Effects of ultraviolet radiation on photosynthetic performance and N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; fixation in &amp;lt;i&amp;gt;Trichodesmium erythraeum&amp;lt;/i&amp;gt; IMS 101

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

doi:10.5194/bg-14-4455-2017

Abstract

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.

Highlights

Global warming is inducing shoaling of the upper mixed layer and enhancing a more frequent stratification of the surface layer, exposing phytoplankton cells which live in the upper mixed layer to higher depth-integrated irradiance including UV radiation (Häder and Gao, 2015)
To deal with UV stress cyanobacteria have evolved a number of defense strategies, including migration to escape from UV radiation, efficient DNA repair mechanisms, programmed cell death, the production of antioxidants, and the biosynthesis of UVabsorbing compounds, such as mycosporine-like amino acids
Our study shows that growth, photochemistry, photosynthesis, and N2 fixation in Trichodesmium sp. are all significantly inhibited by ultraviolet radiation (UVR), including both UV-A and UV-B

Summary

Introduction

Global warming is inducing shoaling of the upper mixed layer and enhancing a more frequent stratification of the surface layer, exposing phytoplankton cells which live in the upper mixed layer to higher depth-integrated irradiance including UV radiation (Häder and Gao, 2015). The increased levels of UV radiation have generated concern about their negative effects on aquatic living organisms, phytoplankton, which require light for energy and biomass production. Cyanobacteria have often been presumed to have evolved under more elevated UV radiation conditions than any other photosynthetic organisms, possibly making them better equipped to handle UV radiation. To deal with UV stress cyanobacteria have evolved a number of defense strategies, including migration to escape from UV radiation, efficient DNA repair mechanisms, programmed cell death, the production of antioxidants, and the biosynthesis of UVabsorbing compounds, such as mycosporine-like amino acids

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Results

Conclusion