ECOLOGICAL PHYSIOLOGY OF AN ASSEMBLY OF PHOTOSYNTHETIC MICROALGAE IN WASTE-WATER OXIDATION PONDS

During the year 1991 an intensive study was made of parameters governi ng phytoplankton dominance and succession in wastewater oxidation pond s in relation to their photosynthetic properties under environmental s tress. The most successful phytoplankton species was Chlorella vulgari s based on its high photosynthetic potential and its relatively modest contribution to suspended solids. It bloomed during spring after whic h euglenoids and cyanobacteria became increasingly important. In late summer the green alga Ankistrodesmus became increasingly dominant. Thi s alga strongly contributed to a rise in total suspended solids and ha d a low photosynthetic capacity. Its dominance appeared to be linked t o a relief of ammonium stress on the system following lower organic lo ading during summer months. A potential for primary productivity in th e presence of sulfide was present throughout the year. The pattern of activity correlated with blooms of Chlorella and coccoid cyanobacteria . Two cell types of Chlorella vulgaris were observed occurring in a st able ratio throughout the year. One type is fully phototrophic and onl y slightly affected by presence of organic solutes like acetate. Its p hotosynthesis has a pronounced resistance to sulfide and ammonia stres s. The second type is auxotrophic and capable of exploiting various or ganic carbon sources. The presence of organic carbon strongly reduces photosynthesis in this type. Both Chlorella types appeared to be highl y adapted to the environmental stress imposed on photosynthetic phytop lankton in the oxidation pond inlet. This stress was defined as being derived from anoxic conditions lead to elevated concentration in volat ile fatty acids (acetate, lactate etc.), ammonium and sulfide.