Arabian Sea Oxygen Minimum Zone Research Articles

Seasonal and depth-wise variations in bacterial and archaeal groups in the Arabian Sea oxygen minimum zone

Seasonal variations, as well as depth-wise distribution patterns of bacterial and archaeal communities in the Arabian Sea oxygen minimum zone (AS-OMZ), were analyzed by examining 16S rRNA gene clones and their sequences. Sampling was carried out at the Arabian Sea Time Series (ASTS) location (17°0.126′ N, 67°59.772′ E) from five different depths during three different seasons. A total of 743 and 256 non-chimeric bacterial and archaeal 16S rRNA gene sequences were analyzed. Most of the bacterial 16S rRNA gene sequences were affiliated to Gammaproteobacteria (39.31%), Alphaproteobacteria (23.56%) and Cyanobacteria (20.2%). The archaeal 16S rRNA gene sequences mostly aligned with Marine Group II (MG-II, Euryarchaeota). An explicit vertical partitioning of bacterial communities between the surface (surface and DCM) and OMZ (250 m, 500 m, and 1000 m) was observed. Also evident was an apparent seasonal variation among surface bacterial communities but a minimal variation among OMZ bacterial and archaeal communities. LINKTREE and canonical correspondence analysis (CCA) indicates that differences in the concentrations of dissolved oxygen (DO) and total organic carbon (TOC) seem to cause the vertical separation among bacterial communities in the ASTS station. A higher number of shared OTUs affiliated mainly to Alteromonadales (bacteria) and Methanosarcinales (archaea) contributed towards seasonally stable community structure in the OMZ depths.

Diversity of culturable nitrate-reducing bacteria from the Arabian Sea oxygen minimum zone

The subsurface waters of the northern Arabian Sea display a pronounced oxygen minimum layer associated with high nitrite maxima which provide an ideal niche for organisms that can respire nitrate. Culture-based studies elaborate the physiological characteristics of the organisms and their metabolic activities in biogeochemical cycles. In this study, the bacterial diversity and nitrate utilizing activity of the culturable heterotrophic bacteria inhabiting the water column oxygen minimum zone of Arabian Sea were investigated. Nitrate-reducing bacteria were isolated from the water column in the central Arabian Sea. Genotypic characterization of the isolates using 16 S rDNA gene sequencing grouped them into three phylogenetic groups i.e. Proteobacteria, Firmicutes, and Actinobacteria. Out of the 56 isolated bacteria, 45 strains belonged to Proteobacteria, 6 to Firmicutes and 5 to Actinobacteria. The nitrate reducing ability of the isolates was tested using Griess test. Thirty-six species belonging to genera Alcanivorax, Alteromonas, Halomonas, Pseudoalteromonas, Marinobacter, Bacillus, and Vibrio were positive for NO2-/NO3- reduction. Our results imply that cultivable bacteria capable of utilizing NO3- available in the system are present in the Arabian Sea oxygen minimum zone and the conditions existing therein must be favorable for their growth and functionality.

Bacterial Community Profiling of the Arabian Sea Oxygen Minimum Zone Sediments using Cultivation Independent Approach

Examines in Marine Biology and Oceanography Bacterial Community Profiling of the Arabian Sea Oxygen Minimum Zone Sediments using Cultivation Independent Approach Baby Divya1*, Annie Feby2 and Shanta Nair3 1,2Department of Zoology, St. Xavier's College for Women, India 3Microbiology Laboratory, National Institute of Oceanography, Goa, India *Corresponding author: Baby Divya, Department of Zoology, St Xavier's College for Women, Kerala, India Submission: November 02, 2017; Published: December 07, 2017 DOI: 10.31031/EIMBO.2017.01.000505 ISSN: 2578-031X Volume1 Issue1

Oxygen Minimum Zone Contrasts Between the Arabian Sea and the Bay of Bengal Implied by Differences in Remineralization Depth

AbstractThe combination of high primary productivity and weak ventilation in the Arabian Sea (AS) and Bay of Bengal (BoB) generates vast areas of depleted oxygen, known as oxygen minimum zones (OMZs). The AS OMZ is the world's thickest and hosts up to 40% of global denitrification. In contrast, the OMZ in the BoB is weaker and denitrification free. Using a series of model simulations, we show that the deeper remineralization depth (RD) in the BoB, potentially associated with organic matter aggregation with riverine mineral particles, contributes to weaken its OMZ. When the RD is set uniformly across both seas, the model fails to reproduce the observed contrast between the two OMZs, irrespective of the chosen RD. In contrast, when the RD is allowed to vary spatially, the contrasting distributions of oxygen and nitrate are correctly reproduced, and water column denitrification is simulated exclusively in the AS, in agreement with observations.

Diversity of sediment-associated Planctomycetes in the Arabian Sea oxygen minimum zone.

We examined the diversity of Planctomycetes in the sediment sample collected from an oxygen minimum zone (OMZ) in the southeast Arabian Sea. A 16SrRNA gene library was constructed using the forward primer specific for Planctomycetes and a universal reverse primer. The 237 sequences obtained were grouped into 130 operational taxonomic units, and the majority of them were clustered with phylum Planctomycetes (45.0%) and unclassified bacteria (27.0%). There were sequences that clustered with distantly separated monophyletic groups such as Latescibacteria (9%), Actinobacteria (6%), Proteobacteria (5%), and others (8%). Among Planctomycetes, 55.7% belonged to family Planctomycetaceae, followed by unclassified Planctomycetes (25.0%) and family candidatus Brocadiaceae (19.2%). The family Planctomycetaceae included the genera Blastopirellula (11.5%), Rhodopirellula (3.8%), and a large number unclassified Planctomycetaceae sequences (40.4%). The members of family candidatus Brocadiaceae included the genera candidatus Scalindua (11.5%), candidatus Brocadia (1.9%) and unclassified genera (5.8). Our study indicates the relatively large diversity of Planctomycetes in sediments underlying the oxygen minimum zone of Arabian Sea. Also, the sequence data generated in the present study may support the efforts on isolation and purification of Planctomycetes from marine environment for understanding their biogeochemical significance.

Climate oscillations reflected within the microbiome of Arabian Sea sediments

Selection of microorganisms in marine sediment is shaped by energy-yielding electron acceptors for respiration that are depleted in vertical succession. However, some taxa have been reported to reflect past depositional conditions suggesting they have experienced weak selection after burial. In sediments underlying the Arabian Sea oxygen minimum zone (OMZ), we performed the first metagenomic profiling of sedimentary DNA at centennial-scale resolution in the context of a multi-proxy paleoclimate reconstruction. While vertical distributions of sulfate reducing bacteria and methanogens indicate energy-based selection typical of anoxic marine sediments, 5–15% of taxa per sample exhibit depth-independent stratigraphies indicative of paleoenvironmental selection over relatively short geological timescales. Despite being vertically separated, indicator taxa deposited under OMZ conditions were more similar to one another than those deposited in bioturbated intervals under intervening higher oxygen. The genomic potential for denitrification also correlated with palaeo-OMZ proxies, independent of sediment depth and available nitrate and nitrite. However, metagenomes revealed mixed acid and Entner-Dourdoroff fermentation pathways encoded by many of the same denitrifier groups. Fermentation thus may explain the subsistence of these facultatively anaerobic microbes whose stratigraphy follows changing paleoceanographic conditions. At least for certain taxa, our analysis provides evidence of their paleoenvironmental selection over the last glacial-interglacial cycle.

Holocene strengthening of the Oxygen Minimum Zone in the northwestern Arabian Sea linked to changes in intermediate water circulation or Indian monsoon intensity?

A high resolution 14,000year old record of benthic foraminifera from ODP Hole 723A was produced to understand changing intensity of the NW Arabian Sea Oxygen Minimum Zone (OMZ) and its links with intermediate water circulation and Indian Monsoon Intensity. During the latest Pleistocene, the OMZ was very weak marked by the abundance of oxic species. From early to mid Holocene period (up to 6500years BP), intense SW monsoon-related high surface productivity led to the development of an OMZ in the Arabian Sea; however, presence of oxic and suboxic species indicate that OMZ intensification was minimized by cyclic incursions of oxygen-rich intermediate waters from the South. The middle Holocene was a transitional period to intense OMZ conditions. The intensification of the OMZ could be due to replacement of oxygen-rich southern source Sub-Antarctic Mode and Antarctic Intermediate Waters (SAMW-AAIW), with oxygen poor Red Sea Water (RSW) from the north during the middle Holocene. As the intensity of the summer monsoon weakened during the late Holocene (~4200years BP onwards) the Arabian Sea OMZ intensified, suggesting total cut off from the oxygenated SAMW-AAIW. Spectral analysis of Information Function (H), benthic foraminifer Bolivina spp. and oxic species of benthic foraminifera indicate that the intensity of the OMZ and deep water conditions respond closely to solar cycles.

Eddies reduce denitrification and compress habitats in the Arabian Sea

AbstractThe combination of high biological production and weak oceanic ventilation in regions, such as the northern Indian Ocean and the eastern Pacific and Atlantic, cause large‐scale oxygen minimum zones (OMZs) that profoundly affect marine habitats and alter key biogeochemical cycles. Here we investigate the effects of eddies on the Arabian Sea OMZ—the world's thickest—using a suite of regional model simulations with increasing horizontal resolution. We find that isopycnal eddy transport of oxygen to the OMZ region limits the extent of suboxia so reducing denitrification, increasing the supply of nitrate to the surface, and thereby enhancing biological production. That same enhanced production generates more organic matter in the water column, amplifying oxygen consumption below the euphotic zone, thus increasing the extent of hypoxia. Eddy‐driven ventilation likely plays a similar role in other low‐oxygen regions and thus may be crucial in shaping marine habitats and modulating the large‐scale marine nitrogen cycle.

Metagenomic analysis of nitrogen and methane cycling in the Arabian Sea oxygen minimum zone.

Oxygen minimum zones (OMZ) are areas in the global ocean where oxygen concentrations drop to below one percent. Low oxygen concentrations allow alternative respiration with nitrate and nitrite as electron acceptor to become prevalent in these areas, making them main contributors to oceanic nitrogen loss. The contribution of anammox and denitrification to nitrogen loss seems to vary in different OMZs. In the Arabian Sea, both processes were reported. Here, we performed a metagenomics study of the upper and core zone of the Arabian Sea OMZ, to provide a comprehensive overview of the genetic potential for nitrogen and methane cycling. We propose that aerobic ammonium oxidation is carried out by a diverse community of Thaumarchaeota in the upper zone of the OMZ, whereas a low diversity of Scalindua-like anammox bacteria contribute significantly to nitrogen loss in the core zone. Aerobic nitrite oxidation in the OMZ seems to be performed by Nitrospina spp. and a novel lineage of nitrite oxidizing organisms that is present in roughly equal abundance as Nitrospina. Dissimilatory nitrate reduction to ammonia (DNRA) can be carried out by yet unknown microorganisms harbouring a divergent nrfA gene. The metagenomes do not provide conclusive evidence for active methane cycling; however, a low abundance of novel alkane monooxygenase diversity was detected. Taken together, our approach confirmed the genomic potential for an active nitrogen cycle in the Arabian Sea and allowed detection of hitherto overlooked lineages of carbon and nitrogen cycle bacteria.

Living (Rose-Bengal-stained) benthic foraminiferal faunas along a strong bottom-water oxygen gradient on the Indian margin (Arabian Sea)

Abstract. Rose-Bengal-stained foraminiferal assemblages (> 150 μm) were analysed along a five-station bathymetric transect across the core and the lower part of the oxygen minimum zone (OMZ) on the Indian margin of the Arabian Sea. Sediment cores were collected using the manned submersible Shinkai 6500 during the RV Yokosuka cruise YK08-11 in the post-monsoon season (October 2008) at water depths ranging from 535 to 2000 m, along a gradient from almost anoxic to well-oxygenated (0.3 to 108 μM) bottom waters. Stained benthic foraminifera were investigated from two different size fractions (150–300 μm and > 300 μm). Stained foraminiferal densities were very high in the core of the OMZ (at 535 and 649 m) and decreased at deeper sites. The faunas (> 150 μm) were dominated (40–80 %) by non-calcareous taxa at all stations. These were mainly species of Reophax and Lagenammina but also included delicate monothalamous taxa (organic-walled "allogromiids", agglutinated saccamminids, psammosphaerids and tubular forms). These new data from the Indian margin are compared to previous studies from the Murray Ridge, the Pakistan margin and the Oman margin. The fact that similar species were found at sites with comparable bottom-water oxygen concentrations but with very different surface water productivity suggests that, within the strongly developed Arabian Sea OMZ, bottom-water oxygen concentration, and not the organic flux to the sea floor, is the main factor controlling the species composition of the foraminiferal communities. Several foraminiferal species (e.g. Praeglobobulimina sp. 1, Ammodiscus sp. 1, Bolivina aff. dilatata) were confined to the core of the OMZ. These species are presently known only from the Arabian Sea. Because of their association with extremely low oxygen concentrations, these species may be good markers for very low oxygen concentrations, and could be used to reconstruct past OMZ variability in the Arabian Sea.

Response of trace metal redox proxies in continental shelf environment: The Eastern Arabian Sea scenario

Major and trace elements (viz. Al, Si, Ca, Na, Fe, Ti, K, S, Mn, V, Cu, Zn, Mo, Ni, Th and U) along with organic carbon and nitrogen content were analyzed in the sediment samples of a gravity core (GC-08/SK-291; 12°34′N: 74°11.47′E) which lies within the upper edge of intense oxygen minimum zone (OMZ) of Eastern Arabian Sea. The aim was to determine how the geochemistry of the redox-sensitive elements (viz. V, Cu, Zn, Mo, Ni, and U) was influenced by the suboxic water column and bottom water condition and whether the sediments show a unique signature of such redox condition. The weathering intensity was also determined and the provenance of sediments has been inferred to discern the uniformity in sediment supply in the studied location.The chemical index of alteration (CIA), Chemical Index of Weathering (CIW) and Al–Ti–Zr ternary diagrams suggest low to moderate source area weathering of granodioritic to tonalitic source rock composition, which is similar to the regional geology of the continental mass nearby. The relative variations of major elements such as Si, Al and Ca suggest that the terrigenous fractions in the studied sediments are diluted by marine carbonates, unlike by biogenic opal, which is the case for most shelf regions of the world experiencing upwelling. The relative distributions of total organic carbon (TOC), total sulfur (TS) and total nitrogen (TN) suggest that the organic matter in the studied sediments is primarily of terrestrial origin irrespective of its presence in the deeper part of the shelf. We adopted the multi-proxy procedure for analyzing the redox condition prevalent during the deposition of sediments (e.g., enrichment of redox-sensitive elements, authigenic U, and ratios of trace metals such as U/Th, V/Cr, V/Mo, Ni/Co and (Cu+Mo)/Zn). The most striking result is a fully oxic signature reflected consistently by all redox proxies despite the occurrence of studied location within the OMZ of Arabian Sea. The redox proxies estimated from the bulk composition as well as the marine fraction of trace metals, do not reveal any significant difference in the depositional condition. The seasonal variation of O2 concentration seems to be the dominant factor which controls the response of the trace metals to the water column anoxia in the studied location. The major implication of the present study is that the trace element redox proxies alone are not always suitable to discern the redox condition of deposition in continental shelf sediments which are characterized by high sediment input from continents and significant seasonal variation of O2 concentration in the water column.

Combining benthic foraminiferal ecology and shell Mn/Ca to deconvolve past bottom water oxygenation and paleoproductivity

The Mn/Ca of carbonate tests of living deep-sea foraminifera (Hoeglundina elegans, Bulimina aculeata, Uvigerina peregrina and Melonis barleeanus) were determined together with pore water manganese along a bottom water oxygen gradient across the lower boundary of the Arabian Sea oxygen minimum zone. Although Mn has long been considered an indicator for contamination, new cleaning protocols and high-resolution laser ablation ICP-MS now allow the reliable analyses of test-associated Mn. Within locations, Mn incorporation between species varies as a function of their in-sediment depth preferences and associated pore water chemistry. Under well-oxygenated bottom water conditions, shallow infaunal species incorporate little Mn in their test, whereas the species collected from deeper habitats show elevated Mn concentrations. With decreasing oxygen contents pore water Mn concentrations and benthic foraminiferal in-sediment distribution change. Whereas Mn/Ca in shallow infaunal species responds moderately to bottom water oxygenation, Mn/Ca of the infaunal species M. barleeanus correlates well to oxygenation. Although high productivity results in a shallower redox cline within the sediment, pore water Mn is retained as long as the bottom water remains oxygenated. Under reduced bottom water oxygen conditions, Mn escapes to the overlying water column and test-associated Mn/Ca decreases also in the infaunal species. By combining pore water chemistry of Mn, calcitic Mn/Ca and foraminiferal ecology, a new conceptual model is presented (TROXCHEM3) that provides a framework for deconvolving past organic matter input and bottom water oxygenation.

Benthic foraminiferal diversity response to the climate induced changes in the eastern Arabian Sea oxygen minimum zone during the last 30 ka BP

A high resolution record of deep sea benthic foraminiferal diversity variations in the eastern Arabian Sea for the last 30kaBP was obtained from two sediment cores (SK17 and MD131) retrieved from the present day Oxygen Minimum Zone (OMZ) in the Indian margin off Goa. The benthic foraminiferal diversity is represented in terms of Sander's rarefaction number (S100), Shannon Wiener Index [H(S)], Equitability (E′) and Alpha Index (α). Records of diversity indices exhibit millennial scale changes during the late glacial and deglacial periods, corresponding to the Northern Hemisphere climatic events. We compared the faunal diversity with proxy records of primary productivity (Corg %) and bottom water oxygen (low-O2 taxa %). We suggest that benthic foraminiferal diversity in the eastern Arabian Sea OMZ is largely controlled by the primary productivity induced organic carbon flux and strength of bottom water oxygenation. The less diverse fauna along with increased percentages of Corg and low- O2 taxa during the last glacial maximum (18–22.5kaBP) suggest eutrophic and oxygen-poor benthic environment, attributed mainly to a strong OMZ associated with intense winter monsoon wind induced productivity and a weak deep ocean circulation. The intervals of distinct increase in diversity closely correspond with North Atlantic cold Heinrich events, when eastern Arabian Sea experienced significant declines in monsoon driven productivity and better deep sea ventilation due to enhanced inflow of Antarctic Intermediate Water (AAIW).

Characterization of phosphorus species in sediments from the Arabian Sea oxygen minimum zone: Combining sequential extractions and X-ray spectroscopy

The bulk phosphorus (P) distribution in sediment samples from the oxygen minimum zone of the northern Arabian Sea was determined using two methods: sequential chemical extraction (the ‘SEDEX’ procedure) and X-ray absorption near-edge structure (XANES) spectroscopy of the phosphorus K-edge. Our results show good agreement between iron (Fe-)associated P and calcium phosphate minerals (Ca-P) determined by both methods. Furthermore, we find that SEDEX exchangeable P likely represents loosely Fe-bound P, and that the SEDEX detrital fraction may consist partly of polyphosphate, i.e. microbially synthesized intracellular phosphate. Below productive waters with relatively high sedimentary organic matter and P contents, polyphosphates may represent an important P sink that is not easily identified by chemical sequential extraction. This study highlights the value of SEDEX as a generally accurate and fast P speciation technique (especially for Fe- and Ca-P) and, for the first time, demonstrates the possibilities of P K-edge XANES as a bulk P speciation tool for marine sediments.

Temporally invariable bacterial community structure in the Arabian Sea oxygen minimum zone

Temporally invariable bacterial community structure in the Arabian Sea oxygen minimum zone

Particle size distribution and estimated carbon flux across the Arabian Sea oxygen minimum zone

Abstract. The goal of the Arabian Sea section of the TARA oceans expedition was to study large particulate matter (LPM > 100 μm) distributions and possible impact of associated midwater biological processes on vertical carbon export through the oxygen minimum zone (OMZ) of this region. We propose that observed spatial patterns in LPM distribution resulted from the timing and location of surface phytoplankton bloom, lateral transport, microbial processes in the core of the OMZ, and enhanced biological processes mediated by bacteria and zooplankton at the lower oxycline. Indeed, satellite-derived net primary production maps showed that the northern stations of the transect were under the influence of a previous major bloom event while the most southern stations were in a more oligotrophic situation. Lagrangian simulations of particle transport showed that deep particles of the northern stations could originate from the surface bloom while the southern stations could be considered as driven by 1-D vertical processes. In the first 200 m of the OMZ core, minima in nitrate concentrations and the intermediate nepheloid layer (INL) coincided with high concentrations of 100 μm < LPM < 200 μm. These particles could correspond to colonies of bacteria or detritus produced by anaerobic microbial activity. However, the calculated carbon flux through this layer was not affected. Vertical profiles of carbon flux indicate low flux attenuation in the OMZ, with a Martin model b exponent value of 0.22. At three stations, the lower oxycline was associated to a deep nepheloid layer, an increase of calculated carbon flux and an increase in mesozooplankton abundance. Enhanced bacterial activity and zooplankton feeding in the deep OMZ is proposed as a mechanism for the observed deep particle aggregation. Estimated lower flux attenuation in the upper OMZ and re-aggregation at the lower oxycline suggest that OMZ may be regions of enhanced carbon flux to the deep sea relative to non OMZ regions.

Polychaete community structure in the South Eastern Arabian Sea continental margin (200–1000 m)

Macrofaunal polychaete communities (>500µm) in the South Eastern Arabian Sea (SEAS) continental margin (200–1000m) are described, based on three systematic surveys carried out in 9 transects (at ~200m, 500m and 1000m) between 7°00′and 14°30′N latitudes. A total of 7938 polychaetes belonging to 195 species were obtained in 136 grab samples collected at 27 sites. Three distinct assemblages were identified in the northern part of the SEAS margin (10–14°30′N), occupying the three sampled depth strata (shelf edge, upper and mid-slope) and two assemblages (shelf edge and slope) in the south (7–10°N). Highest density of polychaetes and dominance of a few species were observed in the shelf edge, where the Arabian Sea oxygen minimum zone (OMZ) impinged on the seafloor, particularly in the northern transects. The resident fauna in this region (Cossura coasta, Paraonis gracilis, Prionospio spp. and Tharyx spp.) were characteristically of smaller size, and well suited to thrive in the sandy sediments in OMZ settings. Densities were lowest along the most northerly transect (T9), where dissolved oxygen (DO) concentrations were extremely low (<0.15mll−1, i.e.<6.7μmoll−1). Beyond the realm of influence of the OMZ (i.e. mid-slope, ~1000m), the faunal density decreased while species diversity increased. The relative proportion of silt increased with depth, and the dominance of the aforementioned species decreased, giving way to forms such as Paraprionospio pinnata, Notomastus sp., Eunoe sp. and lumbrinerids. Relatively high species richness and diversity were observed in the sandy sediments of the southern sector (7–9°N), where influence of the OMZ was less intense. The area was also characterized by certain species (e.g. Aionidella cirrobranchiata, Isolda pulchella) that were nearly absent in the northern region. The gradients in DO concentration across the core and lower boundary of the OMZ, along with bathymetric and latitudinal variation in sediment texture, were responsible for differences in polychaete size and community structure on the SEAS margin. Spatial and temporal variations were observed in organic matter (OM) content of the sediment, but these were not reflected in the density, diversity or distribution pattern of the polychaetes.

Uptake of algal carbon and the likely synthesis of an &amp;quot;essential&amp;quot; fatty acid by &amp;lt;i&amp;gt;Uvigerina&amp;lt;/i&amp;gt; ex. gr. &amp;lt;i&amp;gt;semiornata&amp;lt;/i&amp;gt; (Foraminifera) within the Pakistan margin oxygen minimum zone: evidence from fatty acid biomarker and &amp;lt;sup&amp;gt;13&amp;lt;/sup&amp;gt;C tracer experiments

Abstract. Foraminifera are an important component of benthic communities in oxygen-depleted settings, where they potentially play a significant role in the processing of organic matter. We tracked the uptake of a 13C-labelled algal food source into individual fatty acids in the benthic foraminiferal species Uvigerina ex. gr. semiornata from the Arabian Sea oxygen minimum zone (OMZ). The tracer experiments were conducted on the Pakistan margin during the late/post monsoon period (August–October 2003). A monoculture of the diatom Thalassiosira weisflogii was 13C-labelled and used to simulate a pulse of phytoplankton in two complementary experiments. A lander system was used for in situ incubations at 140 m water depth and for 2.5 days in duration. Shipboard laboratory incubations of cores collected at 140 m incorporated an oxystat system to maintain ambient dissolved oxygen concentrations and were terminated after 5 days. Uptake of diatoms was rapid, with a high incorporation of diatom fatty acids into foraminifera after ~ 2 days in both experiments. Ingestion of the diatom food source was indicated by the increase over time in the quantity of diatom biomarker fatty acids in the foraminifera and by the high percentage of 13C in many of the fatty acids present at the endpoint of both in situ and laboratory-based experiments. These results indicate that

Depth‐related distribution of a key gene of the tetraether lipid biosynthetic pathway in marine Thaumarchaeota

The distribution of isoprenoid glycerol dialkyl glycerol tetraethers (GDGT) lipids synthesized by Thaumarchaeota has been shown to be temperature-dependent in world oceans. Depth-related differences in the ammonia monooxygenase (amoA) of Thaumarchaeota have led to the classification of 'shallow' and 'deep water' clusters, potentially affecting GDGT distributions. Here, we investigate if this classification is also reflected in a key gene of the thaumarchaeotal lipid biosynthetic pathway coding for geranylgeranylglyceryl phosphate (GGGP) synthase. We investigated metagenomic databases, suspended particulate matter and surface sediment of the Arabian Sea oxygen minimum zone. These revealed significant differences in amoA and GGGP synthase between 'shallow' and 'deep water' Thaumarchaeota. Intriguingly, amoA and GGGP synthase sequences of benthic Thaumarchaeota clustered with the 'shallow water' rather than with 'deep water' Thaumarchaeota. This suggests that pressure and temperature are unlikely factors that drive the differentiation, and suggests an important role of ammonia concentration that is higher in benthic and 'shallow water' niches. Analysis of the relative abundance of GDGTs in the Arabian Sea and in globally distributed surface sediments showed differences in GDGT distributions from subsurface to deep waters that may be explained by differences in the GGGP synthase, suggesting a genetic control on GDGT distributions.

Oxygen minimum zone of the open Arabian Sea: variability of oxygen and nitrite from daily to decadal timescales

Abstract. The oxygen minimum zone (OMZ) of the Arabian Sea is the thickest of the three oceanic OMZ. It is of global biogeochemical significance because of denitrification in the upper part leading to N2 and N2O production. The residence time of OMZ water is believed to be less than a decade. The upper few hundred meters of this zone are nearly anoxic but non-sulfidic and still support animal (metazoan) pelagic life, possibly as a result of episodic injections of O2 by physical processes. We report on discrete measurements of dissolved O2 and NO2–, temperature and salinity made between 1959 and 2004 well below the tops of the sharp pycnocline and oxycline near 150, 200, 300, 400, and 500 m depth. We assemble nearly all O2 determinations (originally there were 849 values, 695 of which came from the OMZ) by the visual endpoint detection of the iodometric Winkler procedure, which in our data base yields about 0.04 mL L−1 (~ 2 μM) O2 above the endpoint from modern automated titration methods. We acknowledge that much lower (nanomolar) O2 values have been measured recently with the STOX (Switchable Trace amount OXygen) sensor in the eastern tropical South Pacific, and that similar conditions may also prevail in the Arabian Sea OMZ. In spite of the error in O2 measurements at vanishingly low levels, we argue that the temporal trends of the historic data should still hold. We find 632 values acceptable (480 from 150 stations in the OMZ). The data are grouped in zonally paired boxes of 1° lat. and 2° long. centered at 8, 10, 12, 15, 18, 20, and 21° N along 65 and 67° E. The latitudes of 8–12° N, outside the OMZ, are treated in passing. The principal results are as follows: (1) an O2 climatology for the upper OMZ reveals a marked seasonality at 200 to 500 m depth with O2 levels during the northeast monsoon and spring intermonsoon seasons elevated over those during the southwest monsoon season (median difference, 0.08 mL L−1 [~ 3.5 μM]). The medians of the slopes of the seasonal regressions of O2 on year for each of the NE and SW monsoon seasons are −0.0043 and −0.0019 mL L−1 a−1, respectively (−0.19 and −0.08 μM a−1; n = 10 and 12, differing at p = 0.01); (2) four decades of statistically significant decreases of O2 between 15 and 20° N but an opposing trend toward an increase near 21° N are observed. The mechanisms of the balance that more or less annually maintain the O2 levels are still uncertain. At least between 300 and 500 m, the replenishment is inferred to be due to isopycnal re-supply of O2, while at 200 (or 250?) m it is diapycnal, most likely by eddies. Similarly, recent models show large vertical advection of O2 well below the pycnoclines and oxyclines. The NO2– distribution, taken as an indicator of active NO3– reduction, does not show a trend in the redox environment for a quarter of a century at a GEOSECS station near 20° N. In the entire OMZ, the regression slopes on year within seasons for the rather variable NO2– do not present a clear pattern but by other measures tended to an increase of NO2–. Vertical net hauls collect resident animal (metazoan) pelagic life in the NO2– maximum of the OMZ at O2 levels well below the lower limit of the Winkler titration; the extremely low O2 content is inferred from the presence of NO2– believed to be produced through microbial NO3– reduction. Instead of the difficult measurement by the STOX sensor, the relation between the very low O2 inferred from presence of NO2– and mesozooplankton should be studied with 100 to 150 L bottles rather than nets. The spatial (within drift stations) and temporal (daily) variability in hydrography and chemistry is large also below the principal pycnocline. The seasonal change of hydrography is considerable even at 500 m depth. Future O2 or nutrient budgets for the OMZ must not be based on single cruises or sections obtained during one season only. Steady state cannot be assumed any longer for the intermediate layers of the central Arabian Sea.