Positive effects of protozoa on carbon mineralization by bacteria are
considered here specifically in the context of waste-water treatment.
The following effects are reviewed: (1) changes in the C:N:P ratio of
the substrate; (2) excretion of growth factors; and (3) grazing. Depen
ding on the composition of the substrate, grazing can stimulate biolog
ical breakdown of carbon. Substrate with a high carbon-nutrient ratio
permits mineralization increase due to grazing, while mineralization i
s not enhanced when the substrate has a low carbon-nutrient ratio. The
effects of growth factors and grazing depend on several processes. Th
e availability of bacteria for suspension feeders might be under-estim
ated. A study concerning the exchange between bacteria in the suspende
d liquor and bacteria growing in a floc or biofilm is highlighted.