Marine Crenarchaeota have recently been found to be capable of oxidizing ammonia via an Archaea-specific ammonia monooxygenase (amoA) gene. Archaeal amoA genes have been observed in marine environments at 10-1000 times greater abundance than the amoA homologue from ammonia-oxidizing Bacteria (AOB), suggesting that the ammonia-oxidizing Archaea (AOA) play a major role in the marine nitrogen cycle. Pelagic AOA are most abundant below 100 m depth in the water column and at higher latitudes, especially near the poles. Reasons for this distribution remain largely unknown, but previous studies have pointed to environmental factors, such as salinity, light, temperature, oxygen, and sulfide. Competition with bacteria or phytoplankton has also been thought to influence abundance of AOA in the environment. This study compares the distribution of AOA and AOB in the Mississippi River Plume using quantitative PCR (qPCR) analysis of 16S rDNA and amoA genes from each group combined with different environmental factors measured at the same stations. Crenarchaeal acetyl-CoA carboxylase gene (accA) abundance was also measured to look at distribution of autotrophic metabolism. The enzyme translated from the accA gene is involved in the 3-hydroxypropionate pathway, which enables MG1C to assimilate carbon autotrophically. Surface and deep (200m) samples were also amended with 3H-leucine to measure protein synthesis rates in the Mississippi River Plume; samples from these emendations will also be used for microautoradiography fluorescent in situ hybridization (MAR-FISH) analysis to look at incorporation by single cells, in order to investigate potential heterotrophic metabolism by AOA.