Induction of both acid and alkaline phosphatase activity in two green-algae (chlorophyceae) in low N and P concentrations

Development of cyanobacterial and green algal mass-occurrences in the eutrophied Vaal River (South Africa) has caused concern. The influence of various N and P concentrations and ratios on growth, chlorophyll-a concentration, alkaline and acid phosphatase activity of strains of two green algae, Chlamydomonas and Chlorella were studied. Chlamydomonas frequently develops blooms in the Vaal River, whereas Chlorella occurs commonly in the river but has not been shown to cause problematic mass-occurrences. These strains, isolated from the Vaal River, were grown in fed batch-cultures in N or P-limiting conditions (N:P 1:1 and 160:1 respectively), or in low N and P concentration but corresponding to the Redfield ratio (N:P 16:1). After exposing the algae to the treatments, they were harvested for the enzymatic analyses. The enzyme activities of the algae responded very differently to the nutrient limitation. In both strains acid phosphatase activity was induced by low nutrient concentrations, and especially by N-limitation. Throughout.: the experiment acid phosphatase activities were higher than alkaline phosphatase activities. Chlorella exhibited textbook response in regard to inducible alkaline phosphatases, whereas the bloom-forming Chlamydomonas had comparatively high initial enzyme activities, and increases in its enzyme activities related also to other disturbance rather than nutrient limitation alone. It could be concluded that Chlamydomonas appeared to utilise a K-strategy in its growth, having slower growth rates in optimal conditions but maintaining a higher growth rate in nutrient limitation in comparison to Chlorella. Its capability to form mass-occurrences in the Vaal River may therefore be due to a prolonged competitive advantage that allows it to create blooms in circumstances less favourable to other species.