GROWTH OF BLUE-GREEN ALGAE IN THE MANUKAU (NEW ZEALAND) OXIDATION PONDS--II. EXPERIMENTAL STUDIES ON ALGAL INTERACTION

-An experimental approach was made to the interaction between the blue-green algae Anabaena and Microcystis and the normal oxidation pond dominant Chlorella. Methods employed were dialysis tubing, mixed culture with assay by in vivo absorbance, and by interaction tubes in which the algae were separated by a filter membrane. No dialysable inhibitor material in the oxidation ponds was detected with in situ experiments. In mixed culture Chlorella inhibits Anabaena only when starting concentration of Chlorella is high and that of Anabaena is low. On the other hand at all concentrations Anabaena quickly dominated Chlorella. These results were confirmed in interaction tubes where the algae were separated and the results extended to show Microcystis also inhibits Chlorella growth. Filtrate experiments indicate that both Microcystis and Anabaena produce a filterable substance capable of suppressing Chlorella growth but that Chlorella does not exert any influence on blue-green algae by way of soluble materials. I N T R O D U C T I O N In the previous study {Vincent & Silvester, 1979) it was shown that growth of blue-green algae in a well established sewage oxidation pond was strongly inhibred by high resident populations of green algae. Growth of blue-green algae was only possible during periods when green algal density was low and pond conditions of temperature and pH were suboptimal for green algal growth. The simple interpretation of changing physical conditions of the ponds as the major factor in controlling algal dominance is strongly suggestive, but leaves open the whole problem of the complex interaction phenomena that exist in such a system. A variety of approaches have been made to the study of algal interactions, both in the field and laboratory. In mixed culture experiments interactions between the components are difficult to analyse and different approaches have been made to resolving the interaction mechanisms. Vance (1965) separated the interacting species by a sintered glass filter welded into a glass tube. This approach was extended by Kroes (1971, 1972, 1973) who used a continuous flow systmn with in-line filters. These and other workers have emphasised the importance of extracellular products in algal interaction and competition. It is well established that healthy algal cells liberate a significant proportion of total carbon fixed into their surrounding environment (Fogg, 1971). The diverse range of organic compounds released include * Present address: Ecology Division, DSlR, P.O. Box 415, Taupo, New Zealand. I" Department of Biological Sciences, University of Waikato, Hamilton, New Zealand. carbohydrates, amino acids, organic-phosphates and vitamins, many of which produce effects of possible ecological importance such as growth inhibition, metabolic stimulation or induction of sexual development. For example, several strains of Chiorella, the genus dominating the Manukau ponds throughout most of the year, are known to produce inhibitory compounds termed 'chlorellins' (Scult, 1964). On the other hand Jacob (1961) and Vance (1965) have shown that blue-green algae are capable of suppressing a wide range of eucaryote algae. Interactions between blue-green algae and Chlorella were investigated in the present study using mixed cultures, interaction tubes, and by filtrates to determine the possible competitive advantages of each group. MATERIALS AND M E T H O D S Blue-green algal cultures Anabaena sp. (local isolate) and Microcys~is aeruginosa (Cambridge culture collection) were grown and maintained on ASM inorganic nutrient medium (Gorham et al., 1964). To assess the possible effect of low molecular weight chemicals on growth, polythene or dialysis bags containing algae and nutrient were suspended in the oxidation ponds and assayed after 4 days by cell count. Algal interaction between Anabaena and Chlorella was investigated in mixed culture by growing each alga singly or in combination at various starting concentrations 5 x l0 s, 5 x 10 4 and 5 x i0 s cells per ml. These were shaken under continuous fluorescent light (2500 lux) and constant temperature 280C for 9 days and assayed by "in vivo absorbance' and by final cell count. In vivo absorbance The relative abundanc¢ of Chlorel la and Anabaena in mixed culture was measured by making use of their differ-" ent pigment composition. An opalescent glass screen