Abstract
After in situ incubation at the site for a year, phytoplanktons in surface water were exposed to natural light in temperate lakes (every month); thereafter, the net production rate of photoprotective compounds (mycosporine-like amino acids, MAAs) was calculated using 13C labeled tracer. This is the first report describing seasonal variation in the net production rate of individual MAAs in temperate lakes using a compound-specific stable isotope method. In the mid-latitude region of the Korean Peninsula, UV radiation (UVR) usually peaks from July to August. In Lake Paldang and Lake Cheongpyeong, diatoms dominated among the phytoplankton throughout the year. The relative abundance of Cyanophyceae (Anabaena spiroides) reached over 80% during July in Lake Cheongpyeong. Changes in phytoplankton abundance indicate that the phytoplankton community structure is influenced by seasonal changes in the net production rate and concentration of MAAs. Notably, particulate organic matter (POM) showed a remarkable change based on the UV intensity occurring during that period; this was because of the fact that cyanobacteria that are highly sensitive to UV irradiance dominated the community. POM cultured in Lake Paldang had the greatest shinorine (SH) production rate during October, i.e., 83.83 ± 10.47 fgC·L−1·h−1. The dominance of diatoms indicated that they had a long-term response to UVR. Evaluation of POM cultured in Lake Cheongpyeong revealed that there was an increase in the net MAA production in July (when UVR reached the maximum); a substantial amount of SH, i.e., 17.62 ± 18.34 fgC·L−1·h−1, was recorded during this period. Our results demonstrate that both the net production rate as well as the concentration of MAAs related to photoinduction depended on the phytoplankton community structure. In addition, seasonal changes in UVR also influenced the quantity and production of MAAs in phytoplanktons (especially Cyanophyceae).
Highlights
Ultraviolet radiation (UVR) reaching the Earth’s surface (280–400 nm) is known to inhibit photosynthesis and nutrient uptake [1,2,3]
Besides inhibiting protein synthesis and the activity of several enzymes, it causes DNA damage [4,5,6] in phytoplankton, which results in a decrease in primary productivity of aquatic ecosystems
Several organisms, including phytoplankton, produce photoprotective compounds such as mycosporine-like amino acids (MAAs) and carotenoids in order to protect their UVR-sensitive cellular components; these compounds facilitate their survival in UVR-enriched environments, for, e.g., transparent aquatic ecosystem [3,8]
Summary
Ultraviolet radiation (UVR) reaching the Earth’s surface (280–400 nm) is known to inhibit photosynthesis and nutrient uptake [1,2,3]. Besides inhibiting protein synthesis and the activity of several enzymes, it causes DNA damage [4,5,6] in phytoplankton, which results in a decrease in primary productivity of aquatic ecosystems. The magnitude of UVR mediated damage varies based on the phytoplankton species, i.e., based on the photoprotective mechanisms employed by each organism [4,7]. Several organisms, including phytoplankton, produce photoprotective compounds such as mycosporine-like amino acids (MAAs) and carotenoids in order to protect their UVR-sensitive cellular components; these compounds facilitate their survival in UVR-enriched environments, for, e.g., transparent aquatic ecosystem [3,8]. MAAs are known for their UV-absorbing properties (λmax = 310–362 nm), and are either biosynthesized and/or accumulated by both marine and freshwater primary producers and consumers [9,10,11,12,13]. MAAs are small (
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