Aerobic environmental conditions

Under aerobic environmental conditions nitrogen fixation by cyanobacteria is restricted to heterocystous species. In these organisms, nitrogenase is localized within heterocysts which provide for anaerobic conditions necessary to keep the oxygen-labile enzyme active. Nevertheless, nitrogenase activity has also been observed in non-heterocystous cyanobacteria (Stewart, Lex 1970 Rippka, Waterbury 1977). 

Ba.cteria., A wide variety of bacteria can colonize cooling systems. Spherical, rod-shaped, spiral, and filamentous forms are common. Some produce spores to survive adverse environmental conditions such as dry periods or high temperatures. Both aerobic bacteria (which thrive in oxygenated waters) and anaerobic bacteria (which are inhibited or killed by oxygen) can be found in cooling systems. 

Special consideration shonld be given to the transformation of contaminants in sediments and gronndwater. Under saturated conditions, the solid phase may function as a sink, reservoir, and reactor for contaminants. Contaminant presence, persistence, and transformation in the water phase is controlled by the chemistry of the water body, the surface properties of the materials forming the solid phase (sediments or suspended particles), and environmental conditions (temperature and aerobic or anaerobic stams). 

DEHP is a widely used chemical that enters the environment predominantly through disposal of industrial and municipal wastes in landfills and, to a much lesser extent, volatilization into air (from industrial and end uses of DEHP), carried in waste water from industrial sources of DEHP, and within effluent from municipal waste water treatment plants. It tends to sorb strongly to soils and sediments and to bioconcentrate in aquatic organisms. Biodegradation is expected to occur under aerobic conditions. Sorption, bioaccumulation, and biodegradation are likely to be competing processes, with the dominant fate being determined by local environmental conditions. When DEHP is present in the environment, it is usually at very low levels. It is very difficult to determine these low levels accurately since DEHP is a ubiquitous laboratory contaminant, and laboratory contamination may cause false positives to be reported in the literature. 

The value for Kk, the saturation coefiicient or growth-limiting substrate concentration at which organism growth rate is one-half its maximum value, varies considerably with the reaction, the species mediating the reaction, and environmental conditions. In anaerobic methane fermentations, Kk tends to be much larger, at least with acetate and propionate, than for aerobic oxidations. Values of 10" moles/liter seem appropriate for the former and 10" for the latter. Kk also tends to increase with decrease in temperature. 

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