Judith Bender

Uptake and transformation of metals and metalloids by microbial mats and their use in bioremediation

SummaryConstructed microbial mats, used for studies on the removal and transformation of metals and metalloids, are made by combining cyanobacteria inoculum with a sediment inoculum from a metal-contaminated site. These mats are a heterotrophic and autotrophic community dominated by cyanobacteria and held together by slimy secretions produced by various microbial groups. When contaminated water containing high concentrations of metals is passed over microbial mats immobilized on glass wool, there is rapid removal of the metals from the water. The mats are tolerant of high concentrations of toxic metals and metalloids, such as cadmium, lead, chromium, selenium and arsenic (up to 350 mg L−1). This tolerance may be due to a number of mechanisms at the molecular, cellular and community levels. Management of toxic metals by the mats is related to deposition of metal compounds outside the cell surfaces as well as chemical modification of the aqueous environment surrounding the mats. The location of metal deposition is determined by factors such as redox gradients, cell surface micro-environments and secretion of extra-cellular bioflocculents. Metal-binding flocculents (polyanionic polysaccharides) are produced in large quantities by the cyanobacterial component of the mat. Steep gradients of redox and oxygen exist from the surface through the laminated strata of microbes. These are produced by photosynthetic oxygen production at the surface and heterotrophic consumption in the deeper regions. Additionally, sulfur-reducing bacteria colonize the lower strata, removing and utilizing the reducing H2S, rather than water, for photosynthesis. Thus, depending on the chemical character of the microzone of the mat, the sequestered metals or metalloids can be oxidized, reduced and precipitated as sulfides or oxides. For example precipitates of red amorphous elemental selenium were identified in mats exposed to selenate (Se-VI) and insoluble precipitates of manganese, chromium, cadmium, cobalt, and lead were found in mats exposed to soluble salts of these metals. Constructed microbial mats offer several advantages for use in the bioremediation of metal-contaminated sites. These include low cost, durability, ability to function in both fresh and salt water, tolerance to high concentrations of metals and metalloids and the unique capacity of mats to form associations with new microbial species. Thus one or several desired microbial species might be integrated into mats in order to design the community for specific bioremediation applications.

Uptake, transformation and fixation of Se(VI) by a mixed selenium-tolerant ecosystem

The uptake, transformation and fate of Se, added as selenate to a mixed microbial ecosystem containing Se-tolerant bacteria and cyanobacteria, has been studied in simulated laboratory ponds. There was evidence of concerted activities of the microbial community in the system. Motile bacteria were responsible for transport into a surface mat and the combined actions of the microbial consortium provide chemical conditions under which the Se could be reduced to the elemental form and physically entrained in the mat. Strong qualitative indications of the formation of volatile alkylselenium compounds were also observed. The removal of Se from the water column was rapid and essentially quantitative. The advantages of such biological ecosystems for remediation of Se-contaminated aquatic ecosystems are discussed.

Identification and characterization of Rhodopseudomonas spp., a purple, non-sulfur bacterium from microbial mats

A species of facultative photo-organotrophic, purple, non-sulfur bacterium was isolated from mixed-species microbial mats, characterized and examined for metal tolerance and bioremediation potential. Contributing mats were natural consortia of microbes, dominated by cyanobacteria and containing several species of bacteria arranged in a laminar structure, stabilized within a gel matrix. Constructed microbial mats were used for bioremediation of heavy metals and organic chemical pollutants. Purple, non-sulfur bacteria are characteristically found in lower strata of intact mats, but their contributing function in mats survival and function by mediating the chemical environment has not been explored. The gram-negative rod-shaped bacterium, reported here, produced a dark red culture under phototrophic conditions, reproduced by budding and formed a lamellar intracytoplasmic membrane (ICM) system parallel to cytoplasmic membrane, which contained bacteriochlorophyll a and carotenoids. This strain was found to have multiple metal resistances and to be effective in the reductive removal of Cr(VI) and the degradation of 2,4,6-trichlorophenol. Based on the results obtained from morphology, nutrient requirements, major bacteriochlorophyll content, GC content, random amplified polymorphic DNA-polymerase chain reaction (RAPD-PCR) profile and 16S-rDNA phylogenetic analysis, this member of the microbial mats may be identified as a new strain of the genus Rhodopseudomonas.

Fish feeds from grass clippings

The objective of this research was to develop a quality fish feed that could be produced from simple plant material by way of microbial processing. Silaged grass clippings, applied to shallow ponds, produced floating mats of microbial biomass containing 26% protein and 43% carbohydrate in 7–10 days. The final silage-microbe mat (SMM) showed increases of over 300% in biomass and 100% in protein content over the silage feedstock. Productivity and microbial succession patterns were similar in laboratory ponds and tropical field ponds located in the Dominican Republic. In laboratory experiments SMM-fed tilapia, Oreochromis niloticus, showed significant growth increases over controls (fed commercial catfish fish feed), if they were allowed to graze the SMM together with the gelatinous detrital deposited during SMM culture. SMM produced little fish growth if it was harvested and fed separately in clean-bottom ponds without detrital buildup. These data show that the microbial mat produced from processed grass clippings can produce a nutritious natural fish food in the pond. The detrital trophic level in the sediment (stimulated by SMM culture), is a critical component in this diet.

Deposit of zinc and manganese in an aqueous environment mediated by microbial mats

Microbial mats have been developed to sequester heavy metals from contaminated water. Mixed populations of photosynthetic and heterotrophic bacteria, dominated by Scillatoria spp., were developed for metal tolerance and integrated into a durable, self-sustaining community of microbes stimulated by and attached to ensiled grass. The mat was immobilized on glass wool and layered in flow-through baffled tanks. After allowing 8 weeks for the maturation of the mat, mixed solutions of Zn and Mn (15–16 mg L−1) were passed through a three-tank experimental series. Effluent from each tank was first sampled and then applied to the next tank. This procedure was repeated in triplicate and with six applications of new metal solution per three-tank series. By the third tank, the target metal concentration <1 mg L−1 was always achieved. Mean percentages of the initial influent concentration removed by tanks 1, 2 and 3, respectively, were 72, 93 and 98 for Zn and 78, 97 and 99 for Mn. Mean metal concentrations in the effluents (average of 6 applications) were, for tank 1: Zn (mg L−1) 5.0, Mn (mg L−1) 4.2; for tank 2: Zn 1.6, Mn 0.75; for tank 3: Zn 0.53, Mn 0.19. Mean effluent concentrations from each of the three sequential treatments (average of 6 applications per tank) were for Zn (mg L−1) 5.0, 1.6 and 0.53; for Mn (mg L−1) were 4.2, 0.75 and 0.19. Thus target concentrations were reached in experimental tank 2 for Mn and tank 3 for Zn. Metal removal in the control tank series, containing glass wool only, was 37% for Zn and 5% for Mn (average of 6 applications). Oxygen and redox potential analyses of the mat/glass wool matrix revealed a heterogenous structure of anoxic and oxic zones. Zeta potential analysis of the mat samples identified a mat surface charge ranging from −12.3 to −69.2 mV. Various metal removal mechanisms possibly involved with metal sequestering include surface binding to the mat or to mat exudates trapped within the glass wool, precipitation of the metals with anions present in the oxic/anoxic zones, mat mediation of the water conditions in favor of metal-oxide precipitation and active transport of the metals into the cell.

Effects of genetically different strains of mice on the microsomal activation of 2-aminofluorene

Five strains of mice were examined for microsomal cytochrome P450 (Cyt P450) activity and ability of the microsomal preparation to activate 2-aminofluorene (2-AF) to its mutagenic form. Each strain was assayed under normal and induced conditions. Group A strains known to be easily induced to carcinogenesis by chemicals (C57BL/6J and BALB/cJ) showed higher levels of Cyt P450 and increased mutagenesis activity of the metabolized 2-AF than Group B strains which are known to be high spontaneous tumor producers (CE/J, A/HeJ, C3H/HeJ). Induction of the hepatic microsomes with Aroclor 1254 increased the difference between the two groups.

Erratum to “Identification and characterization of Rhodopseudomonas spp., a purple, non-sulfur bacterium from microbial mats”: [Biomolec. Eng. 18 (2001) 49–56]

Erratum to ‘‘Identification and characterization of Rhodopseudomonas spp., a purple, non-sulfur bacterium from microbial mats’’ [Biomolec. Eng. 18 (2001) 49–56] Sharifeh Mehrabi *, Udoudo M. Ekanemesang , Felix O. Aikhionbare , K. Sean Kimbro , Judith Bender a a Department of Biological Sciences, Clark Atlanta Uni ersity, Atlanta, GA 30314, USA b Department of Microbiology/Biochemistry/Immunology, Morehouse School of Medicine, Atlanta, GA 30310, USA