Liquid fuels have excellent properties in terms of storage, logistics and energy density compared to gaseous fuels or electricity. A major disadvantage of liquid fuels is that a vast majority of them is derived from fossil resources. Currently, the consumption rate of fossil fuels by far outcompetes the natural production rate, resulting in elevated atmospheric CO 2 concentrations. Photosynthetic organisms (plants and algae) xate atmospheric CO 2 using solar energy. CO 2 consumption and emission would be balanced if liquid fuels would be derived from plants or algae. However, growing terrestrial plants for biofuel production means less agricultural land and fresh water remains available for food production. Microalgae can grow under marine conditions and outcompete terrestrial plants in terms of areal productivity. On the other hand, cultivation of microalgae introduces new challenges. Species control is, compared to terrestrial plants, much more difficult. Any cultivation system is prone to contamination by undesired algal species threatening stable production. In this study we show that we can overcome this hurdle by creating a selective environment. Our approach allows for large scale, stable production of biofuel precursors and is therefore a substantial step forward in the production of renewable fuels.

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Exposing a microbial community to alternating absence and presence of carbon substrate in aerobic conditions is an effective strategy for enrichment of storage polymers (polyhydroxybutyrate, PHB) producing microorganisms. In this work we investigate to which extent intermediate storage polymer production is a temperature independent microbial competition determining factor. Eight parallel bioreactors were operated in the temperature range of 20-40°C, but intermediate storage polymer production was only obtained at 25-35°C. Besides PHB production and consumption, cell decay and subsequent cryptic growth on lysis products was found to determine process properties and the microbial community structure at all operational temperatures. At 40°C decay processes cannot be overcome with additional energy from storage polymers, and fast-growing microorganisms dominate the system. At 20°C, highly competitive communities with ambiguous storage properties were enriched. The results described here demonstrate that a rigorous experimental approach could aid in the understanding of competitive strategies in microbial communities.

Anaerobic microbial communities can produce carboxylic acids of medium chain length (e.g., caproate, caprylate) by elongating short chain fatty acids through reversed β-oxidation. Ethanol is a common electron donor for this process. The influence of environmental conditions on the stoichiometry and kinetics of ethanol-based chain elongation remains elusive. Here, a sequencing batch bioreactor setup with high-resolution off-gas measurements was used to identify the physiological characteristics of chain elongating microbial communities enriched on acetate and ethanol at pH 7.0 ± 0.2 and 5.5 ± 0.2. Operation at both pH-values led to the development of communities that were highly enriched (>50%, based on 16S rRNA gene amplicon sequencing) in Clostridium kluyveri related species. At both pH-values, stably performing cultures were characterized by incomplete substrate conversion and decreasing biomass-specific hydrogen production rates during an operational cycle. The process stoichiometries obtained at both pH-values were different: at pH 7.0, 71 ± 6% of the consumed electrons were converted to caproate, compared to only 30 ± 5% at pH 5.5. Operating at pH 5.5 led to a decrease in the biomass yield, but a significant increase in the biomass-specific substrate uptake rate, suggesting that the organisms employ catabolic overcapacity to deal with energy losses associated to product inhibition. These results highlight that chain elongating conversions rely on a delicate balance between substrate uptake- and product inhibition kinetics.

Despite its ecological importance, essential aspects of microbial N2O reduction—such as the effect of O2 availability on the N2O sink capacity of a community—remain unclear. We studied N2O vs. aerobic respiration in a chemostat culture to explore (i) the extent to which simultaneous respiration of N2O and O2 can occur, (ii) the mechanism governing the competition for N2O and O2, and (iii) how the N2O-reducing capacity of a community is affected by dynamic oxic/anoxic shifts such as those that may occur during nitrogen removal in wastewater treatment systems. Despite its prolonged growth and enrichment with N2O as the sole electron acceptor, the culture readily switched to aerobic respiration upon exposure to O2. When supplied simultaneously, N2O reduction to N2 was only detected when the O2 concentration was limiting the respiration rate. The biomass yields per electron accepted during growth on N2O are in agreement with our current knowledge of electron transport chain biochemistry in model denitrifiers like Paracoccus denitrificans. The culture’s affinity constant (KS) for O2 was found to be two orders of magnitude lower than the value for N2O, explaining the preferential use of O2 over N2O under most environmentally relevant conditions.Electronic supplementary materialThe online version of this article (10.1007/s00253-018-9247-3) contains supplementary material, which is available to authorized users.