Warming and growth on urea increase the tolerance of the toxic diatom Pseudo-nitzschia multiseries to UV exposure and so may facilitate establishment of near-surface harmful blooms

Some species within the diatom genus Pseudo-nitzschia spp. produce the neurotoxin domoic acid, and their harmful bloom dynamics and toxin production are expected to be altered as oceanic conditions shift due to climate change. Although previous studies have examined the individual effects of climate-relevant environmental variables on physiology and toxin production, few have considered the interactive effects between them. This multiple variable study examined the simultaneous effects of temperature (20 and 25°C), nitrogen source (nitrate and urea), and light (PAR and UV) on P. multiseries using a matrix experiment. The sensitivity of P. multiseries to UV was first examined by growing cells on nitrate at optimal temperature (20°C) under 0.01 mw/cm² of UVB light for 2, 1, and 0.5 hours per day for two days. For all durations of UV exposure, photosynthetic efficiency (Fv/Fm) decreased from 0.62 to 0.34 after the first day of exposure. While Fv/Fm of cells exposed to UV light >1hr continued to decline to 0.08 after a second UVB exposure, the cells enduring 30 mins of UVB light per day leveled off at 0.32. These findings suggest that P. multiseries is unable to withstand prolonged exposures (>30 minutes) to UVB light, potentially impacting the ability of this species to form near-surface blooms. Although growth rates of nitrate-grown P. multiseries cells incubated under PAR+UV light were strongly inhibited compared to the respective PAR-UV treatments at both temperatures, cells exhibited an increased UV tolerance at 25°C with urea as a nitrogen source. This suggests that a potential future scenario of warming plus increased anthropogenic eutrophication may increase the ability of P. multiseries to tolerate UV light, and thus to form blooms near or at the surface. Domoic acid production data (in prep.) will supplement these findings, providing a clearer picture of how toxin production may be altered in the future as the oceans continue to change.