No abstract

Aims: To investigate the relationships between the operation of the volatile organic compound (VOC) removal biofilter and the structure of microbial communities, and to study the impact on degradation activities and the structuring of microbial communities of biofilter malfunctions related to the qualitative composition of the polluted air. Methods and Results: A microbiological study and a measurement of biodegradation activities were simultaneously carried out on two identical peat-packed columns, seeded with two different inocula, treating polluted air containing 11 VOCs. For both reactors, the spatial structure of the microbial communities was investigated by means of single-strand conformation polymorphism (SSCP) analysis. For both reactors, stratification of degradation activities in function of depth was observed. Oxygenated compounds were removed at the top of the column and aromatics at the bottom. Comparison of SSCP patterns clearly showed a shift in community structure in function of depth inside both biofilters. This distribution of biodegradation activities correlates with the spatialization of microbial density and diversity. Although the operating conditions of both reactors were identical and the biodegradation activities similar, the composition of microflora differed for biofilters A and B. Subdivision of biofilter B into two independent parts supplied with polluted air containing the complex VOC mixture showed that the microflora having colonized the bottom of biofilter B retained their potential for degrading oxygenated compounds. Conclusions: This work highlights the spatialization of biodegradation functions in a biofilter treating a complex mixture of VOCs. This distribution of biodegradation activities correlates with the spatialization of microbial density and diversity. Significance and Impact of the Study: This vertical structure of microbial communities must be taken into consideration when dealing with the malfunctioning of bioreactors. These results are also useful information about changes in microbial communities following natural or anthropogenic alterations in different ecosystems (soils and sediments) where structuring of microbial communities according to depth has been observed.

Biofilters are packed-bed bioreactors where contaminants, once transferred from the gas phase to the biofilm, are oxidized by diverse and complex communities of attached microorganisms. Over the last decade, more and more studies aimed at opening the back box of biofiltration by unraveling the biodiversity-ecosystem function relationship. In this review, we report the insights provided by the microbial ecology approach in biofilters and we emphasize the parallels existing with other engineered ecosystems used for wastewater treatment, as they all constitute relevant model ecosystems to explore ecological issues. We considered three characteristic ecological indicators: the density, the diversity, and the structure of the microbial community. Special attention was paid to the temporal and spatial dynamics of each indicator, insofar as it can disclose the potential relationship, or absence of relation, with any operating or functional parameter. We also focused on the impact of disturbance regime on the microbial community structure, in terms of resistance, resilience, and memory. This literature review led to mitigated conclusions in terms of biodiversity-ecosystem function relationship. Depending on the environmental system itself and the way it is investigated, the spatial and temporal dynamics of the microbial community can be either correlated (e.g., spatial stratification) or uncoupled (e.g., temporal instability) to the ecosystem function. This lack of generality shows the limits of current 16S approach in complex ecosystems, where a functional approach may be more suitable.

The use of hemp fibres as reinforcements in polymer composites requires a thorough understanding of the hemp fibres transformation processes to obtain a constant quality. In this context, the upstream processing termed field retting is considered. Retting allows a subsequent fibre separation from the plant stems by degradation of cementing compounds by microorganisms. This operation depends on weather conditions and is currently empirically carried out in fields, so that a large variability in the hemp fibres quality (color, morphology, biochemical composition, thermal properties and mechanical properties) is resulting. Therefore, the present study aims to investigate the influence of different retting durations (up to 9 weeks) on hemp fibres properties when harvested at the beginning of flowering growth stage to survey their temporal dynamic. Various assessments were applied on fibres: color observations, morphological (optical microscope), surface (ESEM) and biochemical (gravimetry) analyses, spectrocolorimetric measurements (pectins content), thermogravimetric (TGA) analysis, and mechanical in tensile mode testings. The results reveal that increasing the field retting duration leads to a change of color characteristics from light green to grey due to the development of microbial communities (most probably fungal and bacteria) at the stem surface. A separation of the fibres bundle to elementary fibres occurs with the degradation of pectins during retting. An increase of thermal stability of the fibres is also observed. Both increase of cellulose fraction and crystallinity induce an enhancement in tensile properties.