|Other Abstract||The nearshore mariculture industry plays a significant role in ensuring national food security, meeting the diversified dietary needs of the population, and stabilizing and promoting economic development in coastal areas. The intensive input of nutrients from mariculture has the potential to cause seasonal hypoxia in coastal waters, which can destroy ecological stability and affect the biogeochemical cycling of biogenic elements. Nitrogen fixation is important processes for N-input in coastal ecosystems. Nitrous oxide (N2O) is a greenhouse gas that has an important impact on global climate change because of its significant warming effect. N2O emission also serves as an important pathway for N-output in coastal ecosystems. However, the mechanisms of nitrogen fixation and the source-sink processes of N2O in response to seasonal hypoxia are still poorly understood. The contribution of the hypoxic area to the nitrogen flux of the entire marine system and the dynamic changes of microbial communities in related sediment processes with the development of seasonal hypoxia remain to be explored. Given the prolonged expansion of marine hypoxia areas and the rapid and drastic changes in global climate, it is of great significance to investigate the response mechanism of N-input and N-removal to hypoxia in coastal ecosystems in order to maintain the balance of the marine ecological environment.
In this study, we investigated the response mechanisms of nitrogen fixation and nitrous oxide source-sink processes to seasonal hypoxia in the nearshore mariculture zone of Yantai City using nitrogen isotope labeling, functional gene quantification, and omics techniques, in combination with in-situ sample determination and microcosms construction. The aim of this study is to provide references for the ecological assessment of the impact of hypoxia on the ecological function of the coastal zone and a scientific basis for the reduction of greenhouse gas N2O emissions. The main research findings are presented as follows:
(1) In the nearshore mariculture zone of Yantai City, the benthic nitrogen fixation rate ranged from 0.013 to 10.199 μmol kg-1 h-1 and was significantly affected by seasonal hypoxia (ANOVA, p < 0.05). Besides, nitrogen-fixing activity was also significantly correlated with pH, the concentration of Chl-a in the bottom water, and the concentration of NH4+ in the sediment pore water (p < 0.05). Geobacteraceae (63.95%, iron-reducing bacteria) was the most dominant nitrogen-fixing bacteria in nearshore mariculture zone sediments. When hypoxia occurs, the iron-reducing bacteria might collaborate with sulfate-reducing bacteria to achieve the maximum nitrogen-fixing efficiency. Additionally, the abundance and community structure of nitrogen-fixing microorganisms were significantly affected by the total organic carbon (TOC), total nitrogen (TN), and iron contents (Fe (III)) of sediments.
(2) During seasonal hypoxia, N2O was found to be in a supersaturated state in the offshore water column of Yantai City (up to 226%). Under severe hypoxia (August), Station S8 (with a lowest DO, 1.84 mg L-1) had a significantly high net N2O emission potential (2.83 μmol m-2 h-1), and regression analysis indicated that the N2O emission rate from sediments was significantly and negatively correlated with DO (p < 0.05). The qPCR results indicated a significant increase of nirS (nitrite reductase gene) and nosZ (nitrous oxide reductase gene) gene abundance in sediment under hypoxic condition, and nosZ transcription abundance being one order of magnitude higher than that of nosZII gene. The archaea amoA (ammonia monooxygenase gene) transcription abundance was also higher than that from bacteria. Before hypoxia (June), the majority of transcripts in surface sediments were derived from Bacillariophyta (41.9%), and it gradually evolved into a community dominated by Proteobacteria (33.85%) and Desulfobacterota (22.85%) with the occurrence of hypoxia. Thermoproteota and Desulfobacterota are the primary drivers of N2O production and reduction, respectively, in the hypoxic sediments of nearshore mariculture zone. Meanwhile, microorganisms harboring the nosZ gene from Proteobacteria primarily act as the sinks for N2O.
(3) During the DO gradient microcosms simulation incubation, the peak value of N2O emission was reached at 2 h in all treatment groups with DO concentration. The rapid consumption of ammonium and nitrate in the first 2 h indicated that the micro-cosmic system completed the accumulation of N2O rapidly in 2 h. The addition of exogenous ammonium and nitrate increased the potential of sediment to release N2O by 3 and 30 times, respectively. Based on the qPCR results, it was observed that during the process of N2O accumulation (0-2 h), there was a significant increase in nirS gene abundance, whereas during N2O consumption (2-24 h), a notable increase in nosZ and nosZⅡ gene abundance was observed. The metatranscriptomic analysis revealed that Proteobacteria (49.99%) and Desulfobacterota (20.44%) were dominant in all samples. It was speculated that the ammonia oxidation might be dominated by Thermoproteota, and the denitrification might be dominated by Proteobacteria.|