Phosphatase activities of riverine phytoplankton in the Vaal river (South Africa) : physiological responses of nuisance species to different nutrient regimes

Abstract

The theme of this thesis is the ecophysiological basis for development of massoccurrences of phytoplankton species in aquatic environments. Research concerning the reasons for harmful algal bloom (HAB) formation has been intensifying during the past few decades. The need for physiological investigations concerning HAB species has been emphasised in recent literature. The specific objectives of this investigation were to: • study the Vaal River in respect to its phytoplankton species composition, emphasising HAB species, and the environmental as well as physiological characteristics of the river and its phytoplankton community, • study the role of P assimilation . by way of determining the magnitude, size fractionation, seasonal and spatial variation of alkaline and acid phosphatase activities (AP A and AcP A) in the river, • compare the composition of the phytoplankton community, and especially the occurrence of HAB species, in relation to both environmental and physiological parameters, • study the growth, P and N metabolism and photochemistry, in response to varying N and P concentrations and ratios, of key bloom forming species in the river. It was demonstrated that the eutrophic Vaal River has a high percentage of bloom forming species, algal blooms being present throughout the year. The bloom forming taxa occurred in a variety of environmental conditions, thus occupying separate (temporally or spatially) ecological niches in the river. The phosphatase activities in the river were high in comparison to literature, possibly being a physiological indicator of high trophic status. The bacterial and dissolved fraction contained most of the summed phosphatase activity, consisting dominantly of AcPA.

The phytoplankton, on the other hand, was dominated by AP A. The bloom formers were distinguished physiologically from other phytoplankton species occurring in the river by a correlation with high alkaline phosphatase activity measured. Three important bloom forming species from the Vaal River, Chlamydomonas sp., Microcystis aeruginosa and Oscillatoria simplicissima, were chosen for experimental studies, for further investigation of the physiological potential and adaptations of these species. As an example of a non-bloom forming species Chlorella sp. was chosen. Physiological responses, such as growth, cell density, chlorophyll-a content of the cells, endoenzymatic phosphatase activities, ectoenzymatic phosphatase activities, nitrate reductase activity, and photosynthetic performance of these species were studied in vitro. The cells were grown in nutrient replete growth medium ( corresponding to a highly eutrophied environment), as well as N and P limited growth medium with varying N:P ratios. Two different methods were applied for determination of phosphatase enzyme activity. The spectrophotometric method involves pNpp as enzyme substrate and detects endoenzymatic enzyme activity, while the fluorometric method uses fluorogenic MUF-substrates to detect ectoenzyme activity. These methods were used in parallel during the experimental studies, demonstrating the need of more detailed investigation into the origin and functions of phosphatase enzymes. It was demonstrated that the bloom forming species indeed have higher constitutive AP A levels, but also AcP A was present in nutrient replete conditions. In contrast, Chlorella sp., the non-bloom forming species, had very low constitutive AP A and AcP A. After exposing cells to N and :e deplete conditions both AP A and AcP A were strongly induced in all species, suggesting that both enzyme groups are of importance in P-depleted situations. The results indicate that the induction of both phosphatase and nitrate reductase activities function in most species as a general nutrient stress-response, probably preparing the cells for potential supply of organic phosphorus compounds (phosphatases) or nitrates (nitrate reductase), and enabling them to rapidly metabolise these nutrients. Oscillatoria simplicissima was especially well equipped for N-limitation, maintaining a high growth rate despite N and P being supplied in very low concentrations. The filament length in Oscillatoria simplicissima decreased markedly in N limitation, indicating a morphological adaptation mechanism to low N concentrations in the environment. Compared to Chlamydomonas sp., the induction of phosphatases was not as strong, but both N and P limitation induced nitrate reductase activity considerably. The results also implicate differing strategies in the studied species for coping with nutrient limitation of varying degree and type. All bloom formers showed K-strategic ("survival specialist") features either in their nutrient metabolism (Chlamydomonas sp.) or growth (Oscillatoria simplicissima), while the non-bloom forming Chlorella sp. showed characteristics of a r-strategist, having a high growth rate in nutrient replete conditions, whereas nutrient limitation had detrimental and rapid effects on its performance. The use of phosphatases for ecophysiological studies, as well as the implications of the ecophysiological performance of the selected species are summarised and discussed in the concluding chapter.