Attached and Unattached Microbial Communities in a Simulated Basalt Aquifer under Fracture- and Porous-Flow Conditions

Abstract

Bench scale column studies were used to examine the partitioning of microorganisms between groundwater and a geologic medium and to examine the effect of hydrogeology (i.e., porous-versus fracture-flow) on organism partitioning. Replicated columns were constructed with intact basalt core segments that contained natural fractures and with the same basalt crushed into particles. The columns were perfused with groundwater, and upon reaching a steady state, the columns were sacrificed and the attached and unattached communities were analyzed by multiple approaches. The analyses included the total number of cells, the phylogenetic affiliation of the cells (i.e., the ␣, ␤, and ␥ subclasses of the class Proteobacteria and gram positives with high G؉C DNA content) by fluorescent in situ hybridization (FISH), number and taxonomic affiliation by fatty acid methyl ester profiles of culturable heterotrophs, most-probable-number estimates of methanotrophs and phenol oxidizers, and whole-community sole carbon source utilization patterns from Biolog GN microplates. In the packed columns, about 99% of the total biomass (per cubic centimeter of porous medium) was attached to the geologic medium. Lack of equitable units precluded a comparison of attached and unattached biomasses in the fractured columns where the attached biomass was expressed per unit of surface area. Compositional differences in the attached and unattached communities were evidenced by (i) the recovery of Pseudomonas stutzeri, an Enterococcus sp., and Bacillus psychrophilus from the groundwater and not from the basalt, (ii) differences between community carbon source utilization patterns, and (iii) the relative abundances of different phylogenetic groups estimated by FISH in both column types. In the packed columns, attached communities were depleted of members of the ␣-and ␤-Proteobacteria subclasses in comparison to those in the corresponding groundwater. In the fractured columns, attached communities were enriched in gram-positive Bacteria and ␥-Proteobacteria and depleted of ␤-Proteobacteria, in comparison to those in the corresponding groundwater. Segregation of populations and their activities, possibly modified by attachment to geologic media, may influence contaminant fate and transport in the subsurface and impact other in situ applications.Although the presence of microbes in both geologic media and groundwaters is undisputed (44), little work has described the partitioning of populations and activities between communities that colonize geologic media and those that exist planktonically in groundwaters. Clearly, the relative mobility of the microorganisms (i.e., attached or unattached) would have a great influence on processes related to subsurface contaminant fate and transport (29, 57), the stability of geologic radioactive waste repositories (45), microbially enhanced oil recovery, and solution mining. For example, consider the strategies required to deliver a substrate or nutrient to an attached organism versus a planktonic organism that might be ...

Subsurface clade of Geobacteraceae that predominates in a diversity of Fe(III)-reducing subsurface environments

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Utilization of Microbial Biofilms as Monitors of Bioremediation

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The Role of Attached and Free-Living Bacteria in Biodegradation in Karst Aquifers

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