Environmental DNA reveals aquatic biodiversity of an urban backwater area, southeast coast of India.

Strong conservation management needs comprehensive data on biodiversity. Rapid methods that document aquatic biodiversity or assess the health condition of an ecosystem remain scarce. Herein, we have performed a metagenomics study on environmental DNA (eDNA) collected from an urban backwater area - Muttukadu, located in the southeast coast of India. Shotgun metagenomics approach using Illumina®NextSeq500 sequencing yielded 88.4 million raw reads. The processed data was assigned as 80% prokaryotes, 0.4% eukaryotes, ~2% viruses, and ~17% remain unknown. This approach has the potential to identify small micro-eukaryote, unseen species from both estuarine and marine environments. We have identified 156 eukaryote organisms represented from 21 phyla and 112 families, including those that are of conservational significance and ecological importance. Furthermore, our data also demonstrated the presence of pathogenic microorganisms due to sewage mixing with the backwaters. Given its sensitivity, we suggest this approach for an initial assessment of biodiversity structure in an ecosystem for the biomonitoring program.

Geochemical fractionation study in combination with equilibrium based chemical speciation modelling of Cd in finer sediments provide a better description of Cd bioavailability in tropical estuarine systems.

Geochemical fractionation study of cadmium (Cd) in combination with equilibrium based chemical speciation modelling (visual MINTEQ) in finer sediment samples (silt and clay portion), provided a better description of Cd bioavailability for sessile organisms (like oysters) in the Zuari estuary, west coast of India. A substantial fraction of total Cd in the finer sediment samples was present as water soluble, exchangeable and carbonate/bicarbonate forms and showed a strong positive relationship with Cd-bioaccumulation in oysters (Magallana sp.) from the study area. Cadmium bioaccumulation (ranged from 2.4-7.9 mg·kg-1) in soft tissues of the edible oyster was high (higher than the maximum permissible limit for human consumption) throughout the study period of one year. Equilibrium based speciation modelling suggest that sedimentary CdCl2, CdCl+ and Cd2+ were responsible for Cd bioaccumulation in the oysters. This study describes that combination of geochemical fractionation study of metals in finer sediments and equilibrium based speciation modelling can be very useful in comprehending link between sedimentary metal speciation and its bioaccumulation in sessile organisms.

Operationally defined mercury (Hg) species can delineate Hg bioaccumulation in mangrove sediment systems: A case study.

This study investigated the linkage between mercury (Hg) speciation in the surficial sediments from a mangrove ecosystem of the Zuari Estuary, west coast of India, with Hg bioaccumulation in gastropods collected from the same area. Multiple operationally defined protocols and methods were used for determination of Hg speciation study in the mangrove sediments. Moderately low concentrations of Hg were observed in the sediments, ranging from 37.3 ±â€¯1.9 to 79.6 ±â€¯4.0 µg/kg. Geochemical fractionation showed that a significant part of sedimentary Hg was present within the structure of the sediment (residual fraction) and not bioavailable. Non-residual Hg was primarily associated with oxidizable (sedimentary organic matter (SOM) or sulfide) binding phase of the sediments, and ranged from 9.2 ±â€¯0.3 to 78.5 ±â€¯3.9 µg/kg. Concentration of methylmercury (MeHg) (a neurotoxin) in the sediments varied from 1.7 ±â€¯0.1 to 4.4 ±â€¯0.1 µg/kg. l-Cysteine, a suitable complexing ligand, extractable Hg concentration in the sediments ranged from 4.3 ±â€¯0.1 to 15.9 ±â€¯0.3 µg/kg. Statistical analysis suggested that MeHg was adsorbed on Fe/Mn oxyhydroxide phases in the sediments. l-Cysteine was found to extract sedimentary MeHg and thermodynamically less stable Hg-SOM complexes from the sediments. Concentrations of bioaccumulated Hg in soft tissues of the gastropod, Pirenella cingulata, ranged from 57.6 ±â€¯4.4 to 224.4 ±â€¯7.2 µg/kg. Positive correlations existed between the concentration of bioaccumulated Hg in the gastropods and the concentrations of Hg associated with the oxidizable phase, sedimentary MeHg and l-Cysteine extracted Hg in the sediments. This study indicated that operationally defined Hg species can be useful in estimating bioavailable Hg to obligatory deposit feeder in tropical mangrove systems.

Effect of pH on transport and transformation of Cu-sediment complexes in mangrove systems.

Impact of pH variation of overlying water column on transport and transformation of Cu-sediment complexes in the bottom mangrove sediments was investigated by using different metal extraction studies. The total Cu concentration in the studied sediments varied from ~64 ±â€¯1 to 78 ±â€¯2 mg·kg-1. The sequential extraction study showed that a major part of the sedimentary Cu (85-90% of the total sedimentary Cu) was present within the structure of the sediments with minimum mobility and bioavailability. The redistribution of non-residual Cu among the different binding phases of the sediments was observed at different pH. It was found that Cu shifted from the different non-residual binding phases to the organic binding phase of the sediments at higher pH. Partial leaching of sedimentary Cu-SOM complexes (with increasing stability as determined by kinetic extraction study) was observed at higher pH. This study infers that increase in pH of overlying water column may release Cu-SOM complexes and increase the mobility of Cu-complexes in mangrove systems.

Testing the efficiency of temperate benthic biotic indices in assessing the ecological status of a tropical ecosystem.

The objectives of the present study were to evaluate the ecological status of tropical coastal waters using the temperate benthic indices and examine the effect of seasonal variability on the performance of benthic indices. Macrobenthic samples were collected from northwest to southeast coast of India during 2003-2012 and we tested different univariate indices, ecological strategies, indicator species and multimetric indices. AMBI and multimetric indices performed satisfactorily in evaluating the ecological status. Seasonal variability on the biotic indices was observed during the southwest monsoon and fall intermonsoon period due to recruitment. Therefore, we recommended the non-monsoon period (January-May) as a suitable time of the year to use the indices for effective assessment of the Indian coastal waters. Results show that, the temperate benthic indices are efficient in assessing the tropical environmental status. However, complementary use of different indices is suggested for accurate assessment of the environmental status.

Linkage between speciation of Cd in mangrove sediment and its bioaccumulation in total soft tissue of oyster from the west coast of India.

This study established a mechanistic linkage between Cd speciation and bioavailability in mangrove system from the west coast of India. High bioaccumulation of Cd was found in the oyster (Crassostrea sp.) even at low Cd loading in the bottom sediment. Bioaccumulation of Cd in the oyster gradually increased with the increasing concentrations of water soluble, exchangeable and carbonate/bicarbonate forms of Cd in the sediments. Fe/Mn oxyhydroxide phase was found to control Cd bioavailability in the sediment system. Cd-associated with sedimentary organic matter was bioavailable and organic ligands in the sediments were poor chelating agents for Cd. This study suggests that bioaccumulation of Cd in oyster (Crassostrea sp.) depends not on the total Cd concentration but on the speciation of Cd in the system.

Mercury profiles in sediment from the marginal high of Arabian Sea: an indicator of increasing anthropogenic Hg input.

Total Hg distributions and its speciation were determined in two sediment cores collected from the western continental marginal high of India. Total Hg content in the sediment was found to gradually increase (by approximately two times) towards the surface in both the cores. It was found that Hg was preferentially bound to sulfide under anoxic condition. However, redox-mediated reactions in the upper part of the core influenced the total Hg content in the sediment cores. This study suggests that probable increase in authigenic and allogenic Hg deposition attributed to the increasing Hg concentration in the surface sediment in the study area.

Relationship between the lability of sediment-bound Cd and its bioaccumulation in edible oyster.

A linkage between Cd speciation in sediments and its bioaccumulation in edible oyster (Crassostrea sp.) from a tropical estuarine system was established. Bioaccumulation of Cd in edible oyster increased with the increasing lability and dissociation rate constants of Cd-sediment complexes in the bottom sediments. Total Cd concentration in sediment was not a good indicator of Cd-bioavailability. Increasing trace metal competition in sediments increased lability and bioavailability of Cd in the tropical estuarine sediment. Low thermodynamic stability and high bioavailability of Cd in the estuarine sediment were responsible for high bioaccumulation of Cd in edible oysters (3.2-12.2mgkg(-1)) even though the total concentration of Cd in the bottom sediment was low (0.17-0.49mgkg(-1)).

Reduction of mercury (II) by humic substances--influence of pH, salinity of aquatic system.

This study demonstrates that under abiotic dark conditions in aquatic system, humic substances are not only capable of converting Hg(II) to Hg(0) but also able to bind Hg(II) ion. The degree of Hg(II) reduction is significantly influenced by the ratio of -COOH/-OH groups and the sulfur content in the HS, revealing a strong competition between complexation and reduction of Hg(II). This study suggests that abiotic and dark Hg(II) reduction depends on the pH and salinity of aqueous medium. At lower pH (∼ 4.0) and lower salinity (≤ 5.0 PSU), the reduction of Hg(II) to elemental mercury (Hg(0)) was comparatively rapid. Higher -COOH/-OH ratios in HS, favors dark abiotic reduction of Hg(II) as did a lower sulfur (S) content of HS. This study provided a rigorously controlled experimental design that showed that dark abiotic Hg(II) reduction by HS can potentially be important in the aquatic environment and is independent of the photochemical reduction observed in both fresh water and sea water.

Geochemical partitioning of Cu and Ni in mangrove sediments: relationships with their bioavailability.

Sequential extraction study was performed to determine the concentrations of non-residual metal-complexes in the mangrove sediments from the Divar Island, (west coast of India). Accumulation of metal in the mangrove roots (from the same location) was determined and used as an indicator of bioavailability of metal. An attempt was made to establish a mechanistic linkage between the non-residual metal complexes and their bioavailability in the mangrove system. The non-residual fractions of Cu and Ni were mainly associated with Fe/Mn oxyhydroxide and organic phases in the sediments. A part of these metal fractions were bioavailable in the system. These two phases were the major controlling factors for Ni speciation and their bioavailability in the studied sediments. However, Cu was found to interact more strongly with the organic phases than Ni in the mangrove sediments. Organic phases in the mangrove sediments acted as buffer to control the speciation and bioavailability of Cu in the system.

Kinetic speciation and bioavailability of copper and nickel in mangrove sediments.

An attempt was made to establish a mechanistic linkage between chemical speciation of copper and nickel, and their bioavailability in mangrove ecosystem. Kinetic speciation study was performed to determine the concentrations of labile metal-complexes and their dissociation rate constants in mangrove sediments. Concentrations of copper and nickel in the mangrove roots were used as indicators of their bioavailability. It was found that the bioaccumulation of both the metals gradually increased with the increasing concentrations of the labile metal complexes and their dissociation rate constants in the mangrove sediments. This study shows that concentration of labile metal (copper and nickel) complexes and their dissociation rate constants in mangrove sediment can be a good indicator of their bioavailability.

Mercury speciation in coastal sediments from the central east coast of India by modified BCR method.

This is the first study to describe distribution and speciation of Hg in coastal sediments from the central east coast of India. The concentrations of Hg in the studied sediments were found to be much lower than the Hg concentration recommended in coastal sediments by the United State Environmental Protection Agency and the Canadian Council of Ministers of the Environment for the protection of aquatic life. This study suggests that the interactions between Hg and coastal sediments are influenced by particle size (sand, silt and clay) of the sediments and the total organic carbon (TOC) content in the sediments. It was found that the coastal sediments from the central east coast of India could act as a sink for Hg. The availability of strong uncomplexed-Hg binding sites in the coastal sediments was observed.

Changes in metal contamination levels in estuarine sediments around India--an assessment.

This review is the first attempt to comprehend the changes in metal contamination levels in surface estuarine sediments with changing time around India. Contamination factor, geoaccumulation index, pollution load index, effects range low and effects range median analysis were used to evaluate the quality of the estuarine sediments (by using the available literature data). This study suggests that estuarine sediments from the east coast of India were comparatively less contaminated by metals than the west coast. Sediments from those estuaries were found to be more contaminated by metals on which major cities are located. An improvement in estuarine sediment quality (in terms of metal contamination) over time around India was noticed. This study provides managers and decision-makers of environmental protection agency with a better scientific understanding for decision-making in controlling metal pollution in estuarine sediments around India.