Ecosystems, anoxic

Table 2 Relative Effectiveness of H2 and Other Metabolites as Regulators of Microbial Energy Metabolism in a Natural Anoxic Ecosystem ...

The principal distinguishing feature of wetland soils is that they develop under predominantly anoxic conditions. Although anoxia is also sometimes found in other ecosystems, it prevails in wetlands and dominates soil properties. Because of the very large organic matter content of some wetland soils, a rough separation into organic and mineral types based on organic matter content is a useful delineation. 

It is necessary to stress that the direct result of the observed anoxic boundary oscillations for 5-10 m is the change of the volume of the oxic waters of about 5-10%, where the Black Sea oxic ecosystem is situated. Such oscillations are vitally significant and should be studied. 

Ammonium fluxes out of sediments have often been calculated based on measured rates of net ecosystem metabolism (NEM based on dissolved O2 uptake or Die release) (Burdige and Zheng, 1998 Hopkinson et al, 2001). In oxic sediments where NTR is likely to play an important role in transformation of NH4+, total DIN rather than the NH4 flux should be more closely related to the stoichiometry of NEM, whereas in anoxic sediments where DNF is important the estimated DIN flux will not equal that predicted by NEM stoichiometry. Calculation of DNF has often been based upon this missing DIN. 

Overall, the Late Ordovician extinction appears to be the result of purely terrestrial phenomena. High sea-level stands of the early Paleozoic allowed for animal diversification on shallow-water, epicontinental carbonate platforms that proved, however, to be highly sensitive to glacio-eustatic effects on ecospace availability and lateral shifts in the oxic-anoxic interface. Tectonic activity facilitated the establishment of Gondwanan ice sheets which robbed the shallow seas of water, leading to extinction, establishment of recovery ecosystems, and then the destruction of these as the ice sheets melted, perhaps catastrophically. 

Detritus includes non-living particulate, colloidal, and dissolved organic matter, and metabolically size only affects rates of hydrolytic attack [31]. Inland aquatic ecosystems collect organic matter, particularly in dissolved forms, from terrestrial, wetland, and littoral sources in quantities that supplement if not exceed those produced autochthonously. Rates of utilization of that organic matter are slowed by a combination of chemical recalcitrance as well as displacement to anoxic environments. As a result, inland aquatic ecosystems are hetero-trophic and functionally detrital bowls, not algal bowls. 

Bacteria only assimilate dissolved substrates solid substrates are first hydrolysed by extracellular enzymes before being assimilated. Degradation of detritus starts with hydrolytic cleavage of the particulate material into small molecules which can be assimilated by the bacteria. The end-products of extracellular hydrolysis are most amino acids, mono- and disaccharides, and long-chain fatty acids. In aerobic environments these are taken up directly by heterotrophic bacteria, and further metabolism is intracellular. A variable fraction of the detritus in marine ecosystems is never completely remineralised, but accumulates mainly within the anoxic environment, and is gradually transformed into organic complexes refractory to microbial attack (Fenchel and Jorgensen, 1977). 

---