Algal-bacterial symbiosis enhances adaptation to nutrient limitation under low light in urban water: Transcriptomic...

Algal-bacterial co-cultures are promising microbial platforms for maintaining microalgal growth in urban river water without exogenous nutrient supplementation. In this study, a defined Chlorella vulgaris-Ectopseudomonas sp. co-culture and algal monocultures were cultivated in autoclaved urban river water under light intensities of 120 and 60μmol m^-2 s^-1. Starting from an initial algal density of 1.0 × 10⁵ cells mL^-1, the reduced-light co-culture treatment reached the highest algal cell density of approximately 4.9 × 10⁷ cells mL^-1 during cultivation, whereas the higher-light co-culture reached a lower maximum density of approximately 3.4 × 10⁷ cells mL^-1 and declined earlier. Physiological and biochemical measurements at the Day 5 RNA-seq sampling time point showed that the reduced-light co-culture was associated with higher chlorophyll a content, a higher cellular energy state, and lower oxidative stress responses than the corresponding algal monoculture and higher-light treatments. Day 5 RNA-seq revealed clear treatment-associated transcriptomic separation, with PC1 and PC2 explaining 79 % and 11 % of the total variation, respectively. In the reduced-light versus higher-light co-culture comparison, 2244 candidate genes exhibited higher expression and 1534 candidate genes exhibited lower expression under reduced light. Functional enrichment analysis associated these expression-responsive candidate genes with photosynthesis, porphyrin metabolism, nitrogen metabolism, glycerolipid metabolism, and transport-related processes. These results suggest that reduced light intensity supports more sustained algal growth in the co-culture system and is associated with broad chloroplast-related transcriptional adjustment under non-supplemented urban-water cultivation conditions. Further time-series transcriptomic, physiological, biochemical, and metabolomic analyses are needed to further validate these transcriptome-associated interpretations.