposition of organic carbon in surface sediments



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Microorganisms occupy an important position in the ecology of a marine environment. The total organic matter, dissolved, particulate, or bound in protoplasm, must be continually mineralized for life cycles to continue. Thus, the primary activity of bacteria is the decomposition of organic compounds synthesized by plants and animals to simple compounds and elements. Odum and Hoskins (1958) estimate that average annual primary production of organic matter by plants in the shallow bays near the Institute of Marine Science, The University of Texas is 5 gms. per sq. meter surface per day. Bacteria also contribute to organic matter by the synthesis of bacterial cell substances during growth. ZoBell and Feltham (1938) estimate that several milligrams of bacterial organic matter is produced per day per top 5 cm of a square meter of mud in a shallow marine mudflat. The total bacterial population of a marine environment is difficult to determine. Both direct microscopic analyses and standard plate count methods have serious limitations (Jones and Jannisch, 1959). Direct microscopic examination is limited because of the problem of distinguishing between particulate matter and bacteria. Plating procedures are inaccurate because the type of medium, temperature, and duration of incubation are selective for certain groups of organisms. Because no reliable procedures are available to measure total bacterial numbers, Emery and Rittenberg (1952) suggest that bacterial activity may be determined by a study of the time changes of materials known to be affected by microbial activities, such as total organic carbon. A research program was organized to measure the rates of microbial activities in the sediments of highly productive shallow marine bays of the Texas coast by an indirect method involving the study of the changes in total organic matter. Thus, it may be possible to correlate the carbon data with estimates of types of bacteria and total population by both plate and direct microscopic counts. Seasonal variation, lateral variation, vertical variation, and effect of the types of sediments can be estimated. One must assume, however, that the changes in the content of organic matter are directly related to the activities of microorganisms. Large burrowing organisms are not abundant, and therefore it is assumed that the net organic matter in the sediment will not be changed much by the activities of the larger invertebrates. The author is cognizant of such errors. The following thesis is the result of approximately 8 months of research on the changes of organic matter and microorganisms in Red Fish Bay, Texas (Figure 1.)