Effects of light spectrum on the treatment of mariculture wastewater by a microalgae-bacteria symbiotic system

Background With the continuous expansion of marine aquaculture in China, the environmental pressure caused by aquaculture wastewater discharge has become increasingly severe. Microalgae-bacteria symbiotic system (MBSS) is an environmentally sustainable and highly efficient technology for wastewater treatment, the performance is significantly influenced by light conditions. However, the regulatory mechanism of different spectra on the nitrogen-containing pollutants removal remain unclear.

Objective This study aims to explore the effects of four light spectra (white, blue, green, and red light) on mariculture wastewater treatment by MBSS, and evaluate their regulatory effects on the pollutant removal efficiency, chlorophyll content, extracellular polymeric substances (EPS) content and the microbial community structure.

Methods MBSS was constructed with marine Chlorella sp. and compound bacterial agents. Two-stage experiments of batch and continuous operation were carried out under the four LED spectra. Water quality indicators (NH₄⁺-N, NO₂⁻-N, NO₃⁻-N), chlorophyll and EPS content were measured. 16S rRNA high-throughput sequencing and functional prediction were used to analyzed the microbial community structure and metabolic potential.

Results Under all light spectra, NH₄⁺-N removal efficiency exceeded 99.90%. The Group G achieved the highest NO₃⁻-N removal efficiency (80.00%) and total inorganic nitrogen removal efficiency (82.10%), accompanied by slight NO₂⁻-N accumulation (0.15 mg/L). Total chlorophyll content (6.10 mg/L) and total EPS content (8.77 mg/L) in the Group G were higher than those in other groups. Microbial community analysis revealed that the Group G had the highest alpha diversity (Shannon and Simpson indices), and its community structure was reshaped. The relative abundance of the denitrifying bacterium Tropicimonas reached 23.70%, which was 1.93-3.54 times that in the other groups. The predicted abundances of facultative anaerobes (9.50%) and biofilm-forming bacteria (15.40%) were the highest among all groups, whereas the proportion of pathogenic bacteria was lower. Functional predictions indicated that iron respiration and aromatic compound degradation were unique to the Group G, and the abundance of functional genes related to nitrate reduction and nitrogen respiration was also higher than in other groups.

Conclusion Green light promotes the efficient metabolism and functional stability of the MBSS and endows it with the potential to degrade complex organic matter by directionally regulating the microbial community structure, enriching denitrifying functional flora, and promoting the secretion of extracellular polymeric substances (EPS). As a result, it achieves the efficient removal of pollutants in mariculture wastewater. This study provides theoretical underpinnings and data support for the development and determination of operational parameters of light-controlled microalgal-bacterial symbiotic wastewater treatment systems.