Background Marine phytoplankton form the basis of the marine food chain and are essential for the normal functioning of ecosystems. Any disturbance to this component, due to the release and accumulation of toxic compounds can have an impact on higher trophic levels. In this study, we investigate the impact of toxicants on the microalgae isolated from Qatari seawater and cultured under controlled laboratory conditions. Objectives * Develop a toxicity test for Synechococcus sp that can be added to the suite of tests currently available for marine invertebrates and fish. * Perform chronic toxicity tests of three reference toxicants (DCA, SDS and Zn) * Evaluate the sensitivity of Synechococcus sp to three reference toxicants and compare sensitivity to other species used in toxicity tests. Methodology Chronic toxicity tests were carried out in 24-well microplate for a period of 3 days for the DCA and Zn tests and 7 days for SDS test. Algal cultures in logarithmic growth phase (cell density of about 3.105 cells mL−1) were used as inoculum. Each test consisted of at least five test concentrations and a control, in triplicate. A different range of concentrations were used to estimate the range findings for each toxicant. * Cell counting using an hemocytometer was conducted to evaluate the inhibition of microalgal growth * The average specific growth rate and the percent inhibition of growth rate were calculated, the lowest observed effect concentration (LOEC) and the no observed effect concentration (NOEC) were statistically determined. Results A growth inhibition toxicity test was successfully developed for Synechococcus sp, which was isolated from Qatari coastal waters. Prior to conducting the toxicity testing, key environmental parameters including light, temperature and nutrients were optimized to obtain acceptable algal growth rates over 72 hours. Results showed that Synechococcus sp was more sensitive to DCA than SDS and Zn. The growth of Synechococcus sp was found to be stimulated by the SDS at the beginning of the test. The growth inhibition by the SDS on Synechococcus sp was shown by day 3 of the experiment. At a longer exposure time, significant values of the percent inhibition of growth rate were reached compared to the control. Conclusion and Discussion Controlled experiments on microalgae under laboratory conditions provide an opportunity to understand the action of these toxicants in the ecosystem. The growth stimulation in the Synechococcus sp test sample seems to be related to the ability of the microalgae to use the SDS as a source of carbon. Inhibition of cell growth under the influence of high concentrations of SDS may result from the destruction of cellular structures and disruptions of metabolism. The findings in this study showed that Synechococcus sp possess a number of desirable characteristics for use in toxicity assessment. In particular, the algae's high sensitivity to environmentally relevant toxicants makes it a suitable choice for site-specific testing. Therefore, we recommend that they be considered, along with other local organisms, as part toxicity tests in the region.


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