Microalgae-Mediated Electron Transfer and Allocation in <i>Paracoccus denitrificans</i> for Efficient Nitrate Removal

Microalgal-bacterial systems provide a promising strategy to enhance the nitrate removal performance of denitrifiers via the traditionally assumed provision of electron donors or enrichment of functional microorganisms, while the single-species regulatory mechanism remains unclear. Chlorella vulgaris ( C. vulgaris ) could markedly improve the nitrate removal by Paracoccus denitrificans ( P. denitrificans ) even under nutrient-replete conditions, contributing to a 2.48-fold increase in the nitrate removal rate constant compared to the sum of monocultures, indicating non-nutritional regulatory effects. Transcriptomics showed glycolytic genes in P. denitrificans were upregulated by 1.65 to 10.1-fold, accompanied by a 51.0% increase in NADH production. Lactate, as a key intermediate, activated the bacterial electron transport chain (ETC), with ETC-related gene expression upregulated by 2.83 to 2.95-fold. Furthermore, bacterial ammonium induced microalgal glutamate synthesis and release, which likely acted as a signaling molecule to upregulate bacterial nitrate reductase expression. These interactions redirected electrons toward nitrate reduction, increasing carbon utilization efficiency for nitrate removal by 82.75%. Microalgae coordinately optimized bacterial carbon and nitrogen metabolism, reprogramming electron generation, transfer, and allocation to achieve efficient nitrate conversion. This work provides molecular-level mechanistic insights and supports the design of controllable strategies for environmental remediation.