Moderate exposure to ultraviolet radiation under phosphate limitation increases <scp> <i>Trichodesmium</i> </scp> nitrogen fixation and suppresses photosynthesis

Abstract The diazotrophic cyanobacterium Trichodesmium is sensitive to ultraviolet (UV) radiation under phosphate (P)‐replete conditions, but little is known about UV impacts on its physiology under P‐limitation. Here we show that periodic exposures to low or moderate levels of UV (4–12 W m −2 UVA, 0.5–1 W m −2 UVB) under P‐limited (0.5 μ M) conditions enhanced its growth by approximately 31% and increased its N 2 ‐fixation rates by 75–175%. N 2 ‐fixation efficiency (N 2 fixed per O 2 evolved) increased by 41–245% at low and moderate levels of solar radiation with UV compared to under low photosynthetically active radiation (PAR) alone (150 μ mol photons m −2 s −1 ). Particulate organic N‐normalized N 2 ‐fixation rates were enhanced up to 173%, with increasing inhibition of photosynthetic O 2 evolution and quantum yield in P‐limited cells. Exposure to high solar radiation levels with UV (1500 μ mol photons m −2 s −1 PAR, 32 W m −2 UVA, 2 W m −2 UVB) decreased N 2 ‐fixation rates of P‐replete (5 μ M) cells by up to 88% and harmed Trichodesmium in P‐limited cultures, leading to rapid death within 2 weeks. Our results imply that UV irradiances during most sunlit periods other than noon are beneficial to Trichodesmium , especially under P‐limited conditions, offering an underlying mechanism for the common observation of surface Trichodesmium blooms in tropical oceans.