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GCE-LTER Key Findings


Ammonia oxidizers transform the nitrogen cycle

Ammonia oxidizing Archaea (AOA) are members of the guild of microbes that convert ammonia to nitrite. A combination of molecular and ecological measurements suggested that these organisms can be locally abundant, but when we began this research little was known about their spatial or temporal dynamics or their relationship to environmental factors. In recent years the Hollibaugh lab has conducted multiple studies of ammonia oxidizers at the GCE site as well as in polar (PAL-LTER) and other habitats. Hollibaugh et al. (2014) found pronounced mid-summer blooms of AOA at the GCE site that coincided with a peak in nitrite concentration. Analyses of the distribution of both AOA and ammonia-oxidizing betaproteobacteria (AOB) during six cruises in the South Atlantic Bight (Liu et al. 2018) revealed that ammonia oxidation rates were highest in inshore stations, where gene sequences serving as markers for ammonia oxidizers were dominated by Thaumarchaeota (AOA), with both members of the guild peaking in summer months. They concluded that the summer bloom was confined to inshore waters. Schaefer and Hollibaugh (2017) reported experiments that showed that the two steps of nitrification (ammonia- and nitrite-oxidation) become uncoupled between 20 and 30oC, leading to nitrite accumulation, and suggested that the observed nitrite peaks in GA coastal waters may be explained by differences in the responses of these two assemblages to increased summer water temperatures. Although time series of ammonia oxidation rates and AOA populations are rare, field data from 29 temperate and subtropical estuaries and lagoons showed evidence of transient accumulation of nitrite, which suggests that this is a widespread phenomenon. Moreover, the summer accumulation of nitrite was related to water temperature (Fig.). At the PAL-LTER site, Tolar et al. (2016) found that ammonia oxidation in Antarctic waters was ~10-fold greater than previously reported, and that previous global estimates of oceanic nitrification may be underestimates. These insights demonstrate the extent to which the discovery of AOAs has profoundly affected our understanding of nitrogen cycling in aquatic systems.

Plant-Herbivore interactions

Nitrite concentrations compiled from 270 stations in 29 tropical and subtropical estuaries and lagoons, binned by month (left) and temperature (right). Nitrite concentrations are highest during summer months when temperatures exceed 15 C. From Schaefer and Hollibaugh 2018.

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This material is based upon work supported by the National Science Foundation under grants OCE-9982133, OCE-0620959, OCE-1237140 and OCE-1832178. Any opinions, findings, conclusions, or recommendations expressed in the material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.