Distribution of anaerobic ammonia-oxidizing bacteria in a subterranean estuary

Sáenz, J. P.; Hopmans, E. C.; Rogers, D.; Henderson, P. B.; Charette, M. A.; Schouten, S.; Casciotti, K. L.; Sinninghe Damsté, J. S.; Eglinton, T. I. (2012) Marine Chemistry, 136–137, 7–13. DOI: 10.1016/j.marchem.2012.04.004

Summary

Subterranean estuaries — mixing zones between fresh groundwater and seawater within coastal aquifers — can be major nutrient conduits to coastal waters, yet the role of anaerobic ammonium oxidation (anammox) in nitrogen removal within these environments was unknown. We used ladderane lipid biomarkers to demonstrate the presence of anammox bacteria in the Waquoit Bay subterranean estuary (Cape Cod, Massachusetts). Intact ladderane phospholipids, markers for viable cells, showed peak abundance at the upper redox transition zone. Ladderane fatty acids, which persist after cell death, revealed that anammox communities have occupied a broader depth range over time, tracking the vertical migration of redox transition zones.

Key Findings

• Ladderane phospholipid (PC-monoether) concentrations peaked between 1.4 and 1.7 m depth, corresponding to the upper redox transition zone (URTZ) where ammonium and nitrate coexist and oxygen is low.
• The narrow vertical extent (~0.5 m) of the PC-monoether peak suggests anammox activity is restricted to a thin band at the oxycline where ammonium begins to increase.
• Ladderane fatty acids, representing time-integrated biomass, showed three regions of elevated concentration corresponding to the upper, middle, and deep redox transition zones (URTZ, MRTZ, DRTZ), plus high concentrations in surface soil.
• The relative abundances of individual ladderane fatty acids (I: 18%, II: 39%, III: 43%) matched those from cultured anammox bacteria.
• Pore water chemistry at site PZ6 revealed three persistent redox transition zones over 3 years, though their depths migrated vertically by up to 1 m. Anammox bacteria appear to track these migrations.
• Depth offsets between ladderane peaks and current redox transition zone positions are consistent with recent vertical migration of the transition zones, with anammox communities not yet having fully relocated.
• This was the first demonstration that ladderanes are present in permeable coastal aquifer sediments.

Methods

• Study site: Waquoit Bay, Cape Cod, MA — a partially enclosed bay with permeable sandy aquifer. Sediment cores from site PZ6 collected to 7 m depth by pulse auger (July 2006).
• Pore water: drive-point piezometer (Retract-A-Tip) for groundwater sampling; nutrients by flow injection analysis; pH, Eh, dissolved O₂, salinity by YSI multiprobe.
• Ladderane fatty acids: Bligh and Dyer extraction of freeze-dried sediment; saponification; silver nitrate-alumina chromatography; HPLC/APCI-MS² in SRM mode.
• Ladderane PC-monoether: modified Bligh and Dyer with phosphate buffer; HPLC/ESI-MS² in SRM mode.
• Quantification by external standard curves; analytical reproducibility <10% (FA) and <12% (PC-monoether).

Significance

This paper extended the application of lipid biomarker approaches to a novel environment — subterranean estuaries — and demonstrated that anammox can operate in permeable coastal sediments. The dual-biomarker approach (intact phospholipids for living cells vs. fatty acids for historical distribution) provided both a snapshot and a time-integrated view of anammox activity. While this paper is thematically distinct from the hopanoid-membrane work, it showcases the same lipid biomarker expertise applied to nitrogen cycle biogeochemistry.