[A-OS15] Chemical and Biological Oceanography

[AOS15-01] Transoceanic distribution of photoprotective compounds in phytoplankton in the Pacific tropical and subtropical open waters

*SIJUN CHEN¹, Victor S. Kuwahara², Satoru Taguchi³, Tomohiro Yui⁴, Kazutaka Takahashi⁴ (1.Center for International and Local Research Cooperation, Atmosphere and Ocean Research Institute, The University of Tokyo, 2.Faculty of Education and Graduate School of Science & Engineering, Soka University, 3.National Institute of Polar Research, 4.Graduate School of Agricultural and Life Sciences, The University of Tokyo)

Keywords:phytoplankton, ultraviolet, mycosporine-like amino acids, carotenoids, nitrogen fixation

Ultraviolet (UV) radiation can cause considerable damage to DNA and other cellular structures resulting in inhibition of photosynthesis and carbon uptake in phytoplankton. The accumulation of a series of photoprotective compounds under high doses of UV radiation, such as carotenoids and mycosporine-like amino acids (MAAs), is a major adaptation strategy to mitigate the negative effect of UV radiation among planktonic organisms. Both compounds are synthesised by phytoplankton and photosynthetic bacteria. So far, most of the studies on these photoprotective compounds have been conducted in high latitude regions while studies in subtropical and tropical open waters are scarce. Because of ocean warming, thermal stratification will become a stronger barrier to vertical mixing in subtropical and tropical areas which will lead to higher UV exposure of planktonic organisms, the adaptation of the surface planktonic community there are not well investigated. In subtropical oligotrophic areas where surface nitrogen supplies are depleted, the production of photoprotective compounds is limited. In this context, this study aims to reveal the geographical variation of photoprotective compounds in phytoplankton in relation to environmental variables in the Pacific subtropical and tropical open waters to provide important insights into the biogeochemical cycling and biological production across the low-latitude ocean regions.
Water samples for phytoplankton pigments and MAAs, δ15N, and qPCR analyses of major diazotrophs were collected along the cruise track from Tokyo to the eastern South Pacific via Hawaii during the KH-19-6 research cruise on the R/V Hakuho-maru. UV radiation measured continuously by an on-deck UV radiometer showed an increasing towards the equatorial area. The sampling stations were categorized into three groups with clustering analysis based on temperature, salinity, and Chl a which were measured continuously by an automated environment monitoring system. Group 1, roughly corresponding to Longhurst’s province NPTG (North Pacific Tropical Gyre), was characterized by low Chl a concentration and high temperature and salinity. Group 2, aligning with the PNEC (North Pacific Equatorial Countercurrent), exhibited high temperatures with low Chl a and salinity. Group 3 corresponded to the PEQD (Pacific Equatorial Divergence), showing decreasing temperature and high Chl a and salinity. The distribution of photoprotective carotenoids (PPC) showed significantly higher concentration in the PEQD region. Although there were no significant difference in total MAA concentration among the three regions, higher total MAAs to Chl a ratio could be observed towards Hawaii in the NPTG region. PEQD region had a higher MAA diversity than the other two groups. Redundancy analyses showed that in PEQD, dominant MAAs had positive relationship with dinoflagellates, chlorophytes and diatoms. In PNEC, except for cyanobacteria and diatoms, all other phytoplankton groups showed positive correlation with dominant MAAs. While in NPTG, cyanobacteria as well as haptophytes, chlorophytes and pelagophytes showed a strong positive correlation with dominant MAAs. The variation of δ15N as a proxy of N2 fixation activity as well as the distribution of diazotrophs indicates higher diazotrophic activity in the NPTG region. These results indicate that phytoplankton communities in the study area produce different UV protective compounds in response to nitrogen supply processes. In NPTG, the nitrogen-fixing activity by cyanobacteria is deeply related to the production of MAAs, while in PEQD, where nitrate is abundantly supplied through upwelling, a diverse range of taxa are considered to produce PPC and MAAs in response to high UV radiation. On the other hand, in the PNEC, where nutrient levels are low and nitrogen-fixing activity is limited, PPC is likely playing an important role in UV protection, although the details are unclear.