Particulate thiol distribution along a meridional transect in the western North Pacific

We demonstrate the broad distribution of particulate thiols in the western North Pacific and show that their main source is marine phytoplankton. Our analysis indicates that differences in thiol concentrations between ocean areas are significantly influenced by water mass properties, phytoplankton composition, and environmental stress. In the oligotrophic, highly transparent subtropical North Pacific Central Water, we find indications that "preformed" glutathione, retained in particulates derived from dead phytoplankton, makes a significant contribution.

Background

Thiols (cysteine ​​and glutathione) are small organic sulfur compounds found in living organisms that readily bind to metals and play important roles in the metal stress responses and redox reactions of marine microorganisms. The distribution of thiols in the ocean is thought to be involved in the marine biogeochemical cycle, particularly in the mitigation and transport of metals such as Cu, Cd, and Hg. Previous studies have shown that marine phytoplankton have the ability to produce thiols in nutrient-rich environments and that their production increases significantly when marine phytoplankton cells are exposed to heavy metals.

However, research on the spatial distribution of thiols has been limited and the distribution of particulate thiols (p-Cys, p-GSH)1*) in the subtropical and equatorial regions remained unknown due to the difficulty of open ocean observation. Furthermore, there was a lack of information on the extent to which particulate thiols are produced in oligotrophic ocean regions where cyanobacteria dominate (e.g., the subtropical North Pacific gyre).

In this study, we investigated the distribution of particulate thiols in multiple water masses along GEOTRACES GP22*2) (in 2022–2023), which meridionally transects the North Pacific, and conducted laboratory experiments using two major species of phytoplankton with the aim of comprehensively clarifying the dynamics of particulate thiols in the ocean.

Results

This study was conducted by a team consisting of a research group led by Prof. Wong, Kanazawa University, and a research group led by Prof. Obata, the University of Tokyo, also in collaboration with a scientist of Nagasaki University. The researchers analyzed seawater samples collected along the GEOTRACES GP22 transect across the western North Pacific and revealed for the first time the wide distribution of particulate thiols, i.e. cysteine ​​(p-Cys) and glutathione (p-GSH). As a result of the analysis, the indices of particulate thiols normalized to chlorophyll a (p-Cys/Chl-a, p-GSH/Chl-a) were found to vary significantly depending on the water mass, and clear geographical patterns were confirmed with the lowest values ​​in the Pacific Subarctic Upper Water (PSUW) with high values ​​also in the North Equatorial Counter Current (NECC), North Pacific Central Water (NPCW), and North Pacific Transition Zone (NPTZ), reflecting the biological community composition and water mass characteristics. In particular, p-GSH/Chl-a was significantly high in NPCW, suggesting that, in addition to the metal stress from the strong light environment and atmospheric deposition, non-living particulates also contribute.

Furthermore, the results of culture experiments in the laboratory using the marine cyanobacterium Synechococcus sp. and the diatom Thalassiosira nordenskioeldii confirmed that phytoplankton were the major source of particulate thiols and that p-GSH derived from Synechococcus sp. significantly increased under Cu stress conditions. The results reveal that some of the regional differences found in the observation areas can be explained by the physiological responses of phytoplankton. Furthermore, the significantly high level of p-GSH/Chl-a in NPCW cannot be explained solely by an origin from living cells; rather, it is considered that particulates derived from dead phytoplankton, which contain "preformed" glutathione, a form that can remain relatively intact even after chlorophyll has already decomposed, are likely to be a major contributor.

These results indicate that the distribution of particulate thiols in the ocean is formed by the overlap of multiple biogeochemical processes such as the composition of phytoplankton communities, light and metal stress, and the production and decomposition of particulates.

Future prospects

This study has significantly advanced our understanding of the distribution and origin of particulate thiols in the western North Pacific, but it will be important to further clarify their detailed sources and production processes. In particular, analyses using chemical indices such as sulfur isotope ratios are necessary to distinguish between contributions from biogenic, atmospheric, and detrital sources. In addition, research is needed to quantitatively evaluate the preservation and decomposition process of the high concentrations of "preformed" glutathione particulates observed in the North Pacific Central Water (NPCW). Furthermore, it is necessary to clarify how high light regimes and metal stresses such as Cu and Hg contained in atmospheric aerosols affect the thiol responses of phytoplankton by combining observations, culture experiments, and model analysis. It is expected that these efforts will expand the possibilities for using particulate thiols as an index of marine environmental stress and that collaborations with the international GEOTRACES project will increase our knowledge to other ocean regions and deepen understanding of the global organic sulfur cycle and metal dynamics.

 

Glossary

*1) Particulate thiols (p-Cys, p-GSH)

  Low molecular weight organic sulfur compounds contained in suspended particulates in seawater (such as plankton cells and detritus). They show metal-binding and antioxidant capacities and participate in biogeochemical cycles.

*2) GEOTRACES GP22

  The observation transect (survey line) established as part of the international GEOTRACES project runs north-south across the western North Pacific from the subarctic to the equatorial regions. Along GP22, seawater samples were collected from multiple water masses (PSUW, NPTZ, NPCW, NECC, PEW, etc.) by the oceanographic research vessel Hakuho Maru in 2022-2023 to clarify the distribution and circulation of marine metal elements and trace elements. In this study, the distribution and source of particulate thiols (p-Cys, p-GSH) were analyzed using surface to subsurface seawater samples collected along the GP22 transect.

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