Salinity adaptation in the harmful alga, Prymnesium parvum
Richardson, Emily T.
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Golden alga (Prymnesium parvum) is a toxin-producing, euryhaline species responsible for fish-kills worldwide. Although thought to have originated in high salinity habitats, P. parvum abundance in U.S. inland waters and its growth potential in the laboratory show a biphasic relationship to salinity, with peak abundance near 10-15 psu. It is unclear, however, if P. parvum can adapt to long-term exposure to high salinity in terms of growth potential. This information is necessary to understand the spatial distribution of golden alga blooms and especially their absence from Texas coastal habitats. A Texas strain of P. parvum maintained for ~3 years at 5 psu (inland-level salinity) in modified artificial seawater medium (ASM) was subjected to the following treatments over 5 continuous batch cultures: modified ASM at 5 psu (ASM-5), modified ASM with gradually-increasing salinity to 30 psu (ASM-5to30, increased by 5 psu/batch with NaCl), modified ASM at 30 psu increased with NaCl (ASM-30), and Instant Ocean®, a more complex salt mixture, at 30 psu (IO-30). Treatments were conducted in triplicate and each replicate served as inoculum (taken during late-exponential growth) for subsequent cultures. Cell density was measured every 3 days and exponential growth rate (r) and maximum density were determined. Growth rate was reduced when salinity directly increased from 5 to 30 psu in ASM but compensation occurred during the second culture round. Gradual adjustment did not influence this outcome, as inhibition of r was still observed during the fifth cycle when ASM salinity increased from 25 to 30 psu. Inhibition of maximum density was consistently observed in ASM-30 after direct transfer or gradual adjustment. Growth rate and maximum density in IO-30 were generally similar to observations in ASM-5. In conclusion, adaptation to high salinity in ASM was observed for r but not maximum density, and relatively complex salt mixtures (e.g., IO) can compensate for the inhibitory effects of increased salinity. Findings may give insight on P. parvum’s ability to disperse into new environments of varying salinities.