Seagrass (Zostera marina) promotes nitrification potential and selects specific ammonia oxidizers in coastal sediments

Purpose Seagrasses accelerate sedimentation, release oxygen and organic matter through their roots, and compete with ammonia oxidizers for ammonia/ammonium in surface sediments and overlying water, all of which can influence benthic aerobic nitrification. To understand the effects of seagrass vegetation on benthic nitrification, the heterogeneity of nitrification activities and functional microbial communities between seagrass-vegetated and adjacent bare sediments was investigated. Materials and methods Surface (0-5 cm) sediments were sampled from a Zostera marina-colonized coastal lagoon in northern China. The potential nitrification rates (PNR) and relative contributions of ammonia-oxidizing bacteria (PNRaob) and ammonia-oxidizing archaea (PNRaoa) were determined based on the sediment slurry incubation with kanamycin inhibition method. The abundances and community compositions of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) were determined using qPCR, clone library, and high-throughput sequencing. Results and discussion The total PNR (PNRtotal) in the seagrass-colonized sediments were significant higher than those in the bare sediments (P < 0.05), to which PNRaob contributed 59.7% and 55.8% in vegetated and unvegetated sediments, respectively. The concentrations of chlorophyll a and dissolved inorganic nitrogen were key factors determining benthic PNR. AOB outnumbered AOA by one order of magnitude by targeting amoA gene abundance, and no significant difference (P > 0.05) in amoA gene abundance and diversity was observed between the two habitats. Most AOB genotypes were affiliated to Nitrosomonas, of which NL7 was selectively enriched in the vegetated region, while N. cryotolerans prevailed in the bare. Two distinct AOA groups Crenarchaeota 1.1b and Crenarchaeota 1.1a dominated in vegetated and unvegetated sediments, respectively. Dissolved oxygen of overlying water and TOC:TN of sediment significantly influenced AOB community, while AOA community was strongly driven by nitrate and metal iron in sediments. Conclusions Seagrass vegetation substantially enhances nitrification potential and selects specific ammonia oxidizers in coastal sediments.