Biomarkers, chemistry and microbiology show chemoautotrophy in a multilayer chemocline in the Cariaco Basin

Wakeham, S. G.; Turich, C.; Schubotz, F.; Podlaska, A.; Li, X. N.; Varela, R.; Astor, Y.; Sáenz, J. P.; Rush, D.; Sinninghe Damsté, J. S.; Summons, R. E.; Scranton, M. I.; Taylor, G. T.; Hinrichs, K.-U. (2012) Deep Sea Research Part I, 63, 133–156. DOI: 10.1016/j.dsr.2012.01.005

Summary

This large consortium study conducted a comprehensive multidisciplinary investigation of the water column across the redox boundary of the Cariaco Basin, the world’s largest truly marine anoxic basin. Integrating hydrography, water column chemistry, microbial distributions, chemoautotrophy rates, and lipid biomarkers, the study partitioned the 250-m-thick chemocline into five distinct zones. The core of the chemocline was identified as a 25-m-thick suboxic zone where dissolved oxygen and sulfide were both below detection limits, bacterial and archaeal cell numbers were elevated, and tightly coupled cycles of carbon, nitrogen, and sulfur were driven by chemoautotrophy.

Key Findings

• Multivariate statistical analysis resolved five chemically, microbiologically, and biomarker-distinct zones within the chemocline
• The suboxic core (both O2 and H2S below detection) harbored the highest rates of dark carbon fixation and cell abundances
• Bacterial and archaeal lipid biomarker distributions shifted systematically across the redox gradient
• Lipid signatures indicated active sulfur-oxidizing and ammonium-oxidizing communities at specific depth horizons
• Chemoautotrophic carbon fixation was tightly coupled to nitrogen and sulfur cycling in the suboxic zone

Our Contribution

Saenz contributed lipid biomarker analyses as part of the MIT team led by Roger Summons, providing expertise in identifying diagnostic bacterial lipids including Hopanoids and other terpenoids across the redox gradient.

Significance

This study established the Cariaco Basin chemocline as a multi-layered system with distinct biogeochemical regimes, rather than a simple oxic-anoxic transition. The integration of lipid biomarkers with microbiology and chemistry provided a template for interpreting analogous signatures in ancient sedimentary records of ocean anoxic events.