Link between microbial composition and carbon substrate-uptake preferences in a PHA-storing community

Abstract

The microbial community of a fermented molasses-fed sequencing batch reactor (SBR) operated under feast and famine conditions for production of polyhydroxyalkanoates (PHAs) was identified and quantified through a 16 S rRNA gene clone library and fluorescence in situ hybridization (FISH). The microbial enrichment was found to be composed of PHA-storing populations (84% of the microbial community), comprising members of the genera Azoarcus, Thauera and Paracoccus. The dominant PHA-storing populations ensured the high functional stability of the system (characterized by high PHA-storage efficiency, up to 60% PHA content). The fermented molasses contained primarily acetate, propionate, butyrate and valerate. The substrate preferences were determined by microautoradiography-FISH and differences in the substrate-uptake capabilities for the various probe-defined populations were found. The results showed that in the presence of multiple substrates, microbial populations specialized in different substrates were selected, thereby co-existing in the SBR by adapting to different niches. Azoarcus and Thauera, primarily consumed acetate and butyrate, respectively. Paracoccus consumed a broader range of substrates and had a higher cell-specific substrate uptake. The relative species composition and their substrate specialization were reflected in the substrate removal rates of different volatile fatty acids in the SBR reactor.

Figure 1

SBR cycle of operation: (a) VFA and ammonia uptake, PHA storage and active biomass (X) growth; (b) ammonia uptake as estimate of cell growth, estimated VSS and active biomass concentrations and experimental VSS values; (c) VFA-uptake profile during the feast phase. *Marks the point of inflexion of the acetate-uptake curve.

Figure 2

PHA batch accumulation study using the SBR-selected culture fed with fermented molasses using a pulse-wise feeding strategy. *Marks the point of inflexion in the acetate-uptake curve.

Figure 3

Microbial community characterization by FISH. Specific probes for Azoarcus (a), Thauera (b) and Paracoccus (c) are in magenta and other bacteria in blue. Bar=20 μm. The colour reproduction of this figure is available at The ISME Journal online.

Figure 4

MAR-FISH images: (a) Azoarcus and labeled propionate and (b) Paracoccus and labeled butyrate.