Role of Environmental Factors in Regulating Nitrate Respiration in Intertidal Sediments

Abstract

The microbial decomposition of moribund organic matter involves the consumption of an equivalent quantity of mineral oxidant, either directly as in the case of respiratory metabolism, or indirectly as a consequence of fermentation. Oxidants used in respiratory metabolism include O2, NO 3 − and NO 2 − , manganese and ferric oxides, SO 4 − and CO2. Organic matter decomposition within sediments results in the depletion of these oxidants. As a consequence microbial processes which take place in the surface layers of sediments are not only intimately related to the quality and quantity of organic carbon present but to the concentration and availability of potential inorganic electron acceptors. Aerobic respiration, which takes place in the surface layers of sediments, results in the rapid depletion of 02 and alternative electron acceptors, if present, are then successively utilised by different physiological groups of microorganisms thus creating more reduced conditions (Kaplan, 1979). In the absence of 02 many aerobic and facultatively anaerobic bacteria are able to carry out NO 3 − respiration using NO 3 − as terminal electron acceptor (Payne, 1973; Herbert, 1982). In NO 3 − respiration the end-product(s) of reduction depend (Abd. Aziz and Nedwell, 1986) upon the microorganism involved, growth conditions and may be either NO 2 − , NH 4 + , N2O or N2. Whilst NO 3 − respiration to NO 2 − is a fairly widespread property of chemosynthetic bacteria the further reduction of NO 2 − is carried out by fewer species. When the end-products are gaseous the process is termed denitrification and this represents a net loss of nitrogen from an ecosystem. In contrast a number of fermentative bacteria have been shown to reduce NO 2 − to NH 4 + with the resultant conservation of nitrogen in a potentially useful form (Hadjepetrou and Stouthamer, 1965; Dunn et al., 1978; Cole and Brown, 1982; Macfarlane and Herbert, 1982). This process termed ‘NO 3 − ammonification’ has been shown to occur widely in freshwater, estuarine and marine environments.