Abstract
Various types of cyanobacterial mats were predominant in a wetland, constructed for the remediation of oil-polluted residual waters from an oil field in the desert of the south-eastern Arabian Peninsula, although such mats were rarely found in other wetland systems. There is scarce information on the bacterial diversity, spatial distribution and oil-biodegradation capabilities of freshwater wetland oil-polluted mats. Microbial community analysis by Automated Ribosomal Spacer Analysis (ARISA) showed that the different mats hosted distinct microbial communities. Average numbers of operational taxonomic units (OTUsARISA) were relatively lower in the mats with higher oil levels and the number of shared OTUsARISA between the mats was <60% in most cases. Multivariate analyses of fingerprinting profiles indicated that the bacterial communities in the wetland mats were influenced by oil and ammonia levels, but to a lesser extent by plant density. In addition to oil and ammonia, redundancy analysis (RDA) showed also a significant contribution of temperature, dissolved oxygen and sulfate concentration to the variations of the mats’ microbial communities. Pyrosequencing yielded 282,706 reads with >90% of the sequences affiliated to Proteobacteria (41% of total sequences), Cyanobacteria (31%), Bacteriodetes (11.5%), Planctomycetes (7%) and Chloroflexi (3%). Known autotrophic (e.g. Rivularia) and heterotrophic (e.g. Azospira) nitrogen-fixing bacteria as well as purple sulfur and non-sulfur bacteria were frequently encountered in all mats. On the other hand, sequences of known sulfate-reducing bacteria (SRBs) were rarely found, indicating that SRBs in the wetland mats probably belong to yet-undescribed novel species. The wetland mats were able to degrade 53–100% of C12–C30 alkanes after 6 weeks of incubation under aerobic conditions. We conclude that oil and ammonia concentrations are the major key players in determining the spatial distribution of the wetland mats’ microbial communities and that these mats contribute directly to the removal of hydrocarbons from oil field wastewaters.
Highlights
Wastewater reuse is an essential strategy for the conservation of water resources, in developing countries, which suffer from water scarcity throughout the year [1]
Cyanobacterial mat samples were collected from Tracks A, B and C (Fig. 1) in May 2013, in order to span a range of oil exposure levels and plant densities
The constructed wetland cyanobacterial mats harbored a high diversity of microorganisms, as indicated by the OTUs0.03 richness and Chao index estimates, and microbial biogeography in these mats were influenced by both edaphic and biotic parameters
Summary
Wastewater reuse is an essential strategy for the conservation of water resources, in developing countries, which suffer from water scarcity throughout the year (expected to reach 35 countries by 2025) [1]. Constructed wetlands treatment systems are engineered and designed to exploit natural processes, including those of microbial communities associated with wetland plants and soils, for the cleanup of wastewaters [2,3,4]. These systems are low-cost, selfsustaining, operated and have been successfully employed worldwide, over the last six decades, to remove a diverse array of contaminants from municipal, agricultural and industrial wastewaters [2, 5, 6]. In recent years, constructed wetlands have been used for the treatment of residual waters produced from oil fields in order to remove hydrocarbons and to enhance water quality for irrigation purposes and/or safe discharge in aquatic systems [7,8,9,10]. Microorganisms play a central role in the degradation of pollutants and the biogeochemical transformation of nutrients in wetlands, little is known about their diversity and distribution in comparison to soils and aquatic ecosystems, especially with regard to hydrocarbon degradation
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