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Ammonia-oxidizing bacteria (AOB) play an important role in nitrification in estuaries. The aim of this study was to examine the spatial abundance, diversity, and activity of AOB in coastal sediments of the Liaohe Estuary using quantitative PCR, high-throughput sequencing of the amoA gene coding the ammonia monooxygenase enzyme active subunit, and sediment slurry incubation experiments. AOB abundance ranged from 8.54 × 10 to 5.85 × 10 copies g of wet sediment weight and exhibited an increasing trend from the Liaohe Estuary to the open coastal zone. Potential nitrification rates (PNRs) ranged from 0.1 to 336.8 nmol N g day along the estuary to the coastal zone. Log AOB abundance and PNRs were significantly positively correlated. AOB richness decreased from the estuary to the coastal zone. High-throughput sequencing analysis indicated that the majority of amoA gene sequences fell within the Nitrosomonas and Nitrosomonas-like clade, and only a few sequences were clustered within the Nitrosospira clade. This finding indicates that the Nitrosomonas-related lineage may be more adaptable to the specific conditions in this estuary than the Nitrosospira lineage. Sites with high nitrification rates were located in the southern open region and were dominated by the Nitrosomonas-like lineage, whereas the Nitrosospira lineage was found primarily in the northern estuary mouth sites with low nitrification rates. Thus, nitrification potentials in Liaohe estuarine sediments in the southern open region were greater than those in the northern estuary mouth, and the Nitrosomonas-related lineage might play a more important role than the Nitrosospira lineage in nitrification in this estuary.

Hydrothermal carbonization (HTC) combined with anaerobic digestion (AD) is a cost-effective process to simultaneously treat biomass wastes for hydrochar production, energy recovery, and wastewater treatment. However, low fermentation efficiency and high carbon dioxide content in biogas are tricky problems of this process. This study developed a novel strategy based on Fe-modified hydrochar (Fe-HC) for in situ biogas upgrading and fertilizer recovery by AD of macroalgae Laminaria HTC process water. The daily yield and proportion of methane in biogas in the Fe-HC added reactor were 216.6 ± 13.1 mL/g COD fed and 90.3 ± 1.3%, respectively. The conductive Fe-HC increased relative abundances of Trichococcus and hydrogenotrophic methanogens (Methanosarcina and Methanobacterium) and promoted the electron transport system activity, heme c content in extracellular polymeric substances, and sludge granulation. These results implied the Fe-HC-promoted biogas upgrading might be ascribed to the enhancement of H 2 -mediated interspecies electron transfer, potential direct interspecies electron transfer, and trace FeCO 3 precipitation. The effluent from the Fe-HC supplemented reactor was rich in nutrients, including K + , I − , gibberellin (GA 3 ), etc., but low in human potential pathogenic bacteria and heavy metal content, indicating it had excellent potential as a fertilizer. This study provides an ecofriendly and sustainable strategy for energy and fertilizer recovery from macroalgae wastes.

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