Location and continuity of the annual northern Gulf of Mexico hypoxic zone influences microbial diversity

Areas of low oxygen have spread exponentially over the past 40 years, and are cited as a key stressor on coastal ecosystems. The world’s second largest coastal hypoxic (≤ 2 mg O₂ L^-1) zone occurs annually in the northern Gulf of Mexico (nGOM). The July 2013 and 2014 microbial communities in the nGOM were characterized along a hypoxic to oxic gradient. In 2013, the hypoxic zone was continuous, deep and shelfwide, compared to 2014, which was discontinuous, shallower and primarily located at the mouth of the Mississippi River (MR). Previously we reported that the 2013 nGOM hypoxic zone was dominated by Thaumarchaeota and amoA genes. Analysis of the 2014 hypoxic zone 16S rRNA gene iTag data revealed that Thaumarchaeota were again abundant, but not to the same extent as 2013. The dominant Thaumarchaeota, N. maritimus species in 2013 was also abundant in 2014, but its abundance was significantly lower in 2014. In both years, the normalized abundance of this Thaumarchaeota species was inversely correlated with DO concentrations. In contrast to the 2014 iTag data, qPCR of Thaumarchaeota 16S rRNA and amoA genes (1:1 ratio) revealed higher abundances in the hypoxic zone located at the mouth of the MR, as compared to 2013. At this location qPCR showed higher than expected Thaumarchaeota and amoA copy number given the iTag data. We hypothesize that iTag primers did not discern the full breadth of Thaumarchaeota diversity in comparison to qPCR primers, particularly at the mouth of the MR. Taken together, the iTag and qPCR data suggested that the location near the mouth of the MR, for example, and the lack of continuity of the 2014 nGOM hypoxic zone influenced not only Thaumarchaeota abundances, but also the diversity of Thaumarchaeota species. Given that the abundances and diversity of ammonia-oxidizing Thaumarchaeota is influenced by the shape and location of the hypoxic zone, we will next examine changes in function as it corresponds to oxygen concentrations.