The impact of shifts to elevated irradiance on the growth and photochemical activity of the harmful algae Chattonella subsalsa and Prorocentrum minimum from Delaware

The physiological impact of short and long-term shifts from low to high light intensity (30–600 μmol photons m−2 s−1) was assessed for the harmful blooming raphidophyte, Chattonella subsalsa and dinoflagellate, Prorocentrum minimum, isolated from the inland bays of Delaware. Analysis of photochemical activity by active chlorophyll fluorescence indicated a significant decline in the photochemical capacity of photosystem II (PSII), Fv/Fm, in both algal species when shifted to high light, with a greater decline noted in P. minimum. While there was no significant difference in the effective quantum yield in the light acclimated state ( ) under high light between these algae, there were differences in how light energy was utilized between the two species. P. minimum displayed a typical upregulation in light energy dissipation via nonphotochemical pathways by significant elevated nonphotochemical fluorescence quenching and a rapid reduction in electron transport with increasing immediate light exposure. Low light acclimated C. subsalsa maintained greater electron transport during short term high light exposure, and showed minimal change in nonphotochemical energy dissipation despite higher excitation pressure placed upon PSII when shifted to high light for several days. C. subsalsa displayed a significantly higher growth rate than P. minimum following the shift of unialgal cultures from low to high light, while both species grew at equivalent rates when cultured together and shifted to high light. Results from these experiments and analysis of loss and subsequent recovery of PSII in the presence and absence of chloroplast protein translation suggest that during shifts to high light P. minimum relies heavily on photosystem turnover and repair as well as nonphotochemical dissipation most likely from xanthophyll cycle activity. Meanwhile, C. subsalsa appears to down-regulate the number of functional PSII reaction centers yet maintains photochemical activity through lowered electron transport to avoid potential photoinhibition.