Acuticoccus sp. I52.16.1 was isolated from 100 m water depth from the Indian Ocean, and presented a novel Acuticoccus species belonging to the family Acuticoccaceae, class Alphaproteobacteria. The strain I52.16.1 displayed the activities of nitrate reductase, nitrite reductase, and urease. The genome of Acuticoccus sp. I52.16.1 consisted of a circular chromosome (5, 134, 086bp) with a G + C content of 69.7 mol%. The predicted number of coding genes was 4935, including 6 rRNA, 47 tRNA, and 2 sRNA. The 16S rDNA sequence displayed the maximum similarity of 97.58% with Acuticoccus yangtzensis JL1095T, followed by Acuticoccus sediminis PTG4-2T (97.05%), Acuticoccus kandeliae J103T (96.52%), and Acuticoccus mangrove B2012T (95.85%). Acuticoccus sp. I52.16.1 contained clades of genes involved in assimilating ammonium, nitrate, nitrite, and simple organic nitrogen compounds, but lacked the pathway for dissimilatory denitrification. Two distinct types of ureases were also detected, suggesting genetic heterogeneity. This study provided insight into the nitrogen metabolism strategies of heterotrophic bacteria in the oligotrophic ocean surface.

Microbial communities composed of bacteria, archaea and fungi play a pivotal role in driving the biogeochemical cycles in the marine ecosystem. Despite the vastness of the South Indian Ocean, only a few studies reported the simultaneous analysis of bacterial, archaeal and fungal diversity therein, particularly archaeal and fungal communities in deep-sea environments received less attention previously. In this study, microbial diversity, community composition and dynamics in microbial community structure in eight deep-sea sediment samples collected from different sites at varying depths of the South Indian Ocean were explored using Next-Generation Sequencing. In total, 21 bacterial phyla representing 541 OTUs were identified from the eight samples, where phylum Proteobacteria was found as the most abundant bacterial phylum in five out of eight samples. Firmicutes and Chloroflexi were the dominant phyla in the rest of the three samples. In the case of archaea, uncultured species belonging to the phyla Thaumarchaeota and Euryarchaeota were the abundant taxa in all the samples. Similarly, Ascomycota and Basidiomycota were the most abundant fungal phyla present therein. In all the eight samples studied here, about 10-58% and 19-26% OTUs in archaeal and fungal communities were mapped to unclassified taxa respectively, suggesting the lack of representation in databases. Co-occurrence network analysis further revealed that bacterial communities tend to be more dynamic than archaeal and fungal communities. This study provides interesting insights into the microbial diversity, community composition and dynamics in microbial community structure in the deep-sea sediments of the South Indian Ocean.

The dinoflagellates of the genus Gambierdiscus are found in almost all oceans and seas between the coordinates 35° N and 35° S. Gambierdiscus and Fukuyoa are producers of ciguatoxins (CTXs), which are known to cause foodborne disease associated with contaminated seafood. The occurrence and effects of CTXs are well described in the Pacific and the Caribbean. However, historically, their properties and presence have been poorly documented in the Indian Ocean (including the Bay of Bengal, Andaman Sea, and the Gulf). A higher occurrence of these microorganisms will proportionately increase the likelihood of CTXs entering the food chain, posing a severe threat to human seafood consumers. Therefore, comprehensive research strategies are critically important for developing effective monitoring and risk assessments of this emerging threat in the Indian Ocean. This review presents the available literature on ciguatera occurrence in the region and its adjacent marginal waters: aiming to identify the data gaps and vectors.

A taxonomic study was carried out on strain MT13131T, which was isolated from deep-sea sediment of the Indian Ocean during the screening of oil-degrading bacteria. The chain length range of n-alkanes (C8 to C32) oxidized by strain MT13131T was determined in this study. The bacterium was Gram-negative, oxidase- and catalase-positive, single rod shaped, and motile by peritrichous flagella. Growth was observed at salinities of 1-12 % and at temperatures of 10-42 °C. The isolate was capable of Tween 20, 40 and 80 hydrolysis, but incapable of gelatin, cellulose or starch hydrolysis. Phylogenetic analysis based on 16S rRNA gene sequences indicated that strain MT13131T belonged to the genus Alcanivorax, with highest sequence similarity to Alcanivorax marinus R8-12T (96.92 %), other species of genus Alcanivorax shared 92.96-96.69 % sequence similarity. The principal fatty acids were summed feature 3 (C16 : 1ω6c/ω7c), summed feature 8 (C18 : 1ω7c/ω6c), C16 : 0 and C12 : 0 3OH. The G+C content of the chromosomal DNA was 64.2 mol%. Phosphatidylglycerol, phosphatidylethanolamine, three aminolipids and three phospholipids were present. The combined genotypic and phenotypic data showed that strain MT13131T represents a novel species within the genus Alcanivorax, for which the name Alcanivorax mobilis sp. nov. is proposed, with the type strain MT13131T (=MCCC 1A11581T=KCTC 52985T).

Ten surface sediments were collected from the open Indian Ocean at depths below 4000 m in 2011, for the analysis of polybrominated diphenyl ethers (PBDEs) and polychlorinated biphenyls (PCBs). The concentrations of Σ32 PCBs, Σ7 PBDEs, and BDE-209 were 120-514, 49-152, and 7-133 pg/g, respectively. These concentrations are close to the lowest values recorded in the global marine environment. The PCBs had a relatively uniform composition, and were dominated by low chlorinated congeners. The concentrations of di-, tri-, and tetra-PCBs were strongly correlated with the total organic carbon (TOC), suggesting the dissolved PCBs were derived from the atmosphere via diffusive air-water exchange, and absorbed by phytoplankton. A high proportion of BDE209 was only detected in the sediment of the low fan of the Ganga River. There were weak correlations between low brominated BDEs and TOC, implying the degradation of BDE209 is a possible source of lower-brominated BDEs in deep-sea sediments.