Decomposition: Relative Bacterial Heterotrophic Activity on Soluble Organic Matter
The decomposition of organic matter in aquatic ecosystems by microorganisms generally involves two processes: (1) the hydrolytic degradation of high molecular weight organic polymers into compounds of low molecular weight, such as glucose, cellobiose, and amino acids; and (2) the nonhydrolytic oxidative mineralization of low molecular weight organic compounds to inorganic compounds, especially CO2, H2S, NH 4 + , and PO 4 -3 . Measurement of the rates of decomposition and of mineralization of organic matter in natural waters is difficult and has been approached in a number of ways.
To analyze the rates of reactions involved in situ decomposition and mineralization of organic matter, both biochemical and geo-chemical approaches have been taken. Geo-chemical methods involve the chemical analysis of organic and inorganic compounds in water and sediments. Changes in the chemical composition often reflect the biochemical events that have occurred in the environment as a consequence of microbial activities.
Biochemical methods address more directly fundamental questions of the chemical nature, concentrations, and the rates at which substrates are utilized by bacteria for energy. In the previous exercise, we outlined current state-of-the-art methods used to estimate the rates of productivity of the heterogeneous communities of heterotrophic bacteria in situ. Alternative techniques have been used to measure a community response by following the rates of respiratory activity, of utilization of specifically labeled organic substrates, or of polymer degradation by enzymatic activity [e.g., Cunningham and Wetzel (1989)]. Heterotrophic bacterial communities consist of a heterogeneous composite of populations in various physiological states. Moreover, the dissolved organic substrates comprise a spectrum from very labile compounds that are readily reactive with the enzymes of the microflora, to highly recalcitrant substrates that are utilizable only slowly by highly specialized organisms.
Estimates of in situ rates of planktonic community respiration have been made by separating the larger photosynthetic organisms from the bacteria by filtration and then analyzing either the oxygen consumption or CO2 production [see Sorokin and Kadota (1972)]. However, separation of the organisms by filtration does not give consistent results. As we have seen earlier, picophytoplankton (<2 μm) can constitute a major portion of the phytoplank-ton community. Furthermore, under anaerobic conditions, decomposition occurs by multi-faceted fermentation processes. Fermentation uses alternate electron acceptors to produce a variety of reduced metabolic end products, such as methane, volatile fatty acids, and alcohols, in addition to CO2.
This exercise offers a simple approach to measure the relative heterotrophic activity of microbes in natural waters. In situ bacterial communities are analyzed by the uptake and mineralization of a 14C-labeled organic substrate with Michaelis-Menten enzyme kinetics. This technique was proposed originally by Parsons and Strickland (1962) and later developed by Wright and Hobbie (1965, 1966) and Hobbie and Crawford (1969a). Labeled organic compounds are added to natural communities at serially increasing concentration (μg/1) in a closed system so that the 14CO2 evolved by microbial degradation can be recovered. After a period of incubation at in situ temperatures, both the amount of substrate synthesized into cellular components and that portion respired as CO2 are measured. The rate of turnover of the substrate then is evaluated by a model system of enzyme kinetics. The method evaluates the utilization of only one simple substrate at a time and is subject to a number of interpretative difficulties [e.g., Wright (1973) and Caldwell (1977)], some of which will be discussed below. In spite of the availability of other more direct methods of evaluating in situ bacterial productivity and limitations of estimates of relative heterotrophic activity, this method is still used to evaluate comparatively the relative in situ rates of utilization of specific dissolved organic substrates.
KeywordsBacterial Community Organic Substrate Particulate Organic Carbon Mineralization Rate Label Substrate
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- Caldwell, D.E. 1977. The planktonic microflora of lakes. CRC Critical Rev. Microbiol. 5:305–370.Google Scholar
- Hobbie, J.E. and C.C. Crawford. 1969b. Bacterial uptake of organic substrate: New methods of study and application to eutrophication. Verh. Int. Ver. Limnol. 17:725–730.Google Scholar
- Parsons, T.R. and J.D.H. Strickland. 1962. On the production of particulate organic carbon by heterotrophic processes in sea water. Deep-Sea Res. 8:211–222.Google Scholar
- Sorokin, Y.I. and H. Kadota (eds). 1972. Techniques for the Assessment of Microbial Production and Decomposition in Fresh Waters. IBP Handbook No. 23, Blackwell, Oxford. 112pp.Google Scholar
- Wetzel, R.G. 1983. Limnology. 2nd Ed. Saunders Coll., Philadelphia. 860pp.Google Scholar
- Wright, R.T. 1973. Some difficulties in using 14C-organic solutes to measure heterotrophic bacterial activity, pp. 199–217. In: L.H. Stevenson and R.R. Colwell, Editors. Estuarine Microbial Ecology. Univ. South Carolina Press, Columbia.Google Scholar