Articles | Volume 14, issue 19
Research article
11 Oct 2017
Research article |  | 11 Oct 2017

Effects of ultraviolet radiation on photosynthetic performance and N2 fixation in Trichodesmium erythraeum IMS 101

Xiaoni Cai, David A. Hutchins, Feixue Fu, and Kunshan Gao

Abstract. Biological effects of ultraviolet radiation (UVR; 280–400 nm) on marine primary producers are of general concern, as oceanic carbon fixers that contribute to the marine biological CO2 pump are being exposed to increasing UV irradiance due to global change and ozone depletion. We investigated the effects of UV-B (280–320 nm) and UV-A (320–400 nm) on the biogeochemically critical filamentous marine N2-fixing cyanobacterium Trichodesmium (strain IMS101) using a solar simulator as well as under natural solar radiation. Short exposure to UV-B, UV-A, or integrated total UVR significantly reduced the effective quantum yield of photosystem II (PSII) and photosynthetic carbon and N2 fixation rates. Cells acclimated to low light were more sensitive to UV exposure compared to high-light-grown ones, which had more UV-absorbing compounds, most likely mycosporine-like amino acids (MAAs). After acclimation under natural sunlight, the specific growth rate was lower (by up to 44 %), MAA content was higher, and average trichome length was shorter (by up to 22 %) in the full spectrum of solar radiation with UVR, than under a photosynthetically active radiation (PAR) alone treatment (400–700 nm). These results suggest that prior shipboard experiments in UV-opaque containers may have substantially overestimated in situ nitrogen fixation rates by Trichodesmium, and that natural and anthropogenic elevation of UV radiation intensity could significantly inhibit this vital source of new nitrogen to the current and future oligotrophic oceans.

Short summary
Trichodesmium is significant marine N2 fixer. We conducted short- and long-term UV exposure experiment to investigate how UV affects this organism. Our results showed N2 fixation and carbon fixation rates were significantly reduced under UV radiation. As a defense strategy, Trichodesmium is able to synthesize UV-absorbing compounds to protect from UV damage. Our results suggest that shipboard experiments in UV-opaque containers may have substantially overestimated in situ N2 fixation rate.
Final-revised paper