Nitrogen removal performance and microbial community distributions in PCL-based reactors treating mariculture wastewater: Effects of filtration mediums

This study compared denitrification performance and microbial community of PCL-based reactors mixed with three filtration mediums (i.e., gravel [GR], volcanic rock [V] and biochar [BC]) for the treatment of mariculture wastewater. Results showed that PCL mixed with BC had higher TIN removal efficiency and lower NO2--N accumulation (54.46 +/- 19.10-81.84 +/- 9.77%, 0.06 +/- 0.06-3.86 +/- 0.80 mg center dot L-1) than those with GR (51.35 +/- 25.71-63.99 +/- 9.79 %, 0.12 +/- 0.07-5.00 +/- 1.78 mg center dot L-1) and V (44.47 +/- 18.70-67.55 +/- 9.82 %, 0.18 +/- 0.12-10.46 +/- 1.12 mg center dot L-1). Nitrogen removal primarily occurred in the lower part of PCL layer (at 7.0 cm-12.5 cm from the bottom) of three reactors. Mass balance analysis revealed that PCL was predominantly consumed through denitrification (C-D) and microbial growth metabolism (C-m). Compared to GR and V, BC amendment resulted in a 56.17 % increase in C-D while concurrently reducing Cm by 30.95 %. The relative abundance of genera responsible for simultaneously PCL-degradation & denitrification (Simplicispira, 18.92-35.47 %; nor-ank_f_A4b, 1.33-3.05 %; SM1A02, 0.37-0.72 %), denitrification (unclassified_f_Rhodobacteraceae, 14.13-18.87 %; unclassified_k_norank_d_Bacteria, 2.10-5.67 %; Vitellibacter, 3.41-7.83 %; unclassified_c_Alphaproteobacteria, 0.65-1.11 %) and PCL-degradation (IheB3-7, 0.79-2.19 %) were higher in BC. Furthermore, biochar not only upregulated the expression of PCL hydrolysis-related enzymes (EC 2.3.1.16 and EC 1.1.1.35) and pyruvate dehydrogenase (EC 1.2.4.1 and EC 1.8.1.4) to promote TCA cycling, which provides electron donors and energy, but also upregulated the expression of denitrifying enzymes, including nitrate reductase (EC 1.7.99.4), nitrite reductase (EC 1.7.2.1), nitric oxide reductase (EC 1.7.2.5) and nitrous oxide reductase (EC 1.7.2.4). Results of the study confirmed that the use of biochar could indeed improve denitrification performance of PCL-based reactor, which provides a theoretical basis for the establishment of an efficient biological denitrification system.