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

Abstract

Although the marine N 2 -fixers Trichodesmium spp. are affected by increasing pCO 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 pCO 2 levels (410 and 1000 μatm) under natural sunlight, documented the filament length, growth, and chlorophyll content after its acclimation to the pCO 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 pCO 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 pCO 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 pCO 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 pCO 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 pCO 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.

Deoxygenation enhances photosynthetic performance and increases N2 fixation in the marine cyanobacterium Trichodesmium under elevated pCO2

“…This contradicts to some of the previously reported results ( Table 2 ), which show either enhanced or inhibited growth rates of Trichodesmium grown under future ocean acidification conditions. The likely reason responsible for such discrepancies between these studies ( Table 2 ) could be that different light sources (different emission spectra) regulate the diazotroph’s response to elevated p CO 2 even under equal levels of PAR ( Yi and Gao, 2022 ), though such hypothetical explanation should be based on N 2 -fixation action spectra, which has not been documented. In addition, limitation of phosphate and iron ions as well as exposure to solar UV radiation could also result in negative or insignificant effects of future ocean acidification on the N 2 -fixation of Trichodesmium ( Hong et al, 2017 ; Yi and Gao, 2022 ; Zhang et al, 2022 ).…”

“…The likely reason responsible for such discrepancies between these studies ( Table 2 ) could be that different light sources (different emission spectra) regulate the diazotroph’s response to elevated p CO 2 even under equal levels of PAR ( Yi and Gao, 2022 ), though such hypothetical explanation should be based on N 2 -fixation action spectra, which has not been documented. In addition, limitation of phosphate and iron ions as well as exposure to solar UV radiation could also result in negative or insignificant effects of future ocean acidification on the N 2 -fixation of Trichodesmium ( Hong et al, 2017 ; Yi and Gao, 2022 ; Zhang et al, 2022 ). In the present study, when the Trichodesmium cells acclimated to the acidified HC treatment with replete phosphate and iron, its photosynthesis and growth did not show significant changes, though its N 2 -fixation increased by 129% under ambient and by 38% under the reduced p O 2 , respectively ( Figure 3 ).…”

Understanding the Risks of Diffusion of Cyanobacteria Toxins in Rivers, Lakes, and Potable Water

“…The DCMU stopped the cyanobacteria from producing oxygen, which reduced oxygen production—a cyanobacteria indicator—and was used to ascertain the presence of the cyanobacteria [ 303 ]. Others were successful in controlling the growth of bloom-forming M. aeruginosa by employing 2-Hydroxychalcone as a cyanobacterial photosynthesis inhibitor [ 304 ].…”