The Biological Nitrogen Fixation (BNF) is a microbial-mediated process converting atmospheric dinitrogen to ammonia that can play a key role in regulating nitrogen budgets in benthic environments. In this study, BNF measured with the acetylene reduction assay was investigated in vegetated (Zostera marina) and bare subtidal sediments of an upwelling-influenced coastal lagoon located on the west coast of Baja California. The study also quantified the abundance of the nitrogen fixation functional gene (nifH) and determined the relative contributions of sulphate-reducing bacteria (SRB) and cyanobacteria to the total diazotrophic activity. Rates of BNF ranged from 5.1x10^-4 - 0.68 nmol N g^-1 h^-1 and they were significantly higher in sediments with vegetation than in bare sediments and, in general, the diazotrophic activity increased toward stations at the inner lagoon and varied with depth within the sediment. BNF rates were closely associated with the abundance of the nifH gene and were directly affected by the bioavailability of sediment organic carbon. The high contribution of SRB indicates that these benthic organisms can be a significant source of reactive N in coastal lagoons and that sediment heterogeneity shapes the diazotrophic community. These findings broaden our understanding on N-cycling and nutrient management in upwelling-influenced coastal estuarine environments with oyster culture activities.

Authors:

Ariadna Aldrich Rodriguez, Autonomous University of Baja California, Institute of oceanographic investigations, México  (ariadna.aldrich@uabc.edu.mx)

Cristian Hakspiel Segura, Autonomous University of Baja California, Institute of oceanographic investigations, México  (chakspiel@uabc.edu.mx)

Guillermo Samperio Ramos, Autonomous University of Baja California, Institute of oceanographic investigations, México  (guillermo.samperio@uabc.edu.mx)

Francisco Delgadillo Hinojosa, Autonomous University of Baja California, Institute of oceanographic investigations, México  (fdelgadillo@uabc.edu.mx)

Victor Froylan Camacho Ibar, Autonomous University of Baja California, Institute of oceanographic investigations, México  (vcamacho@uabc.edu.mx)

Luisa Falcon, National Autonomous University of Mexico, Ecology Institute, México  (luisaifalcon@gmail.com)

Peter Bernhardt, Old Dominion University (Norfolk)  (PBernhar@odu.edu)

Margaret Mulholland, Old Dominion University (Norfolk)  (mmulholl@odu.edu)