Abstract: Biological effects of ultraviolet radiation (UVR; 280–400nm) 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 (320–400nm) 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.
Fig.1 (A) Specific growth rate (measured during 8th -11th and 12th-15th day) of Trichodesmium IMS101 grown under solar PAR (P) and PAR+UVA+UVB (PAB). Corresponding total solar irradiance during the growth rate measurement were illustrated under horizontal labels. (B) Trichome length (measured at the 11th and 15th day) of Trichodesmium IMS101 grown under solar PAR (P) and PAR+UVA+UVB (PAB). The asterisks indicate significant differences between radiation treatments. Values are the mean ±SD, triplicate cultures.
Fig.2 Photosynthetic carbon fixation rate (A, fmol C cell-1 h-1) and UV-induced C fixation inhibition (B), N2 fixation rate (C, fmol N2 cell-1 h-1) and corresponding UV-induced N2 fixation inhibition (D) of Trichodesmium IMS101 grown under LL and HL conditions. Asterisks above the histogram bars indicate significant differences between LL- and HL-grown cells. Values are the mean ±SD, triplicate incubations.
Reference: Xiaoni Cai, David A. Hutchins, Feixue Fu, and Kunshan Gao*. Effects of ultraviolet radiation on photosynthetic performance and N2 fixation in Trichodesmium erythraeum IMS 101. Biogeosciences, 2017, 14, 4455-4466.