Microalgae-bacteria synergy in photosynthetic bio-electrochemical systems supports nitrogen transformation, microbial dynamics and greenhouse gas mitigation

Specific gaps in this paper, with an experiment that would close each.

• High No systematic variation of cathode dissolved oxygen (DO) independent of circuit configuration; DO ranged 2.52–8.69 mg L⁻¹ across cathode types but was not manipulated as a standalone variable.

  DO strongly correlates with nitrification (AOB AmoA abundance) and methanogenesis suppression, yet the paper confounds DO with cathode type and circuit mode. Isolating DO would clarify whether oxygen is the limiting factor for ammonia oxidation and CH4 suppression.

  Try. Vary cathode DO from 0–10 mg L⁻¹ via aeration or oxygen sparging while holding cathode configuration constant; measure AOB AmoA, methanogen abundance, CH4 flux, and NH4+-N removal.

• High No measurement of cathode potential (Ecathode) or cathodic current density; only anode potentials and half-cell potentials are reported, limiting mechanistic understanding of cathodic nitrogen transformations.

  The paper attributes nitrification to microalgal oxygen production but does not measure whether cathodic potential or current density influences nitrification kinetics or the balance between nitrification and denitrification at the cathode.

  Try. Measure cathode potential and cathodic current density across external resistances; correlate with nitrification gene abundance (AmoA) and NO3−-N production at the cathode.

• Medium No investigation of the effect of influent C/N ratio or nitrogen speciation (NO3− vs. NH4+ vs. organic-N) on nitrogen removal pathways and N2O production.

  The paper uses a fixed wastewater composition; C/N ratio is known to control the balance between nitrification and denitrification in other bioelectrochemical systems, yet is not varied here.

  Try. Test C/N ratios of 5:1, 10:1, and 20:1 at optimal external resistance (100 Ω); measure NO3−-N, NO2−-N, NH4+-N removal and N2O flux; profile narG, napA, nirS, nirK, nosZ gene abundance.

• Medium No direct measurement of N2O reduction kinetics in the cathode; N2O reduction is attributed to the anode, but the cathode may also contribute given its higher DO and presence of denitrifiers.

  The paper states 'Nitrous oxide reduction in the anodic region was attributed to denitrifying bacteria' but does not experimentally isolate or quantify N2O reduction in the cathode compartment under light and dark conditions.

  Try. Measure N2O flux and nosZ gene abundance separately in anode and cathode under light and dark; use 15N-labeled N2O to trace reduction pathways in each compartment.

• Medium No exploration of the trade-off between nitrite accumulation and power output; lower resistance maximizes power but causes NO2−-N accumulation, yet no optimization of resistance to balance both objectives.

  The paper shows that 100 Ω achieves highest power (11.17 mW m⁻²) and highest NO3−-N removal (96.5%) but worst NO2−-N accumulation (−47.6%); intermediate resistances (200–500 Ω) are not tested.

  Try. Test external resistances of 50, 100, 200, 500, 1000, 5000, 10,000 Ω; measure power density, NO3−-N removal, NO2−-N accumulation, and nirS/nirK gene ratio to identify the resistance that minimizes nitrite accumulation while maintaining >80% power output.

• Low No measurement of hydraulic retention time (HRT) or its effect on nitrogen removal and N2O production; HRT is mentioned as a variable in prior PMFC work but not tested here.

  The paper cites prior work showing HRT affects nitrogen removal but does not vary HRT in this study, limiting applicability to different reactor scales and flow rates.

  Try. Test HRT of 2, 5, 10, and 20 days at optimal external resistance; measure NO3−-N, NO2−-N, NH4+-N removal, N2O flux, and functional gene abundance.