Although a great amount of information is available on bacteria inhabiting deep-sea sediments, the occurrence of fungi in this environment has been poorly studied and documented. We report here the occurrence of fungi in deep-sea sediments from ~5000 m depth in the Central Indian Basin
Whereas fungi in terrestrial soils have been well studied, little is known of them in deep-sea sediments. Recent studies have demonstrated the presence of fungal hyphae in such sediments but in low abundance. We present evidence in this study that one of the apparent reasons for the poor detection of fungi in deep-sea sediments is their cryptic presence in macroaggregates. Fungal biomass carbon from different core sections of deep-sea sediments from approximately 5000 m depth in the Central Indian Ocean was estimated based on direct microscopic detection of fungal mycelia. Treatment of sediment samples with ethylenediamine tetra-acetic acid (EDTA) enabled more frequent detection and significantly higher biomass than in samples without such treatment. Treatment with EDTA resulted in detecting various stages of breakdown of aggregates in the sediments, gradually revealing the presence of fungal hyphae within them. Experimental studies of a deep-sea, as well as three terrestrial isolates of fungi, showed that all could grow at 200 bar and 5 degrees C in a nutrient medium and in deep-sea sediment extract. Hyphae of fungi grown in sediment extract under the above conditions showed various stages of accretion of particles around them, leading to the formation of aggregates. Such aggregates showed the presence of humic material, carbohydrate, and proteins. We suggest that fungi in deep-sea sediments may be involved in humic aggregate formation by processes very similar to those in terrestrial sediments. The importance of such a process in carbon sequestration and food web in the deep sea needs to be examined.
The fate of the enormous amount of reactive nitrogen released to the environment by human activities in India is unknown. Here we show occurrence of seasonal stratification and generally low concentrations of dissolved inorganic combined nitrogen, and high molecular nitrogen (N2) to argon ratio, thus suggesting seasonal loss to N2 in anoxic hypolimnia of several dam-reservoirs. However, 15N-experiments yielded low rates of denitrification, anaerobic ammonium oxidation and dissimilatory nitrate reduction to ammonium—except in the presence of methane (CH4) that caused ~12-fold increase in denitrification. While nitrite-dependent anaerobic methanotrophs belonging to the NC10 phylum were present, previously considered aerobic methanotrophs were far more abundant (up to 13.9%) in anoxic hypolimnion. Methane accumulation in anoxic freshwater systems seems to facilitate rapid loss of reactive nitrogen, with generally low production of nitrous oxide (N2O), through widespread coupling between methanotrophy and denitrification, potentially mitigating eutrophication and emissions of CH4 and N2O to the atmosphere.
The Indian Ocean harbors oxygen minimum zones (OMZs) in the Arabian Sea and Bay of Bengal, with dissolved oxygen < 20 µM, located at the mid-depths of the water column. Till date, high-throughput sequence-data on depth-wise distribution of prokaryotic communities have rarely been reported from these OMZs. The present study aimed to characterize the prokaryotic diversity inhabiting Arabian Sea Time Series (ASTS) and India's Idea 2 (II2) in the Arabian Sea, and Bay of Bengal Time Series (BoBTS) in the Bay of Bengal OMZs based on amplicon sequencing of 16S rRNA gene regions, along six sampled depths in the water column. High prokaryotic richness was observed in the Arabian Sea and Bay of Bengal samples. Operational taxonomic units (OTUs) in the range of 1249-3298 were identified, wherein, less prokaryotic diversity was observed at surface and within oxygen minimum depths. At phylum level, most OTUs were affiliated to Bacteroidetes, Chloroflexi, Cyanobacteria, Marinimicrobia, Planctomycetes, and Proteobacteria. Prokaryotic community differed between ASTS, II2 and BoBTS locations along varying physicochemical conditions. Predictive functional profiling of the bacterial communities suggested the involvement of abundant microbes in nitrogen and sulfur metabolism pathways. Bacterial isolates belonging to genera from the clades, δ-Proteobacteria and γ-Proteobacteria, described previously for their participation in biogeochemical cycling of N-and-S in the OMZs were reported from deoxygenated waters of both the basins. Bacteria involved in anammox such as Candidatus Scalindua were found to be relatively high at ASTS and II2 locations in the Arabian Sea. Further studies are required to ascertain the role of abundant bacteria along the dynamic oceanographic processes in the OMZs.
Ten fungal isolates with an ability to degrade crude oil were isolated from select marine substrates, such as mangrove sediments, Arabian Sea sediments, and tarballs. Out of the ten isolates, six belonged to , two to and one each to and as identified using ITS rDNA sequencing. The selected ten fungal isolates were found to degrade the long-chain -alkanes as opposed to short-chain-alkanes from the crude oil. Mangrove fungus #NIOSN-M126 () was found to be highly efficient in biodegradation of crude oil, reducing the total crude oil content by 77% and the individual -alkane fraction by an average of 95.37%, indicating it to be a potential candidate for the development into a bioremediation agent.
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