Novel continuous production of single-cell proteins from purple non-sulfur bacteria using gaseous volatile fatty acids by closed-loop membrane contactor system

Abstract

This study explores the continuous cultivation of purple non-sulfur bacteria (PNSB) in an anaerobic photobioreactor (AnPBR) to produce single-cell protein (SCP) and other valuable co-products, such as pigments and polyhydroxyalkanoates (PHAs), using a gaseous stream of volatile fatty acids (VFA) transferred from a synthetic mixture (70 % acetic, 10 % propionic, 20 % butyric acid; 13.6 ± 1.6 gCOD·L^-1; pH 3.0 ± 0.1) using a closed-loop membrane contactor system. This approach avoids the direct contact between the fermented broth containing VFA and the cultivated biomass. Over 160 days of operation, the system was optimized across four experimental periods to evaluate the impact of varying organic loading rates (OLRs) on biomass yield and quality. During the fourth period, the membrane contactor (MC) operated with a total contact time of 1.92 min and a stripping time of 4 h at a feed temperature of 32 ± 1°C, achieving a VFA mass flux of 8.86 gCOD·m^-2·h^-1 and supporting an OLR of 3.9 ± 0.8 gCOD·L^-1·d^-1. Under these conditions, the AnPBR achieved a biomass production rate of 0.36 ± 0.01 kgX_PB·m^-3·d^-1, with a crude protein content of 62 % and an amino acid profile that met the dietary requirements of aquaculture species. Additionally, the photoheterotrophic metabolism of PNSB enabled the co-production of pigments and polyhydroxyalkanoates (PHAs), adding nutritional and economic value to the biomass. This study highlights the potential of the MC-AnPBR system for sustainable, high-quality SCP production, contributing to circular bioeconomy goals and supporting sustainable aquaculture practice.

Keywords: Acidogenic fermentation; Alternative feed ingredients; Biorefinery; Membrane technology; Resource recovery.