Response of Scenedesmus quadricauda (Chlorophyceae) to Salt Stress Considering Nutrient Enrichment and Intracellular Proline Accumulation.

Aquatic organisms are exposed to a wide range of salinity, which could critically affect their survival and growth. However, their survival and growth response to salinity stress remain unclear. This study evaluates the growth response and intracellular proline accumulation of green algae, Scenedesmus quadricauda, isolated from brackish water, against dissolved salts stress with N and P enrichment. We tested a hypothesis that nutrient enrichment can relieve the dissolved salts stress of algae by accumulating intracellular proline, thereby improving survival and growth. Four levels of salinity (0, 3, 6, 12 psu) were experimentally manipulated with four levels of nutrient stoichiometry (N:P ratio = 2, 5, 10, 20) at constant N (1 mgN/L) or P levels (0.05 and 0.5 mgP/L). In each set of experiments, growth rate and intracellular proline content were measured in triplicate. The highest level of salinity inhibited the growth rate of S. quadricauda, regardless of the nutrient levels. However, with nutrient enrichment, the alga showed tolerance to dissolved salts, reflecting intracellular proline synthesis. Proline accumulation was most prominent at the highest salinity level, and its maximum value appeared at the highest N:P ratio (i.e., highest N level) in all salinity treatments, regardless of P levels. Therefore, the effects of P and N on algal response to salt stress differ.

Akinete germination chamber: An experimental device for cyanobacterial akinete germination and plankton emergence.

Understanding how algal resting cells (e.g. akinetes) germinate and what factors influence their germination rate is crucial for elucidating the development of algal blooms and their succession. While laboratory studies have demonstrated algal germination rate and some key factors affecting the germination, the use of artificially induced akinetes and/or removal of the sediments are obviously limiting in simulating the natural environment when designing such controlled experiments. This study introduce a laboratory Akinete Germination Chamber (AGC) that facilitates research for cyanobacterial akinete germination and emergence in an environment similar to natural conditions while minimizing sediment disturbance. The fundamental difference between AGC method and the conventional microplate method is that AGC incorporates the substrate from the natural environment whereas the microplate method does not employ sediment. Therefore, authors of this study assume that the characteristics of akinete germination between the two methods differ because the sediment influences the germination environment. The present study developed the AGC method as an efficient tool to understand harmful cyanobacterial bloom formation. For validation of the AGC method, this study evaluated akinete germination of Dolichospermum circinale (Anabaena circinalis) with different temperature and nutrient condition and then compared the results with those generated by conventional methods The results showed a marked difference in the maximal germination rate between two methods (78% and 35% in the AGC and the microplate, respectively; p < 0.05) at optimum germination temperature (25 °C for both the AGC and the microplate). The nutrient effect also demonstrated clear difference (p < 0.01) in the germination rate between two methods; 88%, 68% and 78% in the AGC and 15%, 20% and 15% in the microplate with -N+P, +N-P, and +N+P condition of CB medium, respectively. Importantly, both DW and -N-P treatments in the AGC induced a little germination of akinete (4.2 ±â€¯1.4% and 5.0 ±â€¯7.1%, respectively), whereas no germination was occurred in the DW treatment in the microplate, suggesting a possible positive effect of sediment on akinete germination. With these results, this study suspects that these differences were largely attributable to natural sediment. Also sediment-accompanied properties, possibly such as nutrient availability, heat budget, micronutrients, and bacteria might have some potential effects on akinete germination. The AGC method can overcome the limitations of the conventional microplate method, and that it is applicable in studies on pelagic-benthic coupling.