Microbial Extracellular Polymeric Substances (EPS) in Fresh Water Sediments

Microbially produced extracellular polymeric substances (EPS) have been linked with many important ecological functions in natural sediments; yet, most information has been derived from marine systems. The present paper is the first comprehensive study on EPS (i.e., carbohydrates and proteins) dynamics in riverine sediments addressing spatial (six reservoirs and four groyne fields across three European rivers), temporal (all seasons in 2003–2005), and vertical (over a 50-cm sediment depth transect) pattern. The variation in hydrodynamic regime found in the reservoirs and groyne fields was reflected in the biomass and composition of the benthic microorganisms that produce EPS. The microphytobenthic communities consisted mainly of diatoms and a higher algal biomass (up to 248 μg g⁻¹ dry weight, DW) seemed to be indicative for higher amounts of secreted colloidal carbohydrates. Consequently, the model proposed by Underwood and Smith (1998) for the relation chlorophyll–colloidal carbohydrates was also applicable for upper riverine sediment layers. The close relation between algal biomass and bacterial cell counts (10⁸–10⁹ cells g⁻¹ DW) supports the idea of bacterial use of the secreted EPS. However, the data also suggest a contribution to the EPS pool through bacterial secretion of proteins/extracellular enzymes and possibly carbohydrates. Over depth, the relationships between microorganisms and EPS became increasingly decoupled along with increasing ratios of bound (refractory) to colloidal (labile) EPS. These data suggest fresh production of polymeric substances in upper sediment layers and mainly accumulation of refractory, biodegraded material in deeper layers. The high contents of EPS colloidal and bound carbohydrates (0.1–1.8 and 1.3–6.7 mg g⁻¹ DW, respectively) and EPS proteins (0.4–12.9 mg g⁻¹ DW) at the freshwater study sites might indicate an important role in sediment ecology.     

Distribution of Exopolymeric Substances in the Littoral Sediments of an Oligotrophic Lake

Bacteria and algae release exopolymeric substances (EPS) that perform a wide range of important functions in aquatic and terrestrial systems. In this study we measured EPS in sediments at nine littoral sites around a shallow oligotrophic basin, and tested whether the concentration and composition of EPS was related to sediment characteristics. The concentrations of both loosely bound (colloidal) and tightly bound (capsular) EPS carbohydrates ranged up to ~800 µg glucose equiv. cm^–2 and were well within the range of concentrations reported from marine intertidal flats, where EPS play an important role in stabilizing sediments, affecting nutrient exchanges between sediments and the water column, feeding benthic invertebrates, and sequestering and increasing the transfer of contaminants to food webs. Proteins were an important component of the EPS in these littoral sediments, with protein:carbohydrate ratios of ~0.4. In summer, the concentrations of most EPS fractions were positively related (P < 0.05) to the porewater and organic matter content of the sediments. Capsular EPS concentrations were lower in the fall, with a simultaneous increase in colloidal proteins but not in colloidal carbohydrates. This suggests that the carbohydrates in this colloidal EPS may be more labile than the proteins. Our results suggest that exopolymeric substances could be an important, but neglected, component of littoral sediments in lakes.     

Predicting Epipelic Diatom Exopolymer Concentrations in Intertidal Sediments from Sediment Chlorophyll a

Abstract In many intertidal cohesive—sediment habitats, epipelic diatoms are the dominant microphytobenthic organisms. In such sediments, concentrations of colloidal carbohydrate [including the exopolymeric substances (EPS) produced by diatoms during motility] are closely correlated with the biomass (chlorophyll a) of epipelic diatoms. A model describing this relationship (log (conc. coll. carbo. + 1) = 1.40 + 1.02(log (chl. a conc. + 1)) was derived from published data. It was validated against published and unpublished data from 6 different estuaries, and accounted for 64.6% of the variation in sediment colloidal carbohydrate concentrations. The model was valid for intertidal habitats with cohesive sediments where epipelic diatoms constituted >50% of the microphytobenthic assemblage. In sites with noncohesive sediments, or where the microphytobenthic assemblage was dominated by other algal groups, the model was not applicable. The mean percentage of EPS in colloidal carbohydrate extracts varied between 11 and 37% for axenic cultures of epipelic diatoms (with higher values obtained during stationary phase), and between 22.7% and 24.3% for natural sediments dominated by epipelic diatoms. Assuming an EPS percentage of 25% in colloidal extracts yielded an EPS chl. a ratio of 2.62:1. Maximum rates of EPS production in diatom cultures occurred at the beginning of stationary phase (1.6–5.09 μg EPS μg⁻¹ chl a d⁻¹), with Nitzschia sigma having a significantly (P < 0.05) higher rate of production than N. frustulum, Navicula perminuta and Surirella ovata. Similar rates of EPS production were measured in the field. The dynamics of EPS production and loss on mudflats is discussed, with reference to the model and these production rates. Received: 25 February 1997; Accepted: 23 May 1997     

The interaction between water movement,sediment dynamics and submersed macrophytes

Water movement in freshwater and marine environments affects submersed macrophytes, which also mediate water movement. The result of this complex interaction also affects sediment dynamics in and around submersed macrophyte beds. This review defines known relationships and identifies areas that need additional research on the complex interactions among submersed macrophytes, water movement, and sediment dynamics. Four areas are addressed: (1) the effects of water movement on macrophytes, (2) the effects of macrophyte stands on water movement, (3) the effects of macrophyte beds on sedimentation within vegetated areas, and (4) the relationship between sediment resuspension and macrophytes. Water movement has a significant effect on macrophyte growth, typically stimulating both abundance and diversity of macrophytes at low to moderate velocities, but reducing growth at higher velocities. In turn, macrophyte beds reduce current velocities both within and adjacent to the beds, resulting in increased sedimentation and reduced turbidity. Reduced turbidity increases light availability to macrophytes, increasing their growth. Additionally, macrophytes affect the distribution, composition and particle size of sediments in both freshwater and marine environments. Therefore, establishment and persistence of macrophytes in both marine and freshwater environments provide important ecosystem services, including: (1) improving water quality; and (2) stabilizing sediments, reducing sediment resuspension, erosion and turbidity.     

A modified method for studies of electron transport system activity in freshwater sediments

A method for measuring respiratory electron transport system activity (ETSA) in freshwater sediments is presented. It is a modification of the methods used for marine sediments by Christensen & Packard (1977) and Olànczuk-Neyman & Vosjan (1977). The assay is based on the reduction of the electron acceptor 2-(p iodophenyl) 3 (p nitrophenyl) 5 phenyl tetrazolium chloride (INT) by cell-free homogenates of sediment samples. This study shows that the method is sensitive, has good accuracy and can be used for freshwater sediments. The accuracy decreases at high turbidity and as yet has only been tested for sediments of the gyttja type. Untreated samples can be stored at low temperature for several weeks without any loss of activity. ETSA decreases with the depth of sediment and with increased amount of allochthonous material in the sediment.     

The benthic macrofauna along the estuarine gradient of the Schelde estuary

The intertidal benthic macrofauna of the Schelde estuary (The Netherlands and Belgium) was sampled in late autumn of 1990 at 50 stations along the whole salinity gradient (between Vlissingen and Dendermonde), including the freshwater tidal part. All stations were situated in sheltered areas with a relatively muddy sediment. Species richness, diversity and total biomass of the benthic macrofauna decreased along the salinity gradient from Vlissingen to Dendermonde, while total density showed no clear trend. Especially the oligohaline and freshwater tidal part of the Schelde estuary was characterized by a very impoverished benthic community, composed only of Oligochaeta. No other species (freshwater, marine or brackish) was observed in this part of the estuary. The marine part had a more diverse macrozoobenthos structure than that of the brackish part. Species found only in the marine zone areCerastoderma edule, Tharyx marioni, Eteone longa, Nephtys hombergii andCapitella capitata. In the brackish part of the estuary,Corophium volutator was a typical, dominant species. However, a lot of the dominant species were common in both the marine and brackish part of the Schelde estuary (e.g. Heteromastus filiformis, Pygospio elegans, Nereis diversicolor, Macoma balthica). The observed gradient in species composition and dominance is compared with some other European estuaries. The marine and brackish part of the Schelde estuary is quite similar to other european estuaries. The freshwater tidal part, however, was more impoverished.     

Biochemical characterization of extracellular polymeric substances extracted from an intertidal mudflat using a cation exchange resin

The biochemical characterization of Extracellular Polymeric Substances (EPS) excreted in a European intertidal mudflat (Marennes–Oléron Bay) was performed. Experiments were carried out for the first time in situ, by using an improved extraction recently developed. This innovative procedure, using a cation exchange resin (Dowex), allows separating precisely different fractions of EPS, especially pure bound EPS. Moreover, it avoids the contamination of EPS fractions by residual and intracellular polymers, enabling to properly estimate polymeric contents in each fraction. The results were partly similar to conventional results described in the literature and the amount of colloidal carbohydrates (146 μg/g of dry sediment) extracted by the Dowex method fitted well with different EPS estimation in European mudflats. Colloidal carbohydrates were essentially composed of glucose (>50%), a carbon source rapidly consumed by the various communities in the sediment. Pure bound carbohydrates were composed of specific carbohydrates (28% rhamnose, 22% xylose). Residual fractions, considered as containing some refractory bound EPS and mostly other internal polymeric substances, presented a more varied composition rich in carbohydrates: galacturonic acid (20%), mannose (19.5%), glucose (19%), arabinose (15%), xylose (8%), galactose (7%).     

Adaptations of tropical marine microphytobenthic assemblages along a gradient of light and nutrient availability in Suva Lagoon,Fiji

How are microphytobenthic biofilms adapted to the high incident irradiances and temperatures, low inorganic nutrient concentrations and high desiccation stresses on intertidal flats present in tropical environments? This study investigated biofilms subject to different environmental conditions in a range of tropical sites in Suva lagoon, Fiji. PAM fluorescence was used to measure photophysiological responses to the light climate. Biofilm colloidal carbohydrate, extracellular polymeric substances (EPS) and low molecular weight (MW) carbohydrate concentrations and diel carbohydrate production patterns were measured. Average biomass (Chl a) ranged from 15 to 36?mg?m^?2, and was highest in seagrass bed sediments, but biomass was not correlated with water column or sediment porewater nutrient concentrations. Biofilm photophysiology differed significantly along a combined gradient of light and nutrient availability, with F _v/F _m, relative ETR_max and E _k of biofilms highest in mangrove and intertidal main island sites and lowest in subtidal coral reef flats. Subtidal biofilms showed photoinhibition at irradiances > 1000?µmol?m^?2. Significant correlations between Chl a and colloidal carbohydrate concentrations were present (except on intertidal sandflats), and tropical biofilms had higher ratios of colloidal carbohydrate and EPS to Chl a than temperate estuarine biofilms, probably due to a combination of high irradiance and low nutrient availability leading to the production of excess photoassimilates. The percentage of EPS present in the colloidal fraction was highest in coral sand biofilms (42%), which had the lowest nutrient concentrations, compared with other sites (25–32%). Intertidal biofilms predominantly consisted of large motile taxa and showed strong rhythms of vertical migration. During tidal emersion, high sediment temperatures (41?°C), irradiance (>2300?µmol?m^?2?s^?1) and salinity (49‰) stimulated downward migration. In silty sediments, migration resulted in a reduction in photosynthetic activity during the midday period but, in sands with high light penetration (to a depth of > 1700?µm), high production rates of EPS (18.2?µg carbo. µg Chl a^?1 h^?1) and low MW carbohydrate exudates (40.2?µg carbo. µg Chl a^?1 h^?1) occurred. Vertical migration, high E _k and high rates of photoassimilate dumping are all adaptations to living in the tropical intertidal zone. Seagrass and reef flat biofilms consisted of a diverse non-migratory flora of motile and non-motile taxa that were not subject to such extreme temperature and irradiance conditions. Low values of photosynthetic parameters and high colloidal and EPS content indicated that these biofilms were nutrient-limited.     

Impact of microphytobenthos and macroinfauna on temporal variation of benthic metabolism in shallow coastal sediments

The impact of microphytobenthos and different abundances of macrofauna (Nereis diversicolor) on temporal variation of benthic metabolism was investigated in laboratory microcosms. Measurements primarily included diurnal fluxes of O₂ and CO₂ as well as sediment profiles of Chlorophyll a and extracellular polymeric substances (EPS). Net and gross primary production (2-5 and 4-7 mmol CO₂ m^{− 2} h^{− 1}, respectively) were relatively stable in both defaunated and faunated sediment throughout a 12 h light period. The CO₂ release from sediments immediately after onset of darkness ranged from 1.5 to 3.5 mmol CO₂ m^{− 2} h^{− 1} followed by a consistent decrease during the next 12 h in the dark. The decrease was more conspicuous in faunated (about 50%) than defaunated (9%) sediment. Total carbon oxidation was in both cases fuelled primarily by microphytobenthic biomass, while EPS only contributed by 1-4%. Diurnal measurements of Nereis diversicolor ventilation activity showed a significant decrease in the dark that corresponds well to the observed decrease in total metabolic activity. It is concluded that changes in solute exchange associated with animals and burrows (e.g. microbial respiration) is a major controlling factor for total sediment metabolism. In general, the faunal impact was evident as about 50% enhanced CO₂ release in the dark, while net primary production was reduced by 30-50%. The turnover time of produced organic carbon is therefore considerably shorter in the presence than absence of macrofauna. Thus, the daily average exchange of CO₂ was almost balanced in bioturbated sediment with a 43% share of carbon oxidation mediated by direct faunal respiration. Defaunated sediment was net autotrophic with daily primary production exceeding microbial carbon oxidation by 40%. The present study clearly demonstrates that knowledge on interactions between microphytobenthos and macrofauna is essential for understanding carbon dynamics in shallow sediments.     

Microphytobenthos as a biogeomorphological force in intertidal sediment stabilization

The properties and behavior of intertidal marine sediments cannot be understood without taking their biology into account. Biological factors are important for the stability and erosion threshold of intertidal sediments as well as for sediment transport. In this paper I focus on intertidal sediments that are colonized and dominated by phototrophic microorganisms and their impact on the morphodynamics and sediment stabilization. The emphasis is on epipelic diatoms. These organisms exude copious amounts of extracellular polymeric substances (EPS) that may contribute to the stability of the sediment by gluing and binding. I review the factors that lead to the development of such microphytobenthic communities and the processes that lead to the exudation of EPS and its fate in intertidal mudflats. Epipelic diatoms exude EPS partly as the result of unbalanced growth. Extraction of EPS from cultures of epipelic diatoms yields two operational fractions. While one fraction contains largely neutral EPS, which may serve as a carbon- and energy reserve for the organism, the other is acidic and more recalcitrant to degradation. The latter EPS fraction is therefore predominant in the muddy sediment and may be responsible for increasing the erosion threshold. However, since extracted EPS alone is incapable of increasing the erosion threshold, diatoms are apparently actively involved in the structuring of the biofilm matrix. Therefore, sediment stabilization cannot be attributed simply to EPS alone.     

Caging experiment: Relationship between mesh size and artifacts

In marine sediments, cages are widely used to detect effects of predation on infauna. Artifacts due to the use of cages have been, instead, poorly estimated. In this study we have analysed the changes in macrofauna and in sediment composition when cages of different mesh sizes are used to exclude predators. In a shallow micro-tidal flat, the effect of three mesh sizes (2, 6 and 18 mm) was tested on sediment and macrofauna, using cages that excluded predators (EC), partial cages (PC) and natural sediment (un-caged plots, UC). PC allowed the predators to feed on the sediment, but modified the sediment similarly to the complete cages. Changes were measured for the abundance and species composition of macrofauna and for sediment total organic matter (TOM), chlorophyll-a, proteins and water content.The use of cages did not alter TOM and water content, whereas chlorophyll-a and phaeopigments were more abundant under the cages. The deep-burrowing shrimp Upogebia pusilla also increased under cages of any mesh-size. There was, instead, an effect on the abundance of macrofauna only when the smallest mesh size was used. Surprisingly, the exclusion of predators decreased the abundances of Tanais dulongii, Oligochaetes, Brania oculata, whereas the abundance of Aphelochaeta marionii changed when both PC and EC were put in the sediment. Collectively, all these findings indicated that for surface animals the use of small mesh sizes can bias the interpretation of results.     

美国黄松、班克松和油松的抗寒性比较

通过人工冰冻和电导率的测定,对黄土丘陵沟壑区引种载培的美国黄松、班克松和乡土树种油松的抗寒性进行了鉴定,并探讨其抗寒机理.结果表明,班克松的抗寒性比油松强,而美国黄松的抗寒性比油松稍弱.班克松的束缚水/自由水比值高达7.0,组织中ABA含量高达164.3 μg·g^-1 FW;但可溶性糖和K⁺含量较低,分别为12.0%和2 450 μg·g^-1 DW.油松则是可溶性糖、K⁺和ABA含量都较高,分别为18.68%、4 538 μg·g^-1 DW和95.8 μg·g^-1 FW;束缚水/自由水比值较低,为2.58.美国黄松的可溶性糖含量较高,18.05％;但束缚水/自由水比值、K⁺和ABA含量都较低,分别为2.18、2 275 μg·g^-1 DW和63.3 μg·g^-1 FW,可能是其抗寒性较弱的内在原因.班克松较低的叶绿素含量和较高的类胡萝卜素/叶绿素比值对其抗寒性也有贡献.说明3种树种虽然都是抗寒树种,但其内在机理仍有差异.     

Comparison of the Levels of Bacterial Diversity in Freshwater,Intertidal Wetland,and Marine Sediments by Using Millions of Illumina Tags

Sediment, a special realm in aquatic environments, has high microbial diversity. While there are numerous reports about the microbial community in marine sediment, freshwater and intertidal sediment communities have been overlooked. The present study determined millions of Illumina reads for a comparison of bacterial communities in freshwater, intertidal wetland, and marine sediments along Pearl River, China, using a technically consistent approach. Our results show that both taxon richness and evenness were the highest in freshwater sediment, medium in intertidal sediment, and lowest in marine sediment. The high number of sequences allowed the determination of a wide variety of bacterial lineages in all sediments for reliable statistical analyses. Principal component analysis showed that the three types of communities could be well separated from phylum to operational taxonomy unit (OTU) levels, and the OTUs from abundant to rare showed satisfactory resolutions. Statistical analysis (LEfSe) demonstrated that the freshwater sediment was enriched with Acidobacteria, Nitrospira, Verrucomicrobia, Alphaproteobacteria, and Betaproteobacteria. The intertidal sediment had a unique community with diverse primary producers (such as Chloroflexi, Bacillariophyta, Gammaproteobacteria, and Epsilonproteobacteria) as well as saprophytic microbes (such as Actinomycetales, Bacteroidetes, and Firmicutes). The marine sediment had a higher abundance of Gammaproteobacteria and Deltaproteobacteria, which were mainly involved in sulfate reduction in anaerobic conditions. These results are helpful for a systematic understanding of bacterial communities in natural sediment environments.     

Small-scale distribution of interstitial nitrite in freshwater sediment microcosms: the role of nitrate and oxygen availability,and sediment permeability

The spatial distribution of interstitial NO2(-) concentrations was studied in NO3(-)-exposed freshwater sediment microcosms, using pore water extractions as well as ion-selective microsensors. Porewater extractions revealed ecotoxicologically critical NO2(-) concentrations in hypoxic and anoxic sediment layers in which significant NO3(-) consumption took place. In contrast, the use of ion-selective microsensors demonstrated the high capacity of the thin oxic surface layer of the sediments to consume NO2(-) and to produce NO3(-). Two modes of NO3(-) supply to the sediments were compared: In treatments with NO3(-) supply to the overlying water, a subsurface maximum of NO2(-) concentration was observed, coinciding with the site of maximum NO3(-) consumption. When NO3(-) was perfused up through the sediment cores, however, NO2(-) accumulated throughout the entire sediment column. Such spatially extensive NO2(-) accumulations were only observed in sediments poor in organic matter with a relatively high permeability. By manipulating the O2 content of the overlying water, the release of NO2(-) from the sediments could be influenced: In treatments with air-saturated overlying water, the sediments did not release detectable amounts of NO2(-) into the water phase. When kept hypoxic (25% air saturation) instead, significant NO2(-) accumulations were recorded in the overlying water. These findings suggest that in treatments with air-saturated overlying water, NO2(-) that was produced in deeper sediment layers (denitrifying conditions) was completely consumed at the oxic sediment surface (nitrifying conditions) before it could reach the overlying water.     

Effect of biostimulation on the microbial community in PCB-contaminated sediments through periodic amendment of sediment with iron

Reductive dehalogenation of polychlorinated biphenyls (PCBs) by indigenous dehalorespiring microorganisms in contaminated sediments may be enhanced via biostimulation by supplying hydrogen generated through the anaerobic corrosion of elemental iron added to the sediment. In this study, the effect of periodic amendment of sediment with various dosages of iron on the microbial community present in sediment was investigated using phospholipid fatty acid analysis (PLFA) over a period of 18 months. Three PCB-contaminated sediments (two freshwater lake sediments and one marine sediment) were used. Signature biomarker analysis of the microbial community present in all three sediments revealed the enrichment of Dehalococcoides species, the population of which was sustained for a longer period of time when the sediment microcosms were amended with the lower dosage of iron (0.01 g iron per g dry sediment) every 6 months as compared to the blank system (without iron). Lower microbial stress levels were reported for the system periodically amended with 0.01 g of iron per g dry sediment every 6 months, thus reducing the competition from other hydrogen-utilizing microorganisms like methanogens, iron reducers, and sulfate reducers. The concentration of hydrogen in the system was found to be an important factor influencing the shift in microbial communities in all sediments with time. Periodic amendment of sediment with larger dosages of iron every 3 months resulted in the early prevalence of Geobacteraceae and sulfate-reducing bacteria followed by methanogens. An average pH of 8.4 (range of 8.2–8.6) and an average hydrogen concentration of 0.75% (range of 0.3–1.2%) observed between 6 and 15 months of the study were found to be conducive to sustaining the population of Dehalococcoides species in the three sediments amended with 0.01 g iron per g dry sediment. Biostimulation of indigenous PCB dechlorinators by the periodic amendment of contaminated sediments with low dosages of iron metal may therefore be an effective technology for remediation of PCB-contaminated sediments.     

Biomass measurement of methane forming bacteria in environmental samples

Methane-forming bacteria contain unusual phytanylglycerol ether phospholipids which can be extracted from the bacteria in sediments and assayed quantitatively by high performance liquid chromatography (HPLC). In this procedure the lipids were extracted, the phospholipids recovered, hydrolyzed, purified by thin layer chromatography, derivatized and assayed by HPLC. Ether lipids were recovered quantitatively from Methanobacterium thermoautotrophicum and sediments at levels as low as 8 × 10^?14 moles. In freshwater and marine sediments the flux of methane to the atmosphere and the methane levels in the pore water reflects the recovery of the phytanyl glycerol ether lipid ‘signature’. The proportion of the ether phospholipid to the total recoverable phospholipid was highest in anaerobic digester sewage sludge and deeper subsurface freshwater sediment horizons.     

Lack of correlation between surface macrofauna,meiofauna, erosion threshold and biogeochemical properties of sediments within an intertidal mudflat and mangrove forest

This article describes the relationship between 10 selected properties of the sediments (chlorophyll a and b, colloidal and total carbohydrate, water concentration, sediment type, organic matter, erosion threshold and erosion rate) and meio- and macrofauna within and among three different habitats in an urbanized intertidal mudflat/mangrove forest in Tambourine Bay, Sydney Harbour, Australia. Many of the biogeochemical variables were significantly different among habitats, often grading from mudflat to mangrove canopy. In contrast to previous studies, patterns of distribution of macrofauna among habitats were weak. For the meiofauna, only copepods showed any significant difference among habitats, with the greatest numbers in the open mudflat habitat and least under the mangrove canopy. There was a gradient in fauna among the habitats; overall macrofauna abundances were greatest under the mangrove canopy and least on the mudflat, while meiofauna abundance was greatest in the pneumatophore habitat and least under the canopy. Correlations between fauna and properties of sediment were generally weak. When the habitats were analysed separately, some correlations were strengthened but relationships were inconsistent. Thus, while some taxa vary significantly among habitats there was not a strong relationship between biogeochemical properties and either macro- or meiofauna. This suggests that localised factors other than the measured properties of the sediments are driving patterns in fauna at these small scales, which requires further investigation to be unravelled.     

Biostabilization of cohesive sediments: revisiting the role of abiotic conditions,physiology and diversity of microbes,polymeric secretion,and biofilm architecture

In aquatic habitats, micro‐organisms successfully adhere to and mediate particles, thus changing the erosive response of fine sediments to hydrodynamic forcing by secreting glue‐like extracellular polymeric substances (EPS). Because sediment dynamics is vital for many ecological and economic aspects of watersheds and coastal regions, biostabilization of cohesive sediments is one of the important ecosystem services provided by biofilms. Although the research on biostabilization has gained momentum over the last 20 years, we still have limited insights principally due to the complex nature of this topic, the varying spatial, temporal, and community scales examined, oversimplified ecohydraulic experiments with little natural relevance, and the often partial views of the disciplines involved. This review highlights the current state of our knowledge on biostabilization and identifies important areas for future research on: (A) the influence of abiotic conditions on initial colonization and subsequent biofilm growth, focusing on hydrodynamics, substratum, salinity, nutrition, and light climate; (B) the response of microbes in terms of physiological activity and species diversity to environmental settings as well as biotic conditions such as competition and grazing; and (C) the effects of the former on the EPS matrix, its main constituents, their composition, functional groups/substitutes, and structures/linkages. The review focuses specifically on how the numerous mutual feedback mechanisms between abiotic and biotic conditions influence microbial stabilization capacity, and thus cohesive sediment dynamics.     

Assessment of persistent organochlorine compounds contamination on the Lake Victoria water and sediments: a case study in Tanzania

The current study was conducted to establish the levels of organochlorine pesticides (OCPs), its degradation products, and indicatory polychlorinated biphenyls (PCBs) in water and sediments from Lake Victoria in Tanzania. Seven indicator PCBs were identified in sediments, and five were identified in water. The PCBs loading ranged from lower than the limit of detection (LOD) to 10.28 µg kg^?1 DW and 0.95 to 2.24 µg l^?1 in sediments and water, respectively. PCBs, CB 138 and CB 153 dominated the load, because of their chemical stability and high degree of chlorination. In addition, ten OCPs were identified in sediments, and seven OCPs were identified in water. The levels of organochlorine compounds in sediments were higher than in water samples, implying that sediments may serve as a sink for these compounds. The presence of these compounds is probably as a result of either historical use, long-range environmental transport or environmental persistence. The levels of organochlorine compounds in the lake water and sediments are below the maximum residue limit set by EU and FAO, except for aldrin and dieldrin. Based on the Threshold Effect Concentration (TEC) for freshwater ecosystems, aldrin and dieldrin are the only OCPs that seem to be a threat to the lake environment.