Hydrothermal Alteration Promotes Humic Acid Formation in Sediments: A Case Study of the Central Indian Ocean Basin

Sarma, Nittala S.; Kiran, Rayaprolu; Reddy, M. Rama; Iyer, Sridhar D.; Peketi, A.; Borole, D.V.; Krishna, M. S.

doi:10.1002/2017jc012940

Cited by 8 publications

(9 citation statements)

References 88 publications

(115 reference statements)

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“…If dissolved Fe bound to organic ligands causes extensive vertical and lateral transport of dissolved Fe in the abyssal Indian Ocean, there is a possibility that FOM370/440 could also be affected by the hydrothermal vents, because humic-like FDOM is likely a major Fe-binding organic ligand in the dark ocean ( Laglera and van den Berg, 2009 ; Yamashita et al, 2020 ). A few studies indicate that hydrothermal systems act as a sink of DOM ( Hawkes et al, 2016 ; Yang et al, 2017 ), while some studies indicate that the systems behave as a source of DOM ( Yang et al, 2012 ; Sarma et al, 2018 ). In this study, the regions were not found where both turbidity and FOM370/440 levels were high along the Leg 2 transect ( Figures 6L , 8A ).…”

Section: Discussionmentioning

confidence: 99%

Determining the Distribution of Fluorescent Organic Matter in the Indian Ocean Using in situ Fluorometry

et al. 2020

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In order to determine the dynamics of marine fluorescent organic matter (FOM) using high-resolution spatial data, in situ fluorometers have been used in the open ocean. In this study, we measured FOM during the Global Ocean Ship-based Hydrographic Investigations Program (GO-SHIP) expedition from early December 2019 to early February 2020, using an in situ fluorometer at 148 stations along the two meridional transects (at ∼80 and ∼57°E) in the Indian Ocean, covering latitudinal ranges from ∼6°N to ∼20°S and ∼30 to ∼65°S, respectively. The FOM data obtained from the fluorometer were corrected for known temperature dependence and calibrated using FOM data measured onboard by a benchtop fluorometer. Using the relative water mass proportions estimated from water mass analyses, we determined the intrinsic values of FOM and apparent oxygen utilization (AOU) for each of the 12 water masses observed. We then estimated the basin-scale relationship between the intrinsic FOM and the AOU, as well as the turnover time for FOM in the Indian Ocean (410 ± 19 years) in combination with the microbial respiration rate in the dark ocean (>200 m). Consistent to previous estimates in the global tropical and subtropical ocean, the FOM turnover time obtained is of the same order of magnitude as the circulation age of the Indian Ocean, indicating that the FOM is refractory and is a sink for reduced carbon in the dark ocean. A decoupling of FOM and AOU from the basin-scale relationship was also observed in the abyssal waters of the northern Indian Ocean. The local variability may be explained by the effect of sinking organic matter altered by denitrification through the oxygen-deficient zone on enhanced abyssal FOM production relative to oxygen consumption.

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Section: Discussionmentioning

confidence: 99%

Determining the Distribution of Fluorescent Organic Matter in the Indian Ocean Using in situ Fluorometry

et al. 2020

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“…Humic ligands form iron complexes of intermediate strength 13,14 , bracketed by stability constants of the stronger siderophores and weaker polysaccharides. All major sources of iron to the water column also supply humic-like material [15][16][17][18] , with terrestrial-derived humics often dominating iron complexation in estuarine and coastal waters 13,14 . Furthermore, the stability of iron-humic complexes allows iron to be transported long distances by ocean currents 19 .…”

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confidence: 99%

A call for refining the role of humic-like substances in the oceanic iron cycle

Whitby

Planquette

Cassar

et al. 2020

Sci Rep

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Primary production by phytoplankton represents a major pathway whereby atmospheric CO2 is sequestered in the ocean, but this requires iron, which is in scarce supply. As over 99% of iron is complexed to organic ligands, which increase iron solubility and microbial availability, understanding the processes governing ligand dynamics is of fundamental importance. Ligands within humic-like substances have long been considered important for iron complexation, but their role has never been explained in an oceanographically consistent manner. Here we show iron co-varying with electroactive humic substances at multiple open ocean sites, with the ratio of iron to humics increasing with depth. Our results agree with humic ligands composing a large fraction of the iron-binding ligand pool throughout the water column. We demonstrate how maximum dissolved iron concentrations could be limited by the concentration and binding capacity of humic ligands, and provide a summary of the key processes that could influence these parameters. If this relationship is globally representative, humics could impose a concentration threshold that buffers the deep ocean iron inventory. This study highlights the dearth of humic data, and the immediate need to measure electroactive humics, dissolved iron and iron-binding ligands simultaneously from surface to depth, across different ocean basins.

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“…Intriguingly, a near‐Redfield ratio might indicate in situ organic synthesis by RuBisCO pathway in chemolithotrophs, rather than being a detrital signature. In situ production of organic matter by chemosynthetic processes in these sediments is well known by now (Das et al, , ; Sarma et al, ). An involvement of Fe 2+ and possibility of thermophilic microbes regulating the humic acid pool have been indicated in fracture zone areas of CIB (Sarma et al, ).…”

Section: Discussionmentioning

confidence: 94%

“…In situ production of organic matter by chemosynthetic processes in these sediments is well known by now (Das et al, , ; Sarma et al, ). An involvement of Fe 2+ and possibility of thermophilic microbes regulating the humic acid pool have been indicated in fracture zone areas of CIB (Sarma et al, ). A recent study in the Arabian Sea oxygen minimum zone has used a combination of community sequencing and isotope studies of sulfur (Fernandes et al, ).…”

Section: Discussionmentioning

confidence: 94%

Implications of Microbial Thiosulfate Utilization in Red Clay Sediments of the Central Indian Basin: The Martian Analogy

Singh

Kshirsagar

Das

et al. 2019

Geochem Geophys Geosyst

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Microbial thiosulfate utilization and S-disproportionation could be important mechanisms of sulfate-formations on Earth and Mars. Sulfates on Mars date back to late-Noachian to Hesperian period. In contrast, the large sulfur/sulfate formations on Earth evolved under different chronological sequences. The S-cycle was provoked intermittently, permitting multiple appearances of the S-oxidizers on an evolutionary timescale. Hydrothermally altered deep-oceanic red clay sediments of the Central Indian Basin were examined as potential analogue for sulfur (S) oxidation on Mars. The basin sediments supported an active microbial S-metabolism that exhibited S-disproportionation coupled to microbial carbon-fixation through intermediate processes like thiosulfate utilization. Sulfur-oxidizers/thiotrophic denitrifiers were isolated in large numbers at circum-neutral pH, from these cold and dark abyssal Fe-oxide dominated organic-C starved clay. Experimental simulations under psychrophilic and thermo-tolerant conditions revealed the coexistence of an anaerobic, thermal component under the predominantly oxic, circum-neutral seafloor conditions. Multiple causative factors like hydrothermal seafloor circulation, in situ volcanism, and fracture zone reactivation could drive the widespread S-cycle activity in the Central Indian Basin, albeit at a low scale. It is postulated that these conditions are analogous to Great Oxidation Event situations on Earth, when S-oxidizers evolved and flourished. Experimental studies on microbial thiosulfate flux are few in spite of intense scientific interest in microbial S-disproportionation. To the best of our knowledge, this is a new report on regression model development for microbial thiosulfate flux. These clay-systems and their component microbes could serve as analogue to the ancient well-hydrated Noachian Mars and throw light on planetary hydration and desiccation mechanisms.The Central Indian Basin (CIB) red clays are Fe-oxide dominated, well supplied by oxygenated seawater, usually organic-C-and calcite-depleted environment. The predominantly circum-neutral pH environment often shows sharp pH gradients from 5 to 10. Thus, red clay sediments of CIB were examined for their ability to oxidize/utilize/disproportionate thiosulfate in a Fe-dominant setting. Examining thiosulfate utilization in red clay sediments could provide a glimpse of the hydration-desiccation cycles on Earth and Mars, through mechanisms involving sulfide-sulfate-oxide mineral transitions.

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Hydrothermal Alteration Promotes Humic Acid Formation in Sediments: A Case Study of the Central Indian Ocean Basin

Cited by 8 publications

References 88 publications

Determining the Distribution of Fluorescent Organic Matter in the Indian Ocean Using in situ Fluorometry

Determining the Distribution of Fluorescent Organic Matter in the Indian Ocean Using in situ Fluorometry

A call for refining the role of humic-like substances in the oceanic iron cycle

Implications of Microbial Thiosulfate Utilization in Red Clay Sediments of the Central Indian Basin: The Martian Analogy

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