Abstract
AbstractIncreased stratification and mixed layer shoaling of the surface ocean resulting from warming can lead to exposure of marine dinitrogen (N2)‐fixing cyanobacteria to higher levels of inhibitory ultraviolet (UV) radiation. These same processes also reduce vertically advected supplies of the potentially limiting nutrient phosphorus (P) to N2 fixers. It is currently unknown how UV inhibition and P limitation interact to affect the biogeochemical cycles of nitrogen and carbon in these biogeochemically critical microbes. We investigated the responses of the important and widespread marine N2‐fixing cyanobacteria Crocosphaera (strain WH0005) and Trichodesmium (strains IMS 101 and GBR) to UV‐A and UV‐B under P‐replete and P‐limited conditions. Growth, N2 fixation, and carbon dioxide (CO2) fixation rates of Trichodesmium IMS 101 and Crocosphaera were negatively affected by UV exposure. This inhibition was greater for Trichodesmium IMS 101 than for Crocosphaera, which fixes N2 only during the night and so avoids direct UV damage. Negative effects of UV on both IMS 101 and Crocosphaera were less in P‐limited cultures than in P‐replete cultures. In contrast, no UV inhibition was observed in GBR, regardless of P availability. UV inhibition was related to different amounts of UV‐absorbing compounds produced by these isolates. Responses to UV radiation and P availability interactions were taxon‐specific, but our results indicated that in general, UV radiation effects on Trichodesmium and Crocosphaera range from negative to neutral. UV inhibition and its interactions with P limitation may thus have a substantial influence on the present day and future nitrogen and carbon cycles of the ocean.
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