Algae Biofilm Reduces Microbe-Derived Dissolved Organic Nitrogen Discharges: Performance and Mechanisms

Microbe-derived
dissolved organic nitrogen (mDON) can readily induce
harmful phytoplankton blooms, and thus, restricting its discharges
is necessary. Recently, algae biofilm (AB) has attracted increasing
interest for its advantages in nutrient recovery. However, its features
in mDON control remain unexplored. Herein, AB’s mDON formation
and utilization performance, molecular characteristics, and metabolic
traits have been investigated, with activated sludge (AS) as the benchmark
for comparisons. Comparatively, AB reduced mDON formation by 83% when
fed with DON-free wastewater. When fed with AS’s effluent,
it consumed at least 72% of the exogenous mDON and notably reduced
the amount of protein/amino sugar-like compounds. Irrespective of
the influent, AB ultimately produced more various unsaturated hydrocarbon
and lignin analogues. Redundancy and network analysis highlighted
the algal–bacterial synergistic effects exemplified by cross-feeding
in reducing mDON concentrations and shaping mDON pools. Moreover,
metagenomics-based metabolic reconstruction revealed that cyanobacteria Limnothrix and Kamptonema spp. facilitated
mDON uptake, ammonification, and recycling, which supplied the extensive
nitrogen assimilatory demand for amino acids, vitamins, and cofactors
biosynthesis, and therefore promoted mDON scavenging. Our findings
demonstrate that regardless of the secondary or tertiary process,
cyanobacteria-dominated AB is promising to minimize bioavailable mDON
discharges, which has implications for future eutrophication control.