The microbial biogeochemistry of nitrogen in the coastal ocean: Interactions at the interface of the Yaqui Valley - Gulf of California, Mexico

Nitrogen acts as an essential nutrient and a critical substrate in much of the ocean. In estuaries and in the coastal ocean, the biogeochemistry of this element is strongly influenced both by human activity and by microbial activity. This dissertation explores both of these influences at the coastal interface of the Yaqui Valley, an intensive, developing world agricultural system, and the Gulf of California, one of the most productive and nutrient-rich regions of the ocean. Results presented herein suggest that the Gulf is acutely vulnerable to nitrogen-rich agricultural runoff from the Yaqui Valley, as pulses of runoff following discrete fertilization/irrigation events in the Valley fuel large phytoplankton blooms in the open waters of the Gulf of California. These phenomena occur despite the presence of highly diverse and active denitrifier communities within the sediments of Bahia del Tobari; denitrification is expected to remove most of the nitrogen entering this estuarine system in runoff before it reaches the Gulf. Molecular evidence for the widespread presence of ammonia-oxidizing Archaea in marine water columns and sediments---including those of Bahia del Tobari---is also reported. Considering the abundance of Archaea in the ocean, these findings suggest that they play an important role in the global nitrogen cycle and nitrogen-cycling within individual ecosystems. Analysis of spatial and temporal variability in community composition and richness of ammonia-oxidizing Archaea and Bacteria within Tobari lends further support to this idea. Taken together, these findings highlight our still limited understanding of nitrogen biogeochemistry and how it is influenced by human and microbial activity.