Carbon and ammonium limitation and nitrogen fixation alter the nutritional profile of purple non-sulphur bacteria grown on H2 and CO2.

Purple non-sulphur bacteria biotechnology represents a promising microbial platform to support the protein transition towards more sustainable food systems, particularly when cultivated photoautohydrogenotrophically using carbon dioxide and green hydrogen. This study explored how carbon and/or nitrogen limitations, as well as biological nitrogen fixation can steer the nutritional composition and growth kinetics of purple non-sulphur bacteria. The essential amino acids fraction remained stable at ∼41 %. Under carbon limitation, carbon-rich isoleucine decreased (-27 %) while nitrogen-rich histidine increased (+18 %), and phosphorus content rose by 40 %. Carbon resp. nitrogen limitation reduced bacteriochlorophyll a and carotenoids by 52 % resp. 34 %. Biological nitrogen fixation raised phenylalanine and tyrosine content by 39 %, but reduced the maximum growth rate 2.8-fold; however, final biomass levels were comparable to NH4+-based cultures. These findings highlight the nutritional tunability and metabolic versatility of purple non-sulphur bacteria, reinforcing their potential as a circular, low-carbon platform for sustainable microbial protein production.