Effects Of Aquatic Acidification On Nitrogen Removal In Estuarine And Coastal Environments

Estuarine ecosystem is an important transition zone connecting the ocean and land.In recent decades,with the rapid population growth and socio-economic development,a large amount of reactive nitrogen has been transported to estuarine and coastal areas through rivers,atmospheric deposition and groundwater,causing great ecological and environmental hazards to estuarine and coastal ecosystems,such as eutrophication,frequent occurrence of harmful algal red tides,and seasonal hypoxia.As a result,the biogeochemical cycling of nitrogen in estuaries has become a hot topic and academic frontier in the field of international geological research.In the context of global climate change,the continuous increase of atmospheric carbon dioxide（CO₂）concentration and the resulting acidification potentially and persistently affect the nitrogen cycle in estuaries.In particular,the acidification of estuaries and coasts is more rapid and severe due to the further influence of anthropogenic inputs from land-based sources,complex biogeochemical processes and hydrological conditions.Microbially mediated nitrogen removal processes,mainly denitrification and anaerobic ammonia oxidation,can convert bioavailable reactive nitrogen into nitrogen gas（N₂）,which is ecologically important for reducing nitrogen load in coastal wetlands.However,the effects of estuarine and coastal acidification on the nitrogen removal process and its mechanism are not clear.In view of this,the paper investigated the effects and mechanisms of acidification on nitrogen removal in estuarine and coastal areas based on the ¹⁵N stable isotope tracing technique and molecular biology techniques,using Chongming Dongtan wetland of the Yangtze Estuary as a typical study area and an in-house constructed acidification simulation system.The main results were as follows:（1）Aquatic acidification in estuarine and coastal areas generally suppressed the rate of denitrification of sediments.After two months of acidification simulations,denitrification rates were inhibited by about 28%at a pH reduction of about 0.27,and the rate of denitrification inhibition increased significantly with increasing acidification,reaching 90%at a pH reduction of about 1.46.However,acidification of the estuary significantly promoted the rate of anaerobic ammonia oxidation.After two months of acidification simulations,the anaerobic ammonia oxidation rate increased approximately 1-fold at a pH reduction of approximately 0.27.The anaerobic ammonia oxidation rate showed a facilitation effect in the acidification range of pH reduction from 0.13～1.46,with the facilitation leading the increase and then decreasing.Thus,aquatic acidification in estuarine and coastal environments promoted the contribution of nitrogen removal by the anaerobic ammonia oxidation process,however,as the denitrification process remained the main nitrogen removal pathway,aquatic acidification inhibited the total nitrogen removal rate overall.（2）Aquatic acidification in estuarine and coastal areas significantly contributed to the emission of the greenhouse gas N₂O during denitrification.After two months of acidification simulations,the rate of N₂O emissions increased by about 87%at a pH decrease of about 0.27.The contribution of aquatic acidification to N₂O emissions during denitrification increased and then decreased with increasing acidification,with the highest contribution of 9.3～10.4-fold for N₂O emissions under acidification condition with a pH decrease of 0.62～0.93.Macrogenomic analyses further suggest that the differential response of the relative abundance of N₂O production（nor）and consumption（nos Z）functional genes to aquatic acidification may lead to the accumulation of N₂O,a greenhouse gas,and thus promote N₂O emissions.（3）Aquatic acidification may affect the abundance and microbial structure of denitrifying and anaerobic ammonia-oxidizing bacteria,which in turn affect the rates of denitrification and anaerobic ammonia oxidation and their contributions to total nitrogen removal.The results showed that aquatic acidification in the estuary generally reduced the abundance of denitrifying microorganisms,while the abundance of anaerobic ammonia-oxidizing bacteria tended to increase under acidification conditions.The differential response of denitrifying microorganisms and anaerobic ammonia oxidizing bacteria abundance to aquatic acidification may be an important reason for the elevated contribution of anaerobic ammonia oxidation process to the total nitrogen removal under aquatic acidification conditions.In addition,aquatic acidification in the estuary significantly altered the structure of denitrifying and anaerobic ammonia-oxidizing bacteria,with the relative abundance of denitrifying bacteria such as Arthrobacter,Flavobacterium,and Gaiella decreasing significantly（P<0.05）under acidified conditions when pH decreased by approximately 0.13～1.46,while Pseudomonas,Lysobacter,and Bacillus,Lysobacter,Bacillus and other denitrifying bacteria showed a significant increase in relative abundance（P<0.05）.The dominant bacteria changed from Candidatus Scalindua to Candidatus Kuenenia after about two months of acidification culture for anaerobic ammonia-oxidizing bacteria.