Tropical Estuarine Sediments Research Articles

Distribution and fractionation of metals in tropical estuarine sediments, NW Borneo: Implication for ecological risk assessment

Sediments were collected from 36 sites along the estuary of the Sibuti River in Sarawak, Malaysia, during the South-west and North-east monsoons, and then subjected to metal fractionation for Pb, Cd, Mn, Ba, Zn, Se, Co, Fe, Cu, Al and Cr. Metal concentrations were analyzed for 5 different fractions using the sequential extraction method. Pb, Mn and Cd possessed the greatest mobility, while major percentage of metals found were associated with residual fraction of sediments. Cd, Se and Cr, Fe were found dominantly associated with the carbonate and reducible fractions respectively, whereas Zn was found highly associated with oxidizable fraction of sediments. The factor analysis showed that the absorption of Mn in the upper part of estuary and most of the transition of metals in both seasons was regulated by the synthesis of Mn-oxyhydroxides catalyzed by microorganisms. On the other hand, association of organic matter with Fe and Al-oxyhydroxides work as major mechanism for absorption and settlement in intermediate and lower part of the estuary. Despite being associated with exchangeable fraction, Mn was found to be from geogenic sources along with all other metals except Cd, which is believed to be sourcing from agricultural lands. Ecological risk indices suggest that estuarine sediment is highly contaminated with Se, Co, Cu, Cd and pose considerable to very high risk at 90 % of the stations. Such enrichment is mainly resulting from the oxidation of metals from pyrite concretions and shale beds found in Sibuti and Lambir formations of the river basin. ERL and ERM values suggest Cu and Cr pose maximum threat towards the sediment dwelling living organisms. RAC reported Cd, Mn, Ba and Se in high risk category due to their potential to mobilize frequently, whereas Al, Fe and Cr present no risk towards benthic community.

Abundance, geo-microbial role and community diversity of sulfate-reducing bacteria in tropical estuarine sediments

Sulfate-reducing bacteria (SRB), a diverse taxonomic group of anaerobic microorganisms that oxidize sulfur compounds as terminal electron acceptors, drive the key biogeochemical process of sulfate reduction. However, little is known about the diversity of SRB in tropical coastal habitats. Thus, the goal of this study is to explore and compare the community diversity of the Ashtamudi and Vembanad estuaries and evaluate the sedimentary sulfate reduction rate. The microbiome profiling of SRB was carried out using Next Generation Sequencing (NGS) techniques (Illumina sequencing) based on gene amplification of the dissimilatory sulfate reductase (dsr) enzyme to confirm the diversity and dominant taxa of SRB. The diversity of SRB represented by the Shannon alpha diversity index of the Ashtamudi estuary (6.55) was two-fold that of the Vembanad estuary. In the Vembanad estuary, the marine zone (4.09) harboured more diversity than the freshwater zone (1.825). Higher diversity and lower abundant taxa in the Ashtamudi estuary pointed out the presence of a rare biosphere that has not yet been studied. It was interesting to note that both estuaries have the same SRB phylum diversity. At the level of species, however, there was a noticeable difference. This differential diversity of SRB has an apparent influence on methane production during the breakdown of organic matter in addition to sulfate reduction. Several species of SRB have an important role in the biodegradation of persistent pollutants and the removal of heavy metals.

Occurrence, distribution and sources of phthalates and petroleum hydrocarbons in tropical estuarine sediments (Mandovi and Ashtamudi) of western Peninsular India

The present study provides baseline information on the concentration levels, distribution characteristics and pollution sources of environmental contaminants, such as phthalic acid esters (PAEs or phthalates) and petroleum hydrocarbons in surface sediments of the tropical estuaries (Mandovi and Ashtamudi) from western Peninsular India. Total PAEs (∑5PAEs), hopanes, steranes and diasteranes concentrations from Ashtamudi estuary ranged from 7.77 to 1478.2 ng/g, n.d.-363.2 ng/g, n.d.-121.5 ng/g and n.d.-116.6 ng/g, respectively. Likewise, PAEs (∑6PAEs), steranes and diasteranes concentrations from Mandovi estuary ranged from 60.1 to 271.9 ng/g, 2.33–40.1 ng/g and 2.28–23.0 ng/g, respectively. The PAEs comprising di-isobutyl phthalate (DIBP), dibutyl phthalate (DBP), an isomer peak for DBP, di(2-ethylhexyl) phthalate (DEHP), di-isononyl phthalate were dominant in Ashtamudi estuary sediments, while PAEs including diethyl phthalate, DIBP, DBP and its isomer, DEHP, di(2-ethylhexyl) terephthalate were detected in the Mandovi sediment samples. The results of this study show an insignificant correlation of TOC with PAEs, and indicates that the varying spatial distributions of the PAEs in both the estuaries can be the result of discharge sources. The higher concentration of PAE congeners was noticed in Ashtamudi, a Ramsar wetland site, that can be attributed to land-based plastic waste. The petroleum biomarkers were abundantly present in Mandovi estuary due to anthropogenic activities such as boating and spillage from oil tankers. The findings of the present study will serve as a reference point for future investigation of organic contaminants in Indian estuaries, and calls for attention towards implementing effective measures in controlling the pervasion of the PAEs and petroleum biomarkers.

Glycerol dialkyl glycerol tetraether signatures in tropical mesotidal estuary sediments of Qua Iboe River, Gulf of Guinea

The continuous supply of land derived organic carbon by rivers is significant to the ocean carbon cycle. In river-estuary systems, the composition and distribution of sedimentary organic matter (OM) can provide detailed information on the balance of land-derived inputs and in situ productivity. This insight is significant for an improved understanding of anthropogenic activity and the changing climate. In the Gulf of Guinea (GoG), high rainfall, coupled with flooding and a warmer climate have direct impacts on OM distributions. Glycerol dialkyl glycerol tetraethers (GDGTs) have been used extensively to investigate OM. In this study, we focused on the widespread occurrence of isoprenoid GDGTs (isoGDGTs), branched GDGTs (brGDGTs), H-tetraethers (H-isoGDGTs and H-brGDGTs), hydroxyl GDGTs (OH-GDGTs), isoGDGT isomers, methylated isoGDGTs (Me-GDGTs), butanetriol and pentanetriol tetraethers (BDGTs and PDGTs), in short sediment cores collected from the inner and outer Qua Iboe River Estuary, located in the GoG. We further determined total organic carbon (Corg), total nitrogen (TN), long chain hydrocarbons (LCA; C27–C33n-alkanes), dinosterol, brassicasterol and phytol, to estimate OM sources, flooding activity, OM degradation, to reconstruct pH and to determine past tropical temperatures. The distribution patterns of GDGTs, use of principal component analysis (PCA), heatmap hierarchical cluster analysis, correlation models, the ratios of isoGDGTs/brGDGTs, ring index (RI), brGDGT-IIIa/IIa, branched and isoprenoid tetraether (BIT), Corg/TN and terrestrial-marine biomarker ratio (TMBR) were all sensitive for indicating terrestrial vs marine OM contributions to the total OM of the tropical estuarine sediments. Marine OM persisted from subsurface to bottom sediment layers (8–38 cm) in the outer estuary site, while a mixed OM source was observed in inner estuary sediments. The Cren/Cren’ ratio >58.5 (up to 83.8), pointed to a predominance of Thaumarchaeota group I.1a microbes. The Methane Index (MI) and isoGDGT-0/Cren ratio indicated significant imprints of a non-methanotrophic archaeal community. A positive linearity of total BDGTs and PDGTs plotted against MI and isoGDGT-0/Cren suggested that these compounds are produced by the same group of the non-methanogenic archaeal community in this tropical estuary. Thus, Methanomassiliicoccus luminyensis is not the only source of BDGTs and PDGTs in the marine environment. Further, the calculated CBT5me derived pH reflected in situ measured sediment pH (6.3). TEX86H-derived sea surface temperature (SST), within error, reflected water column temperature. Calculated brGDGT mean annual air temperatures (MAATs) reflected average annual atmospheric temperatures. Ring index OH-GDGTs temperature proxies (RI-OH SSTsummer and RI-OHꞌ SST) reflected mean dry season (November to February) temperatures. A newly derived MAAT proxy, calculated from H-brGDGT, also reflected the tropical annual air temperature (23–28 °C). Altogether, as expected, the OM sources are traced largely to both terrestrial and marine inputs. The terrestrial OM was deposited by excess flooding and surface runoff occasioned by torrential monsoon rains in recent years, triggered by climate change, while mesotidal currents introduced marine OM into the Qua Iboe River Estuary system. This study further contributes to our understanding of OM distributions in warmer climate conditions.

Distribution and Sources of Carbon, Nitrogen and Their Isotopic Compositions in Tropical Estuarine Sediments of Mtoni, Tanzania

Distribution and Sources of Carbon, Nitrogen and Their Isotopic Compositions in Tropical Estuarine Sediments of Mtoni, Tanzania

Spatiotemporal variations of organic matter sources in two mangrove-fringed estuaries in Hainan, China

Mangrove systems represent important long-term sinks for carbon since they have much higher carbon burial rates than terrestrial forests or typical coastal ecosystems. However, quantifying the sources of organic matter (OM) in estuarine and coastal sediments, where mangroves occur but are not the only source of OM, is challenging due to the variety of OM sources and diverse transport processes in these dynamic environments. The sources of OM in subtidal surface sediments of two mangrove-fringed estuaries in Hainan Province, China, were investigated using the mangrove-specific biomarker taraxerol and other lipid biomarkers, as well as stable carbon isotopes. Mixing models based on the concentration of taraxerol, plant wax n-alkanes and δ13COM indicate that terrestrial non-mangrove plant OM accounted for 40–57% of the OM in the two estuaries, phytoplankton OM accounted for 24–45%, and OM from mangroves comprised 15–19% of the total. Terrestrial plants contributed 10–21% more of the OM to subtidal sediments of Bamen Bay, which is on the wetter, eastern side of Hainan Island, than to Danzhou Bay, but the phytoplankton OM fraction was 16–24% lower than that in Danzhou Bay subtidal sediments. In both estuaries, mangrove and phytoplankton OM fractions increased seaward while the terrestrial OM fraction decreased. On a seasonal basis, lipid biomarker concentrations indicated a small change of OM sources. The biomarker and carbon isotope approach used here can be applied to semi-quantitatively estimate spatial and temporal variations of the sources of organic carbon in tropical estuarine and coastal sediments, a major sink for carbon in the ocean.

Interactions of sulfur and methane-oxidizing bacteria in tropical estuarine sediments

The bacterial oxidation of sulfur and methane is central to the biogeochemical processes in sediments such as the tropical mangrove sediments. However, there is a lacuna of information on the seasonal interactions including the influence of monsoons which is a major driver of seasonal change, on sulfur-oxidizing bacteria (SOB) and methane-oxidizing bacteria (MOB), their activity and the environmental variables. To understand these interactions, the analysis was carried out on sediment samples that were sampled monthly for a year from Chorao mangrove, Goa, southwest coast of India. SOB (3.8×105CFU g-1) and MOB (0.90×105CFU g-1) had maximum average abundance in the surface sediments in the post-monsoon and monsoon season, respectively. The mean sulfur-oxidation activity (SOA) of 2.63mMday-1 and methane-oxidation activity (MOA) of 110.94mMday-1 were highest in surface sediments during the post-monsoon season. Generally, the activity of SOB and MOB in surface sediments of post-monsoon was 2.2 times(×) and 2.8× respectively higher than that in the monsoon season. Among the environmental parameters analyzed, protein and sulfide concentrations significantly (p < 0.001) influenced SOA and MOA, respectively. There was a significant difference in SOA (p < 0.003) and MOA (p < 0.036) in surface sediments between the monsoon and the post-monsoon season. During the monsoon season, when the system is a sink of terrestrial/anthropogenic material, the interrelationship of SOB with MOA (r = 0.617, p < 0.001) and SOB with SOA (r = 0.489, p < 0.05) aids in maintaining the homeostasis of the system.

Molecular characteristics of sedimentary organic matter in controlling mercury (Hg) and elemental mercury (Hg0) distribution in tropical estuarine sediments

Sedimentary organic matter (SOM) plays an important role in hosting and reducing HgII in marine/estuarine sediment. This study provides a better understanding on the influence of nature of SOM, in regulating sedimentary mercury (Hg) and elemental mercury (Hg0) distribution, and speciation in the Zuari and Mandovi Estuaries that are representative of monsoon fed tropical estuaries, located in the central west coast of India. Salinity of the overlying water column controlled the physical and chemical characteristics of SOM in the estuarine systems. The high molecular weight (MW) SOM dominated at the mid and upstream (low salinity region) of the estuaries, whereas, the low MW SOM prevailed at the downstream (high salinity region). Sediment Hg showed more affinity towards the SOM of high MW. Increasing MW of SOM increased total sedimentary HgT in both the estuaries. SOM with low MW in the estuarine sediment displayed a negative relationship with the sediment Hg concentration. Distribution of Hg0 concentration in the estuarine sediment suggests that reduction of HgII in presence low MW SOM was a dominant process. It was also found that distribution and speciation of Hg0 in the estuarine sediment depends on the quantity, quality of the SOM, and the total sediment Hg loading. This study demonstrated that the competition between Hg-SOM complexation and HgII reduction by SOM controls HgII/Hg0 distribution in tropical estuarine sediment systems.

Partitioning of Cd, Zn and Fe in the Tissues and Cytosols of Blood Cockles (Anadara granosa) from the Gulf of Thailand

Anadara granosa is a bivalve blood cockle occurring abundantly in tropical estuarine intertidal sediments. The accumulation of Cd, Zn and Fe and their partitioning at the sub-cellular level in the cytosol has been investigated in the gills, digestive gland, body and foot of A. granosa from seven estuaries in the Gulf of Thailand. Cadmium and Zn were bioaccumulated predominantly in the gills with smaller concentrations distributed equally throughout the other tissues. Iron was present at relatively low concentrations in gills but was higher, and more evenly distributed, in the digestive gland, body and foot. The concentrations of metallothionein-like protein (MTLP) were found to be in the range 1–11 mg g−1 (dry weight) with the highest values being in the digestive gland at all sites. Partitioning of the metals in the cytosols between low molecular weight (LMW) compounds, MTLP and high molecular weight compounds (HMW) indicated that 16–39% of total Cd, 6.5–15% of total Fe and < 6% of total Zn were bound with MTLP. However, 23–27% of total Zn was bound with the HMW fraction, ascribed potentially to its association with the blood pigment haemoglobin (Hb). The metal-handling strategy, involving Zn, is likely important to blood cockles in supporting their uptake of dissolved oxygen in under-saturated tropical waters.

Correction to: Probabilistic risk assessment and risk mapping of metals in tropical estuarine sediments using the Aquarisk model.

The original version of this article unfortunately contained a mistake in the article title.

Probabilistic risk assessment and risk mapping of metals in tropical estuarine sediments using the Aquarisk model.

Total sediment concentrations of Cd, Cu, Pb, Zn, As, and Hg obtained from the Ankobra, Sakumo II, and Volta estuaries in Ghana were used to generate contaminant probability density distributions and species sensitivity distributions in AQUARISK. Results of the tier 1 assessment showed Cu, Cd, Zn, and Pb were not of concern in the Ankobra as their measured values and the 99th percentile of the fitted distributions were lower than the SQG low-trigger values. Mercury (Hg) and As were however, identified to be of concern in this estuary. In the Sakumo II estuary, Cu, Cd, Pb, and Hg have been identified to be of concern because their concentrations are higher than the SQG low-trigger values. Hg has been identified as the only metal of concern in the Volta estuary. The total proportion of species likely to be affected by the combined concentration of Cd, Cu, and Zn measured from Ankobra, Sakumo II, and Volta were 14%, 16%, and 12%, respectively, according to the Bur III distributional analysis of theecotoxicology data. The measured median sediment concentrations of As and Hg in the Ankobra estuary greatly exceeded the median sediment concentration targets to achieve a 5% or less exceedence of the SQG low value. Similarly, in the Sakumo II estuary, the measured median sediment concentrations of Cu, Pb, and Hg greatly exceeded the median sediment concentration targets to achieve a 5% or less exceedence of the SQG low. For the Volta estuary however, other metals except Hg fall below the target values.

Correction to: The mobility of the trace metals copper, zinc, lead, cobalt, and nickel in tropical estuarine sediments, Ebrie Lagoon, Côte d’Ivoire

Trace metals accumulate in the food web and can pose high risks to human health and aquatic biota. We investigated seasonal and spatial variations of Cu, Zn, Pb, Co, and Ni mobility in surface sediments (0–5 cm) and their potential toxicity. The sampling was carried out in three embayments of the Ebrie Lagoon (Cote d’Ivoire) during the rainy and dry seasons. The sequential and kinetic extraction methods were used to assess Cu, Zn, Pb, Co, and Ni mobility and their potential toxicity. Analysis of variance (ANOVA) was performed to ascertain spatial and seasonal differences. The results showed that contrarily to Co, distribution patterns of Zn and Cu in the sediment fractions showed no spatial and seasonal variability. On the other hand, Pb and Ni distribution patterns varied highly across the bays but not seasonally. Nevertheless, repeated measurements over a long period of time should be carried out to better understand the seasonal variability of metal distribution in sediment phases. Cu, Pb, Co, and Ni were abundant in residual fraction (56.93–71.66%) in the bays. On the contrary, a very high portion of Zn (up to 70%) could be remobilized. Cu, Pb, Co, and Ni formed mainly inert complexes in sediments, while Zn formed labile complexes. Zn could pose high risk to biota, Co and Ni a medium risk, and Cu and Pb a low risk. A combination of both sequential and kinetic methods revealed that both Zn and Pb formed labile complexes in the exchangeable and iron/manganese oxide fractions. Sequential extraction showed high Pb concentrations in the iron/manganese oxides. Thus, Zn and Pb may be highly available, which confirmed fish Pb concentrations exceeding WHO safe limits found by previous studies. Therefore, potential mobility of metals could be better assessed by the exchangeable and iron/manganese oxide fractions obtained by sequential extraction methods.

The mobility of the trace metals copper, zinc, lead, cobalt, and nickel in tropical estuarine sediments, Ebrie Lagoon, C\xf4te d\u2019Ivoire

PurposeTrace metals accumulate in the food web and can pose high risks to human health and aquatic biota. We investigated seasonal and spatial variations of Cu, Zn, Pb, Co, and Ni mobility in surface sediments (0–5 cm) and their potential toxicity.Materials and methodsThe sampling was carried out in three embayments of the Ebrie Lagoon (Côte d’Ivoire) during the rainy and dry seasons. The sequential and kinetic extraction methods were used to assess Cu, Zn, Pb, Co, and Ni mobility and their potential toxicity. Analysis of variance (ANOVA) was performed to ascertain spatial and seasonal differences.Results and discussionThe results showed that contrarily to Co, distribution patterns of Zn and Cu in the sediment fractions showed no spatial and seasonal variability. On the other hand, Pb and Ni distribution patterns varied highly across the bays but not seasonally. Nevertheless, repeated measurements over a long period of time should be carried out to better understand the seasonal variability of metal distribution in sediment phases. Cu, Pb, Co, and Ni were abundant in residual fraction (56.93–71.66%) in the bays. On the contrary, a very high portion of Zn (up to 70%) could be remobilized. Cu, Pb, Co, and Ni formed mainly inert complexes in sediments, while Zn formed labile complexes. Zn could pose high risk to biota, Co and Ni a medium risk, and Cu and Pb a low risk.ConclusionsA combination of both sequential and kinetic methods revealed that both Zn and Pb formed labile complexes in the exchangeable and iron/manganese oxide fractions. Sequential extraction showed high Pb concentrations in the iron/manganese oxides. Thus, Zn and Pb may be highly available, which confirmed fish Pb concentrations exceeding WHO safe limits found by previous studies. Therefore, potential mobility of metals could be better assessed by the exchangeable and iron/manganese oxide fractions obtained by sequential extraction methods.

An improved SPE-LC-MS/MS method for multiclass endocrine disrupting compound determination in tropical estuarine sediments

Estuary sediments are one of the important components of coastal ecosystems and have been regarded as a sink for various types of organic pollutants. Organic pollutants such as endocrine disrupting compounds (EDCs) which have been associated with various environmental and human health effects were detected in the estuary sediment at trace level. Considering various interferences that may exist in the estuarine sediment, a sensitive and selective method, capable of detecting multiclass EDC pollutants at the trace levels, needs to be developed and optimized to be applied for environmental analysis. A combination of Soxhlet extraction followed by offline solid phase extraction (SPE) cleaned up with detection based on LC triple quadrupole MS was optimized and validated in this study. The targeted compounds consisted of ten multiclass EDCs, namely, diclofenac, primidone, bisphenol A, estrone (E1), 17β-estradiol (E2), 17α-ethynylestradiol (EE2), 4-octylphenol (4-OP), 4-nonylphenol (4-NP), progesterone, and testosterone. The method showed high extraction efficiency with percentage of recovery from 78% to 108% and excellent sensitivity with detection limit between 0.02ngg−1 and 0.81ngg−1. Excellent linearity from 0.991 to 0.999 was achieved for the developed compounds and the relative standard deviation was less than 18%, an indication of good precision analysis. Evaluation of the matrix effects showed ionization suppression for all the developed compounds. Verification of the method was carried out by analyzing the estuarine sediment collected from Langat River. The analyzed estuarine sediments showed a trace concentration of diclofenac, bisphenol A, progesterone, testosterone, primidone, and E1. However, E2, EE2, 4-OP, and 4-NP were below the method's detection limit. Diclofenac exhibited the highest concentration at 2.67ngg−1 followed by bisphenol A (1.78ngg−1) while E1 showed the lowest concentration at 0.07ngg−1.

Structural dynamics of microbial communities in polycyclic aromatic hydrocarbon-contaminated tropical estuarine sediments undergoing simulated aerobic biotreatment.

Coastal sediments contaminated by polycyclic aromatic hydrocarbons (PAHs) can be candidates for remediation via an approach like land farming. Land farming converts naturally anaerobic sediments to aerobic environments, and the response of microbial communities, in terms of community structure alterations and corresponding effects on biodegradative activities, is unknown. A key goal of this study was to determine if different sediments exhibited common patterns in microbial community responses that might serve as indicators of PAH biodegradation. Sediments from three stations in the Lagos Lagoon (Nigeria) were used in microcosms, which were spiked with a mixture of four PAH, then examined for PAH biodegradation and for shifts in microbial community structure by analysis of diversity in PAH degradation genes and Illumina sequencing of 16S rRNA genes. PAH biodegradation was similar in all sediments, yet each exhibited unique microbiological responses and there were no microbial indicators of PAH bioremediation common to all sediments.

Nitrogen fixing bacterial diversity in a tropical estuarine sediments.

Microorganisms play a significant role in biogeochemical cycles, especially in the benthic and pelagic ecosystems. Role of environmental parameters in regulating the diversity, distribution and physiology of these microorganisms in tropical marine environment is not well understood. In this study, we have identified dinitrogen (N2) fixing bacterial communities in the sediments by constructing clone libraries of nitrogenase (nifH) gene from four different stations in the Cochin estuary, along the southeastern Arabian Sea. N2 fixing bacterial clones revealed that over 20 putative diazotrophs belong to alpha-, beta-, gamma-, delta- and epsilon- proteobacteria and firmicutes. Predominant genera among these were Bradyrhizobium sp. (α-proteobacteria), Dechloromonas sp. (β-proteobacteria); Azotobactor sp., Teredinibacter sp., Methylobacter sp., Rheinheimera sp. and Marinobacterium sp. (γ-proteobacteria); Desulfobacter sp., Desulfobulbus sp. and Desulfovibrio sp. (δ -proteobacteria); Arcobacter sp. and Sulfurospirillum sp. (ε-proteobacteria). Nostoc sp. was solely identified among the cyanobacterial phylotype. Nitrogen fixing Sulfate reducing bacteria (SRBs) such as Desulfobulbus sp., Desulfovibrio sp., Desulfuromonas sp., Desulfosporosinus sp., Desulfobacter sp., were also observed in the study. Most of the bacterial nifH sequences revealed that the identities of N2 fixing bacteria were less than 95% similar to that available in the GenBank database, which suggested that the sequences were of novel N2 fixing microorganisms. Shannon-Weiner diversity index of nifH gene ranged from 2.95 to 3.61, indicating an inflated diversity of N2 fixing bacteria. Canonical correspondence analysis (CCA) implied positive correlation among nifH diversity, N2 fixation rate and other environmental variables.

Partitioning of metals in different binding phases of tropical estuarine sediments: importance of metal chemistry.

Distribution of metals in different binding phases of estuarine sediments provides chemically significant description of metal-sediment interactions. This study describes the influences of ligand field stabilization energy (LFSE), Jahn-Teller effect, and water exchange rate (k-w) on metal distribution in different binding phases of estuarine sediments. It was found that Cu had highest affinity for organic binding phases in the studied sediments followed by Ni and Pb. However, Pb showed strong association with Fe/Mn oxide phases followed by Ni and Cu. Faster k-w of Cu (II) (1 × 10(9) s(-1)) increased the rate of complex formation of Cu(2+) ion with ligand in the organic phases. The Cu-ligand (from organic phase) complexes gained extra stability by the Jahn-Teller effect. The combined effects of these two phenomena and high ionic potential increased the association of Cu with the organic phases of the sediments than Ni and Pb. The smaller ionic radii of Ni(2+) (0.72 Å) than Pb(2+) (1.20 Å) increase the stability of Ni-ligand complexes in the organic phase of the sediments. High LFSE of Ni(II) (compared with Pb(2+) ions) also make Ni-organic complexes increasingly stable than Pb. High k-w (7 × 10(9) s(-1)) of Pb did not help it to associate with organic phases in the sediments. The high concentration of Pb in the Fe/Mn oxyhydroxide binding phase was probably due to co-precipitation of Pb(2+) and Fe(3+). High surface area or site availability for Pb(2+) ion on Fe oxyhydroxide phase was probably responsible for the high concentration of Pb in Fe/Mn oxyhydroxide phase. Increasing concentrations of Cu in organic phases with the increasing Cu loading suggest that enough binding sites were available for Cu in the organic binding phases of the sediments. This study also describes the influence of nature of sedimentary organic carbon (terrestrial and marine derived OC) in controlling these metal distribution and speciation in marine sediment.

Speciation of metals and their distribution in tropical estuarine mudflat sediments, southwest coast of India

Two sediment cores collected from a mudflat sedimentary environment of Swarna estuary (S3) and Gurpur estuary (MF6), representing the middle estuarine region, Karnataka, India, were investigated to understand bioavailability of metals and their toxicity. The subsamples were analyzed for sand, silt, clay, organic carbon and total metal concentration of Aluminum (Al), Iron (Fe), Manganese (Mn), Nickel (Ni), Zinc (Zn), Copper (Cu), Cobalt (Co) and Chromium (Cr) at 2cm intervals. Sediments (average) are relatively coarser in Gurpur estuary whereas silt, clay, organic carbon along with the studied metals (except Mn and Cu) is noted to be higher in the Swarna estuary. Significant correlations were observed of Al, Fe with finer sediments and most of the trace metals in both the cores indicating a mainly lithogenic source. Further, metal speciation analysis carried out for Fe, Mn, Ni, Zn, Cu, Co and Cr on selected samples of both the cores indicated that Fe was largely associated with the residual fraction with 93±0.5% in Gurpur and 84±6% in Swarna estuary. The concentration of studied metals in the residual fraction in the sediments of Gurpur estuary was in the order Mn(90±5%)>Cr(85±1%)>Ni(72±5%)>Zn(69±3%)>Cu(57±5%)>Co(55±2%) and; Cr(80±7%)>Mn(77±10%)>Ni(76±7%)>Zn(67±10%)>Cu(67±10%)>Co(50±7%) in Swarna estuary. When the total (bulk) metals were compared with the Sediment Quality Values (SQV) following Screening Quick Reference Table (SQUIRT), Co values of both the cores fell above Apparent Effect Threshold (AET) values. When the sum of the average bioavailable fractions in sediments was considered, Co values exceeded the AET in core S3 (Swarna estuary). When the variations are viewed with depth, bioavailability of Mn, Ni, Cu and Co in Gurpur estuary indicated anthropogenic addition in recent years whereas in Swarna estuary most of the studied metals showed diagenetic remobilization and diffusion to the water column from surface sediments. Metal speciation study indicated no harm to the aquatic life, except of Co, in Swarna estuary wherein it posed a high risk of toxicity to organisms associated with the sediments.

Dynamics of the methanogenic archaea in tropical estuarine sediments.

Methanogenesis may represent a key process in the terminal phases of anaerobic organic matter mineralization in sediments of coastal lagoons. The aim of the present work was to study the temporal and spatial dynamics of methanogenic archaea in sediments of tropical coastal lagoons and their relationship with environmental changes in order to determine how these influence methanogenic community. Sediment samples were collected during the dry (February, May, and early June) and rainy seasons (July, October, and November). Microbiological analysis included the quantification of viable methanogenic archaea (MA) with three substrates and the evaluation of kinetic activity from acetate in the presence and absence of sulfate. The environmental variables assessed were temperature, pH, Eh, salinity, sulfate, solids content, organic carbon, and carbohydrates. MA abundance was significantly higher in the rainy season (106–107 cells/g) compared with the dry season (104–106 cells/g), with methanol as an important substrate. At spatial level, MA were detected in the two layers analyzed, and no important variations were observed either in MA abundance or activity. Salinity, sulfate, solids, organic carbon, and Eh were the environmental variables related to methanogenic community. A conceptual model is proposed to explain the dynamics of the MA.

Archaea in Past and Present Geobiochemical Processes and Elemental Cycles

For a long time, Archaea have been considered as an ancient prokaryotic group comprising specialists restricted to narrow ecological niches. This opinion might have been supported by the characteristics of the first well-investigated isolates, being strictly anaerobic (methanogens), halophilic (haloarchaea), or thermophilic (various groups). This is, however, just the tip of the iceberg. Archaea are abundant in all ecosystems. Representatives of the whole domain span the widest range of ecological adaptations from psychrophilic to hyperthermophilic. They tolerate the widest range of pH as well as salt concentrations and use all types of substrates comprising all kinds of organic molecules as well as reduced inorganic compounds. Nowadays, a multitude of experimental tools is available allowing researchers to study the important role of Archaea and other microorganisms in environmental processes. Most important, analysis of environmental DNA and RNA opens a direct way to assess diversity in specific habitats. The pioneering findings of Carl R. Woese for assessing microbial taxonomy by ribosomal RNA sequence analysis and comparison, together with recently developed powerful amplification and sequencing techniques, widely opened the doors for detection of new hitherto unknown archaeal groups. In particular, the phyla Crenarchaeota and Thaumarchaeota and their significance in environmental cycles could be determined by these techniques. Other tools gain in importance for the assessment of environmental processes. With the increasing availability of annotated genomes, the analysis of metagenomes and metaproteomes becames possible and now helps us to understand the metabolic processes and networks within microbial communities. The identification of lipids allows bulk quantification of specific prokaryotic groups in recent habitats and even in fossil settings. Finally, microscopic techniques along with functional analysis helps us to understand some of these environmental processes also at structural levels, for example, in microbial mats. This special issue of the journal Archaea addresses several aspects of the metabolic, structural, and ecological diversity of Archaea from a wide range of research fields. The contributions by B. Wemheuer et al. point out the diversity of Bacteria and Archaea in hot springs from the Kamchatka Peninsula, revealing the dominance of Proteobacteria, Thermotogae, and Thaumarchaeota. In a second publication, B. Wemheuer et al. address the diversity in a marine habitat with haloarchaea (Halobacteria) as the predominant archaeal group. One of the key processes in sediments is methanogenesis. M. R. Torres-Alvarado et al. show in their study the changes in the abundance and activity of methanogens during dry and rainy seasons in tropical estuarine sediments. The contribution of L. A. Fernandez-Guelfo et al. addresses the directed processes of methanogenesis in an anaerobic reactor suitable for wastewater treatment. The whole anaerobic food chain at different temperatures was evaluated by A. Siggins et al. in an anaerobic reactor with trichloroethylene as a substrate. X. L. H. Lim et al. used the lipid archaeol in anoxic water saturated soil as a biomarker for the biomass of methanogens. Another marker for methanogens was evaluated by C. Wrede et al. for microbial biofilms. Methyl coenzyme M reductase was detected in situ by respective antibodies and was found to be a suitable indicator for the enzymes catalyzing methanogenesis as well as the reverse pathway anaerobic oxidation methane (AOM). Another research article by C. Wrede et al. addresses iron biomineralization in the course of AOM. Finally, a review article on Archaea in symbiosis briefly summarizes the diverse symbiotic interactions of Archaea with organisms of all other domains. We hope that this issue will support to the statement stressed previously: Archaea, being as diverse as Bacteria, make their contribution to all important global elemental cycles. Michael Hoppert Martin Kruger Joachim Reitner Charles Cockell