Ammonia oxidation is the first step in the nitrification pathway that eventually leads to loss of fixed nitrogen (Figure 1).  Conversion of ammonia to oxidized nitrogen changes the chemical form of nitrogen available to biota and it produces the substrates for denitrification, a process that can remove excess nitrogen from ecosystems.  Some of the nitrogen processed through this pathway is also released as nitrous oxide (N₂O), a greenhouse gas 400-fold more powerful than carbon dioxide. 

Our data indicate that blooms of Ammonia Oxidizing Archaea (AOA) are regular, seasonal (August) features of bacterioplankton community dynamics in Georgia coastal waters.  These blooms are accompanied by dramatic, transient increases in concentrations of nitrite (Figure 2).  The questions we addressed in this project are: What factors cause the bloom? What are the geochemical consequences of this bloom? How widespread is this phenomenon?  We conducted a sampling program coupled with experimental manipulations to follow interactions between members of the nitrifier community and to identify the factors controlling the dynamics of the bloom.

We found that the rapid development of the water column bloom is consistent from year to year (Figure 2).  A survey of the distribution of AOA revealed 1) that AOA are the dominant ammonia oxidizer in Georgia coastal waters; and 2) that the bloom develops in and is confined to a relatively narrow band of turbid water immediately adjacent to the coastline.  The identity of the water column AOA population is similar to that of AOA found in salt marsh sediments, suggesting that seasonal variation in resuspension by bioturbation may drive water column population dynamics.  We concluded that it does not because the seasonality of sediment AOA differs from that of the water column population; the water column population is actively growing as reflected by measured ammonia oxidation rates and accumulation of nitrite; and the composition of the two communities is subtly different (Figure 3). 

The geochemical consequence of the bloom is a spike in nitrite concentration.  We also measured N₂O concentrations in water samples collected over a year and found that this gas was supersaturated with respect to atmospheric equilibrium for much of the summer.  The N₂O distribution did not coincide with the AOA bloom, suggesting that other processes, such as denitrification, may contribute to N₂O production.

The bloom appears to be triggered by a pervasive and fairly simple cause.  We performed statistical analyses of our data sets and determined that AOA abundance increased in response to shifts in a set co-varying environmental variables.  We examined the speciation of copper (Cu) because Cu bioavailability has been proposed to limit growth of AOA populations.  Cu²⁺ concentrations were extremely low during the bloom because most of the Cu was associated with humic material and reduced sulfur species.  We conclude that AOA can access this Cu and that bioavailability of Cu does not limit the growth of this AOA population.  

The dominant factors correlating with AOA abundance were water temperature (T) and dissolved oxygen (DO) concentrations.  We performed a laboratory experiment to determine the dependence of nitrification on temperature and found optimal growth between 25 and 32 ^oC, with decreased growth at T>35 ^oC and mortality at T>40 ^oC.  Surprisingly, the nitrite-oxidizing bacteria in the sample did not respond to temperature as strongly as AOA, resulting in the accumulation of nitrite when samples were incubated at 25-32 ^oC, consistent with field observations (Figure 4).  Assuming that nitrite peaks in other data sets might indicate similar seasonal cycles of nitrification, we examined data from 270 stations in 29 sites (81,217 records) to determine: 1) the geographic extent of the phenomenon; and 2) whether DO or T is the dominant factor in the seasonality we observed.  Our conclusion is that the bloom occurs widely in estuaries and it is driven by simple seasonal shifts in T (Figure 5).

Estuarine temperatures can be expected to rise in the future as a consequence of climate change.  Our analysis suggests that warmer water, and especially higher peak summer temperatures, will result in higher estuarine ammonia oxidation rates and significant nitrite accumulation that may have wider impacts. Elevated levels of nitrite in the water column have been shown to be more effective than nitrate in stimulating production of N₂O. Coastal areas may contribute significantly to oceanic N₂O emissions, and thus increased emissions due to temperature-induced nitrite spikes may contribute to a positive feedback loop for global warming. Another potential effect of increased nitrite concentrations is toxicity to aquatic organisms. Shifts in the form of dissolved inorganic nitrogen available to phytoplankton may cause shifts in the composition of those communities, with potential consequences for the food chain and for public health if harmful algae are affected.  Although our analysis focused on coastal waters, we expect that similar dynamics apply in terrestrial ecosystems with potential consequences for N-loss from crops.

Last Modified: 02/02/2017
Modified by: James T Hollibaugh