Axenic cultures of Nitrosomonas europaea and Nitrobacter winogradskyi in autotrophic conditions: a new protocol for kinetic studies.

As a part of a natural biological N-cycle, nitrification is one of the steps included in the conception of artificial ecosystems designed for extraterrestrial life support systems (LSS) such as Micro-Ecological Life Support System Alternative (MELiSSA) project, which is the LSS project of the European Space Agency. Nitrification in aerobic environments is carried out by two groups of bacteria in a two-step process. The ammonia-oxidizing bacteria (Nitrosomonas europaea) realize the oxidation of ammonia to nitrite, and the nitrite-oxidizing bacteria (Nitrobacter winogradskyi), the oxidation of nitrite to nitrate. In both cases, the bacteria achieve these oxidations to obtain an energy and reductant source for their growth and maintenance. Furthermore, both groups also use CO2 predominantly as their carbon source. They are typically found together in ecosystems, and consequently, nitrite accumulation is rare. Due to the necessity of modeling accurately conversion yields and transformation rates to achieve a complete modeling of MELiSSA, the present study focuses on the experimental determination of nitrogen to biomass conversion yields. Kinetic and mass balance studies for axenic cultures of Nitrosomonas europaea and Nitrobacter winogradskyi in autotrophic conditions are performed. The follow-up of these cultures is done using flow cytometry for assessing biomass concentrations and ionic chromatography for ammonium, nitrite, and nitrate concentrations. A linear correlation is observed between cell count and optical density (OD) measurement (within a 10 % accuracy) validating OD measurements for an on-line estimation of biomass quantity even at very low biomass concentrations. The conversion between cell count and biomass concentration has been determined: 7.1 × 10¹² cells g dry matter (DM)⁻¹ for Nitrobacter and 6.3 × 10¹² cells g DM⁻¹ for Nitrosomonas. Nitrogen substrates and products are assessed redundantly showing excellent agreement for mass balance purposes and conversion yields determination. Although the dominant phenomena are the oxidation of NH4⁺ into nitrite (0.95 mol mol N⁻¹ for Nitrosomonas europaea within an accuracy of 3 %) and nitrite into nitrate (0.975 mol mol N⁻¹ for Nitrobacter winogradskyi within an accuracy of 2 %), the Nitrosomonas europaea conversion yield is estimated to be 0.42 g DM mol N⁻¹, and Nitrobacter winogradskyi conversion yield is estimated to be 0.27 g DM mol N⁻¹. The growth rates of both strains appear to be dominated by the oxygen transfer into the experimental setups.

Analysis of a continuous culture of Fibrobacter succinogenes S85 on a standardized glucose medium.

Continuous cultures of Fibrobacter succinogenes S85 were performed on a standardized fully synthetic culture medium with glucose as carbon source at a dilution rate (D = 0.02 h(-1)) in a 5-L bioreactor. The culture was stabilized during 20 days and demonstrated the ability of Fibrobacter succinogenes to grow in this synthetic medium. CO(2) partial pressure and redox potential probes were used to check the anaerobic state of the culture. The biomass yield was calculated 0.206 g (g glucose)(-1) and the production yield of succinate, the major end-product, was 0.63 mol (mol glucose)(-1). The consistency of the experimental data was checked by proton and mass (C, N) balances. The results were satisfactory (90-110% recovery) leading to derive a stoichiometric equation representative of the growth on glucose. The stoichiometric coefficients were calculated using data reconciliation and linear algebra methods enabling to obtain a complete modeling of all conversion yields possible.

Growth monitoring of Fibrobacter succinogenes by pressure measurement.

In life support systems, such as the MELiSSA (Micro-Ecological Life Support Alternative) project, developed by the European Space Agency, the aim is to understand and assemble artificial ecosystems for ensuring human subsistence in space. Fibrobacter succinogenes, an anaerobic bacterium, was used for the degradation of vegetable wastes produced in higher plants chambers, but the process does not allow the monitoring of biomass concentration and degradation rates. This study proposes a growth and a degradation monitoring technique using pressure measurements. First, volatile fatty acids (VFA) production was compared with biomass growth and with CO(2) production. The experiments were carried out in batch and fed-batch processes on glucose and on vegetables. The results have shown that a link could be established between VFA production, degradation rate and gas pressure measurements. Thus, the pressure could be used both as a relevant variable for online evaluation of biomass growth and of degradation of complex vegetable wastes.

Degradation of plant wastes by anaerobic process using rumen bacteria.

An operational reactor has been designed for the fermentation of a pure culture of Fibrobacter succinogenes with the constraints of strict anaerobic condition. The process is controlled by measurements of pH, redox, temperature and CO2 pressure; it allows an efficient degradation (67%) of lignocellulosic wastes such as a mixture of wheat straw, soya bean cake and green cabbage.