Short-term algal toxicity test based on phosphate uptake
Introduction
Algal assay is one of the important methods to evaluate the ecotoxicity of chemicals. However, widely used standard methods [1], [2] require several days in order to determine the growth rate, based on cell increase, with adequate accuracy. Therefore, it is not suitable as a short-term, preliminary screening method. Therefore short-term algal assays have been attempted by changing the effect parameter form the growth rate to the photosynthetic response of algae as measured by carbon uptake [3], [4] or oxygen evolution [5], [6], [7].
Phosphate uptake is also possible to be one of the parameter candidates. It is known that the algal growth rate is related to the cell phosphorus content [8], [9] and that the phosphorus content in some algal cell is affected by culture conditions such as nutrient condition [10], [11]. As for the effect of toxicant, Kuwabara [12] reported that the cellular phosphorus concentration of Selenastrum capricornutum increased markedly with zinc ion activity.
It is also reported that the phosphate uptake rate is affected by the cell phosphorus content. Some kinetic approaches have been made [13], [14], [15]. Nyholm [16] tried to evaluate the toxicity of two toxicants on phosphate uptake rate of S. capricornutum, and noted it was a more sensitive parameter than algal growth. Kuwabara [12], on the other hand, reported that zinc had no apparent effect on phosphorus uptake rate during exponential growth. This difference may be caused due to the experimental conditions or the characteristic of the toxicant. Priet et al. [11] noted that phosphorus starvation is effective in achieving a much higher phosphate uptake rate. These suggest that phosphate uptake can be a useful parameter if culture conditions are adequately chosen.
The first purpose of this paper is to clarify the timing of growth and phosphate uptake of S. capricornutum cultured under similar conditions with a conventional algal assay and the effect of toxicant on them. The second is to propose a short-term algal test method based on phosphate uptake and compare its sensitivity with that of a traditional one.
Section snippets
Batch culture
The test algae S. capricornutum (NIES-35) was batch-cultured in AAP medium [2]. In order to examine the effect of a toxicant on the phosphate uptake rate, concentrations of zinc nitrate were added to the culture media between 0 and 0.3 mg/l as a toxicant.
The 5-day-precultured algal cells were inoculated into 100 ml of sterilized medium contained in multiple 200 ml Erlenmeyer flasks. The initial algal concentrations were varied from 0.8 to 8 mg/l dry mass and were equivalent to 4×102–4×103 cells/ml.
Phosphate uptake as an effect parameter
Fig. 1 shows the time courses of algal mass and phosphate concentration in the control medium during batch culture. Although a change in biomass was observed throughout the incubation period, phosphate uptake was almost finished in 50 h. S. capricornutum batch cultured under similar conditions with a conventional algal assay took up phosphate more rapidly than it started growing. This means that phosphate uptake is possibly a more suitable effect parameter for short-term assay.
In order to use
Conclusions
From the observation of the time course of algal mass and phosphate concentration in batch culture of S. capricornutum, it was confirmed that phosphate uptake occurs earlier than algal growth. It was also observed that some toxicants affect the phosphate uptake rate. Based on these results, a short-term algal toxicity test based on phosphate uptake was designed.
The results of this test for some heavy metals, MCPP and MWI fly ash leachate were similar to the results of a conventional algal assay
Acknowledgements
This work was partially supported by the Japanese Ministry of Education Culture, Sports, Science and Technology (21st Century COE Program, Research and Education on Integrated River Basin Management in Asian Monsoon Regions).
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