Ecophysiological and Transcriptomic Response of Prymnesium parvum to Environmental Stressors: The Role of Phosphorus Availability and Glyphosate

Prymnesium parvum is a harmful algal bloom (HAB) species responsible for large-scale fish kills and significant economic losses globally. This species is capable of thriving in various environmental conditions, especially in eutrophic waters. Furthermore, the rise of anthropogenic pollution and nutrient-rich runoff from agricultural and urban areas have led to increased risk of HABs, including those of P. parvum. This study aims to comprehensively explore the physiological and transcriptomic response of P. parvum to phosphate (P) availability. Moreover, the following objectives were established to shape the three research chapters that compose this dissertation: 1) determine the influence of different concentrations and constant supply of P on the growth and toxicity of P. parvum, 2) determine the physiological and transcriptomic response of P. parvum to P limitation across its growth curve, and 3) determine the effect of the phosphonate pesticide glyphosate on the growth and toxicity of P. parvum under P-sufficiency and P-limitation. To address the first objective, cultures of P. parvum were grown in modified artificial seawater medium at salinity of 5 psu and under the following sodium phosphate concentrations: 0 µg ml-1, 2.5 µg ml-1, 5 µg ml-1, and the control. Steady-state conditions were maintained for 0, 2.5, and 5 µg ml-1, except for control, which had an initial phosphate concentration of 5 µg ml-1 added only at the beginning of the experiment. For cultures under 2.5 and 5 µg ml-1, phosphate was repleted every three days to keep the concentrations constant. The cell density was recorded and acute daphnia toxicity tests were performed every three days during a 15-day period. To address the second objective, cultures were grown under phosphate-sufficient and phosphate-limited conditions. Cell density and toxicity were recorded every three days for 15 days. Additionally, RNA samples from each replicate (n = 3) were taken every three days from each experimental condition from day 6 to day 15 (total n = 24). Short reads were de novo assembled for subsequent differential expression analysis. For the third objective, four concentrations of glyphosate were tested: 0, 0.1, 0.25, and 0.5 mg l-1. Growth and toxicity were recorded as previously mentioned. The continuous supply of P accelerated P. parvum’s growth during the early phase. Additionally, it appeared to reduce P. parvum’s toxicity. Cultures with complete P deprivation and those with no continuous supply (control) exhibited the highest toxicity in early and late growth phases respectively. P-limitation significantly impacted P. parvum’s growth, also characterized by higher toxicity compared to P sufficiency. The transcriptomic analysis revealed higher abundance of genes under P limitation and more specifically, the upregulation of stress-related genes, potentially as an adaptive strategy for prioritizing survival mechanisms. Genes involved in phosphate uptake were also upregulated, suggesting enhanced mechanisms to acquire external phosphate, especially during the early growth phase. Genes involved in metabolic processes required for rapid growth were more active at the early growth phases, while genes involved in survival and stress response were more active in the stationary growth phase regardless of P supply. Upregulation of phospholipid metabolism and cell integrity and stress response were significantly correlated with toxicity. Lastly, glyphosate at lowest concentration tested (0.1 mg l-1) stimulated P. parvum’s growth in P-sufficiency and enhanced its toxicity under both P conditions but with a more substantial effect under P-sufficiency. Together, these findings confirm the critical role of P availability in P. parvum’s growth, toxicity, and transcript abundance, emphasizing the alga’s adaptive mechanisms under nutrient stress. The potential of glyphosate to enhance growth and toxicity is consistent with a previously suggested role of agrochemicals in promoting HABs. Future research integrating multi-omics approaches, studying interactions with other aquatic organisms, and examining the effects of various anthropogenic pollutants are necessary to enhance our understanding of HAB dynamics and develop more effective mitigation strategies.