Although fungi can use soluble organic matter in competition with Bacteria, they seldom compete well in suspended growth cultures under normal conditions, and thus do not usually constitute a significant proportion of the microbial community. " On the other hand, when the supplies of oxygen and nitrogen are insufficient, or when the pH is low, fungi can proliferate, causing problems similar to those caused by filamentous bacteria. In contrast to suspended growth cultures, fungi commonly play an important role in attached growth cultures, making up a large part of the biomass.7" Under certain conditions, however, they can also become a nuisance in such systems by growing so heavily as to block interstices and impede flow.

Protozoa play an important role in suspended growth cultures by grazing on colloidal organic matter and dispersed bacteria, thereby reducing the turbidity remaining after the biofloc has been removed by sedimentation. Protozoa are also known to contribute to bioflocculation, but their contribution is thought to be less important than that of the floc-forming bacteria." Although some protozoa can utilize soluble organic compounds for growth, it is doubtful that they can compete effectively with bacteria in that role and thus soluble substrate removal is generally considered to be due to bacterial action. Protozoa also play a significant role in attached growth bioreactors where the protozoan community is usually richer than it is in suspended growth cultures. Nevertheless, their role appears to be similar to that in suspended growth cultures.

Other Eucarya in suspended growth cultures are usually limited to rotifers and nematodes, but their presence depends very much on the way in which the culture is grown. Although these organisms feed upon protozoa and biofloc particles, their contribution to biochemical operations using suspended growth cultures is largely unknown because little change in process performance can be attributed to their presence. In contrast, because attached growth bioreactors provide a surface upon which higher organisms can graze, it is not uncommon for such reactors to have highly developed communities of macroinvertebrates in addition to rotifers and nematodes.22 The nature of those communities depends largely on the physical characteristics of the bioreactor and in some cases the presence of the higher community has no deleterious effect on system performance. In other cases, however, the grazing community can disrupt development of the primary biofilm that is responsible for the removal of the pollutants, leading to a deterioration in system performance.

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