Citation:

James, J. AND R. Devereux. Nitrogen Cycling Genes and Biogeochemistry of Three Small Eutrophic Lakes. ASM Microbe 2019, San Francisco, CA, June 20 - 24, 2019.

Impact/Purpose:

The Jackson Lakes (JL) project will investigate nitrogen, sulfur and carbon cycling pathways in Jackson Lakes. The site is in the Bayou Chico watershed and consists of three lakes. Two of the lakes, SE and SW, are hydrologically connected to Bayou Chico as evidenced by anoxic bottom water having elevated salinity and dissolved sulfide. The third lake, NE, is an artesian, spring-fed system filled with freshwater and independent of Bayou Chico and Jackson Stream. This NE lake may also become hypoxic and contribute to elevated methane production, a potent greenhouse gas. The lakes have 100 μM ammonium in bottom waters making these excellent sites near the GED to develop methods new to the division that are needed to measure nitrogen (N), sulfur (S) and carbon (C) cycles. This project will employ field and laboratory measurements that integrate geochemical and molecular microbiological approaches to monitor the changes in N cycling rates and microbial community structure.Successful completion of this effort will contribute to major advances in the capabilities of GED to investigate C, N, and S cycles and evaluate the effects of nutrients on coastal ecosystems using a novel, interdisciplinary approach. The results will be used to develop a nitrogen cycle model that incorporates C, N, and S pathways and validate the measurements by providing a genomic and nutrient data base for model calibration. The project will further the use molecular methods to characterize active nitrogen-cycling microbial communities and identify potential microbial indicators of stressed and recovered systems. Use of gene abundances in modeling microbial processes is a unique and forward thinking effort for modeling nutrient cycling, which would be useful to program and regional partners.

Description:

Our previous work has shown that the Jackson Lakes, in the Bayou Chico Watershed in NW FL, USA, are biogeochemically different and have different microbial communities, despite their proximity and common origin. All three lakes are polymictic, but the NE lake is fully freshwater, while the SE and SW lakes have saline bottom waters and are stratified. This study was undertaken to investigate key functional genes involved in the nitrogen cycle of the microorganisms in the lakes, and how they correlate with the different biogeochemical conditions over the course of a year. Methods: The lakes were sampled quarterly starting in September of 2016 through October 2017. Water column chemical profiles were obtained for a suite of analytes including salinity, O2, NH4+, NO3-/NO2-2, particulate nitrogen, total nitrogen, and N2O. Each sampling event took place at the same area in each lake. Water was collected at five depths, from 3.5-7.5 m below the surface, focused around the oxycline. The water was filtered using .22 µm filters and the DNA extracted from the filters. QPCR analysis was performed using established primers for key components of the nitrogen cycle. Nitrification was investigated using amoA for bacteria and archaea (commonly, AOB and AOA). Denitrification genes included nirS, nirK, and nosZ, as well as nrfA, which is involved in dissimilatory nitrate reduction to ammonium (DNRA). The anaerobic ammonium oxidation, or annamox, pathway was investigated with hzo. Copy numbers were calculated per ml of lake water. Results: Genes for nitrification activity in the Jackson Lakes were detected and quantified only when using the AOA gene primers. No AOB were found above the limit of detection using QPCR. This was not entirely surprising as the 16S rRNA gene data suggested that AOB organisms were not in our samples. While AOA genes were found, on average, at the low range of 103 copies per ml of water, genes belonging to other specialists such as nosZ and nrfA were an order of magnitude higher. These genes responded to changes in substrate concentration and were variable between samples. The denitrification genes nirS and nirK were found in greater abundance, especially nirK, which was not surprising as these genes are not unique to specialist organisms. As for annamox, no hzo was detected above the limit of detection in any of our samples. Overall, 50.6% of the similarities in gene copy numbers in samples, across seasons, lakes, and depths, was correlated to a combination of temperature, dissolved oxygen, N2O, and NH4. Archaea were responsible for nitrification.

Record Details: