Cyanobacteria, which are promising platforms for sustainable bioproduction, are often limited by environmental and metabolic constraints under monoculture conditions. This study investigated the possible factors by which heterotrophic bacteria enhance cyanobacterial growth by co-culturing Synechococcus elongatus PCC 7942 and the following four cyanobacterium growth-promoting bacteria (CGPBs) in a membrane-separated Beppu flask system in a co-culture medium containing 5 g/L glucose to support heterotrophic bacterial activity: Rhodococcus sp. AF2108, Ancylobacter sp. GA1226, Xanthobacter sp. AF2111, and Shewanella sp. OR151. All CGPBs stimulated S. elongatus PCC 7942 growth, but AF2108 had the strongest effect by 11.5-fold increase of chlorophyll a . A stable isotope analysis revealed increases in δ 13 C and δ 15 N, indicating increased bicarbonate assimilation and nitrogen turnover. A transcriptomic analysis detected the upregulation of genes associated with the Calvin–Benson cycle, tricarboxylic acid cycle, glutamine synthetase–glutamate synthase cycle, and amino acid biosynthetic pathways along with the differential expression of iron-related ATP-binding cassette transporter genes. These results suggest that AF2108 promotes S. elongatus PCC 7942 growth through diffusible factors under glucose-containing co-culture conditions. These factors may include respiration-derived CO₂ and other unidentified bacterial metabolites, which together may influence photosynthesis, redox balance, and nutrient turnover. The study provides an exploratory model of non-contact cyanobacterium–bacterium interactions relevant to microbial consortia design.
Tan et al. (Mon,) studied this question.