Impact of ultraviolet radiation nearly overrides the effects of elevated pCO2 on a prominent nitrogen‐fixing cyanobacterium

Abstract

AbstractAlthough the marine N2‐fixers Trichodesmium spp. are affected by increasing pCO2 and by ultraviolet radiation (UVR) in their habitats, little is known on their potential responses to future ocean acidification in the presence of UVR. We grew Trichodesmium at two pCO2 levels (410 and 1000 μatm) under natural sunlight, documented the filament length, growth, and chlorophyll content after its acclimation to the pCO2 treatments, and measured its carbon and N2 fixation rates under different solar radiation treatments with or without UVR. We showed that the elevated pCO2 did not significantly alter the diazotroph's growth, filament length, or pigment content, and its photosynthetic rate was only affected by solar radiation treatments rather than the pCO2 levels. The presence of UV‐A and UV‐B inhibited photosynthesis by 10–22% and 17–21%, respectively. Inhibition of N2 fixation by UV‐B was proportional to its intensity, whereas UV‐A stimulated N2 fixation at low, but inhibited it at high, intensities. Elevated pCO2 only stimulated N2 fixation under moderate levels of solar radiation. The simulated depth profile of N2 fixation in the water column showed that UV‐induced inhibition dominated the combined effects of elevated pCO2 and UVR at 0–30 m depth and the combination of these factors enhanced N2 fixation at 30–60 m depth, but this effect diminished in deeper water. Our results suggest that Trichodesmium could be influenced more by UVR than by pCO2 and their combined action result in negative effects on N2 fixation under high solar radiation, but positive effects under low to moderate solar radiation.