Impact of ultraviolet radiation nearly overrides the effects of elevated p CO 2 on a prominent nitrogen‐fixing cyanobacterium

• Published in: Limnology and Oceanography
• Main Authors: Yi, Xiangqi, Gao, Kunshan
• Other Authors: National Natural Science Foundation of China
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2023
• Subjects: Ocean acidification
• Online Access: http://dx.doi.org/10.1002/lno.12294; https://onlinelibrary.wiley.com/doi/pdf/10.1002/lno.12294; https://onlinelibrary.wiley.com/doi/full-xml/10.1002/lno.12294; https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.1002/lno.12294

Abstract Although the marine N 2 ‐fixers Trichodesmium spp. are affected by increasing p CO 2 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 p CO 2 levels (410 and 1000 μ atm) under natural sunlight, documented the filament length, growth, and chlorophyll content after its acclimation to the p CO 2 treatments, and measured its carbon and N 2 fixation rates under different solar radiation treatments with or without UVR. We showed that the elevated p CO 2 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 p CO 2 levels. The presence of UV‐A and UV‐B inhibited photosynthesis by 10–22% and 17–21%, respectively. Inhibition of N 2 fixation by UV‐B was proportional to its intensity, whereas UV‐A stimulated N 2 fixation at low, but inhibited it at high, intensities. Elevated p CO 2 only stimulated N 2 fixation under moderate levels of solar radiation. The simulated depth profile of N 2 fixation in the water column showed that UV‐induced inhibition dominated the combined effects of elevated p CO 2 and UVR at 0–30 m depth and the combination of these factors enhanced N 2 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 p CO 2 and their combined action result in negative effects on N 2 fixation under high solar radiation, but positive effects under low to moderate solar radiation.