9:00 AM - 9:15 AM
[AOS13-01] Particulate thiol dynamics in the Western North Pacific: Insights from the GEOTRACES
GP-22 transect
Keywords:phytoplankton, pigment, environmental stress, metal toxicity
Introduction
Thiols are a group of organosulfur compounds that binds strongly to trace metals such as copper (Cu), cadmium (Cd), and mercury (Hg) in aquatic environments. Cu, Cd, and Hg, being toxic to marine microorganism at high concentration, are detoxified by thiols derived from these microorganisms, and maintained at non-toxic levels in eutrophic waters. Consequently, the tolerance of marine microorganism towards Cd and Cu toxicities may be linked to their ability to biosynthesize thiol species such as cysteine (Cys) and glutathione (GSH) as part of their detoxification mechanism. A high concentration of thiols in phytoplankton cells may indicate oxidative stress in the microorganism, particularly that caused by metal toxicity. This suggests the potential use of thiols as biomarkers for identifying metal stress in marine microorganism communities. A major advantage of using thiols as biomarkers is their capacity to enable in-situ measurement of metal-induced oxidative stress, a feat that has been challenging to achieve through direct chemical measurements.
In this study, we determined the distribution of particulate thiols, specifically Cys and GSH, in both surface and subsurface waters of the North Pacific from the tropical to subarctic regions along GEOTRACES GP22 transect. We also assessed the pigment composition in the phytoplankton community as a proxy for identifying the dominant species phytoplankton in the various water masses of the western North Pacific. Furthermore, to explore potential sources of thiols in our study region, we incubated a marine cyanobacterium and a diatom under controlled laboratory conditions in a land-based laboratory and compared their intracellular thiol contents with those of their natural counterparts.
Methods
Particulate samples were collected along the GEOTRACES GP22 transect, spanning from the tropical region to the subarctic region of the western Pacific Ocean, onboard R/V Hakuho Maru between June and September 2022, and between June and July 2023. Seawater samples from the upper 100 m and at the chlorophyll maximum layer were collected and filtered through GF/F filters (0.7 µm pore size) in an acid-cleaned polycarbonate filtration unit under gentle negative pressure. Thiols were extracted using an extraction buffer containing 5 mM DTPA and stored frozen until analysis. Analysis was performed using fluorescence detection following derivatization and separation via HPLC. Photosynthetic pigments were extracted in methanol, filtered, and then analyzed using HPLC. Additionally, thiol and pigment content analyses were conducted for laboratory-cultured cyanobacterium (Synechococcus sp.) and diatom (T. nordenskioeldii).
Results and Discussion
Water masses in the surface waters of the Western North Pacific are distinguished into Pacific Equatorial Water (PEW), North Equatorial Counter Current (NECC), North Pacific Central Water (NPCW), North Pacific Transition Zone (NPTZ), and Pacific Subarctic Upper Water (PSUW) (Fig). Notably, particulate GSH (p-GSH) concentration to chlorophyll a ratio (p-GSH/Chl-a) was elevated in the NPCW, ranging from 22 to 40 mmol/g Chl-a (Fig). This was in contrast to the NECC (10 – 20 mmol/g Chl-a), NPTZ (1 -18 mmol/g Chl-a), PEW (< 6 mmol/g Chl-a), and PSUW (< 6 mmol/g Chl-a), all of which exhibited lower concentration ratios. Correspondingly, a high zeaxanthin to chlorophyll a ratio (> 0.6) was observed in the NPCW (Fig), indicative of a significant presence of cyanobacteria, particularly Prochlorococcus.
There is a positive correlation between p-GSH and chlorophyll a concentration in the PSUW (R2 = 0.45). This suggests that phytoplankton are a likely source of p-GSH in the open ocean. Our incubation studies revealed that p-GSH/Chl-a in cultured cyanobacteria was ~2.4 mmol/g Chl-a, which is consistent with observations in the PSUW but is lower than that in the NPCW. This suggests an additional source of p-GSH in the NPCW. Furthermore, the ratios of p-GSH to p-Cys were much higher in the NPCW (ranging from 9.5 to 105) compared to ~6.6 in the other water masses. Our analysis also revealed the presence of “preformed” GSH of ~ 6 mmol/g Chl-a, which might be attributed to non-biogenic suspended particles. Further studies are needed to elucidate the source of this p-GSH in the NPCW.
Thiols are a group of organosulfur compounds that binds strongly to trace metals such as copper (Cu), cadmium (Cd), and mercury (Hg) in aquatic environments. Cu, Cd, and Hg, being toxic to marine microorganism at high concentration, are detoxified by thiols derived from these microorganisms, and maintained at non-toxic levels in eutrophic waters. Consequently, the tolerance of marine microorganism towards Cd and Cu toxicities may be linked to their ability to biosynthesize thiol species such as cysteine (Cys) and glutathione (GSH) as part of their detoxification mechanism. A high concentration of thiols in phytoplankton cells may indicate oxidative stress in the microorganism, particularly that caused by metal toxicity. This suggests the potential use of thiols as biomarkers for identifying metal stress in marine microorganism communities. A major advantage of using thiols as biomarkers is their capacity to enable in-situ measurement of metal-induced oxidative stress, a feat that has been challenging to achieve through direct chemical measurements.
In this study, we determined the distribution of particulate thiols, specifically Cys and GSH, in both surface and subsurface waters of the North Pacific from the tropical to subarctic regions along GEOTRACES GP22 transect. We also assessed the pigment composition in the phytoplankton community as a proxy for identifying the dominant species phytoplankton in the various water masses of the western North Pacific. Furthermore, to explore potential sources of thiols in our study region, we incubated a marine cyanobacterium and a diatom under controlled laboratory conditions in a land-based laboratory and compared their intracellular thiol contents with those of their natural counterparts.
Methods
Particulate samples were collected along the GEOTRACES GP22 transect, spanning from the tropical region to the subarctic region of the western Pacific Ocean, onboard R/V Hakuho Maru between June and September 2022, and between June and July 2023. Seawater samples from the upper 100 m and at the chlorophyll maximum layer were collected and filtered through GF/F filters (0.7 µm pore size) in an acid-cleaned polycarbonate filtration unit under gentle negative pressure. Thiols were extracted using an extraction buffer containing 5 mM DTPA and stored frozen until analysis. Analysis was performed using fluorescence detection following derivatization and separation via HPLC. Photosynthetic pigments were extracted in methanol, filtered, and then analyzed using HPLC. Additionally, thiol and pigment content analyses were conducted for laboratory-cultured cyanobacterium (Synechococcus sp.) and diatom (T. nordenskioeldii).
Results and Discussion
Water masses in the surface waters of the Western North Pacific are distinguished into Pacific Equatorial Water (PEW), North Equatorial Counter Current (NECC), North Pacific Central Water (NPCW), North Pacific Transition Zone (NPTZ), and Pacific Subarctic Upper Water (PSUW) (Fig). Notably, particulate GSH (p-GSH) concentration to chlorophyll a ratio (p-GSH/Chl-a) was elevated in the NPCW, ranging from 22 to 40 mmol/g Chl-a (Fig). This was in contrast to the NECC (10 – 20 mmol/g Chl-a), NPTZ (1 -18 mmol/g Chl-a), PEW (< 6 mmol/g Chl-a), and PSUW (< 6 mmol/g Chl-a), all of which exhibited lower concentration ratios. Correspondingly, a high zeaxanthin to chlorophyll a ratio (> 0.6) was observed in the NPCW (Fig), indicative of a significant presence of cyanobacteria, particularly Prochlorococcus.
There is a positive correlation between p-GSH and chlorophyll a concentration in the PSUW (R2 = 0.45). This suggests that phytoplankton are a likely source of p-GSH in the open ocean. Our incubation studies revealed that p-GSH/Chl-a in cultured cyanobacteria was ~2.4 mmol/g Chl-a, which is consistent with observations in the PSUW but is lower than that in the NPCW. This suggests an additional source of p-GSH in the NPCW. Furthermore, the ratios of p-GSH to p-Cys were much higher in the NPCW (ranging from 9.5 to 105) compared to ~6.6 in the other water masses. Our analysis also revealed the presence of “preformed” GSH of ~ 6 mmol/g Chl-a, which might be attributed to non-biogenic suspended particles. Further studies are needed to elucidate the source of this p-GSH in the NPCW.