Author:

Abstract:

Most environmental habitats are colonized by complex microbial communities where micro-organisms interact both in space and in time. Microbial interactions can be both antagonistic and cooperative. Often cooperative interspecies interactions are synergistic in nature, i.e., one or more bacterial strains are functioning together, producing a result unobtainable individually. Such synergistic interactions are often metabolic which include cross-feeding of energy resources or co-factors and occur for instance in the degradation of complex xenobiotic compounds. An example of cooperative interactions in xenobiotic degradation occurs in a three-species bacterial consortium that consists of Variovorax sp. strain WDL1, Comamonas testosteroni strain WDL7 and Hyphomicrobium sulfonivorans strain WDL6 which together synergistically mineralize the phenylurea herbicide linuron. Tt was investigated whether this synergism during linuron degradation extends towards the metabolism of regular carbon sources such as carbohydrates, carboxylic acids, amino acids, … . For that purpose, the metabolic performance and metabolic range of the individual consortium members were first compared with those of paired and three-species combinations of them in Biolog GN2 MicroPlate assays. The combinations showed an increase in metabolic performance for most of the tested carbon sources and the additional respiration of carbon sources for which respiration was unobtainable by the individual strains. When one of the three strains was replaced by bacterial strains “foreign” to the consortium either belonging to the same genus or other genera, mainly antagonistic effects occurred. These findings made us investigate whether degradation of complex molecules in natural dissolved organic matter may involve synergism within microbial communities. This hypothesis was tested by determining bacterial growth and the biodegradable dissolved organic carbon for members of that consortium, either in isolation or as a triple-species combination. For different natural dissolved organic matter samples, the biodegradable fraction decreased with increasing aromaticity which is considered to be an important indicator for recalcitrance. In case of a low aromaticity, the biodegradable fraction was 40 to 50% for all inocula. However, for a high aromaticity, this fraction was >23% for the consortium members in combination compared to a biodegradable fraction of <16% when in isolation. The observed increase in this fraction according to aromaticity as well as the increase in growth, indicated that for both samples with low and high aromaticity, a synergism existed within the consortium. Since growth of all consortium members in combination increased compared to isolation, it appeared all strains benefited from this cooperation, although a shift in community composition was observed according to aromaticity. The GN2 MicroPlate assay suggests a specialization of the consortium in synergistically degrading carbon sources different from linuron. This feature can contribute in consolidating the consortium composition when linuron is absent or present at poor concentrations. While the carbon removal and associated assimilation from natural dissolved organic matter indicated that consortia benefit more than individual pesticide degrading bacteria of dissolved carbon resources available in an environment, due to metabolic cooperation which can be a crucial factor when it concerns growth and survival of pollutant degrading bacteria in the environment.