Bermuda Atlantic Time-series Study (BATS) Oxycline

Oceanic time-series measurements have shown that oxygen levels have generally declined over the past 50 years. At the Bermuda Atlantic Time-series Study (BATS) site, the oxygen minimum value (the measure of dissolved oxygen in the water) has decreased by 5 µmol/kg since 1988 and is driven mainly by bacterioplankton respiration. A project associated with BATS and BIOS-SCOPE looked at bacterioplankton dynamics within the upper and lower oxyclines (oxygen concentration gradients) at the BATS site. High-resolution sampling was initiated during stratification in 2014 and 2015, resulting in a secondary peak in bacterioplankton within the upper oxycline (Fig 1)

The bacterioplankton community within the water column was investigated using fluorescence in-situ hybridization (FISH), catalyzed reporter deposition FISH (CARD-FISH), and Illumina sequencing of both the V1-V2 and V4 regions of the 16s ribosome. Dominant lineages were separated by depth:

• The Pelagibacterales, SAR11 in the Epipelagic;
• The Archaea, Thaumarcheota in the Upper Oxycline;
• The Deltaproteobacteria SAR324 in the Lower Oxycline;
• The Cloroflexi-type SAR202 and the Archaea, Euryarchaeota in the Bathypelagic

Focusing on the upper oxycline, Thaumarcheota contributed up to 12% of the total bacterioplankton. Thaumarcheota can oxidize ammonia and this lineage has a higher affinity for ammonia that competing bacteria, making this archaeon well-suited to the oligotrophic (low nutrient) open ocean. The amoA gene is responsible for ammonia oxidation and, at BATS, this gene had a higher intensity and expression in the upper oxycline where ammonium levels are also measurable, suggesting that the first step of nitrification (in which ammonia is oxidized to nitrites, NO2-, by ammonia-oxidizing bacteria) is possible.

Chemoautotrophy is a mode of growth in which energy is derived from chemical processes rather than light. Chemoautotrophy, including carbon fixation, can be measured in the dark Atlantic Ocean with chemoautotrophic fixation occurring in the same order of magnitude as heterotrophic fixation. The chemoautotrophic Deltaproteobacteria SAR324 comprise between 6-17% of total prokaryotic abundance from 200-2000 m in the North Atlantic and contains the RuBisCO enzyme, suggesting participation in the carbon cycle. At BATS, SAR324 contributed up to 15% of the total bacterioplankton community and the cbbL gene responsible for the first set of photosynthesis was present and expressed within the lower oxycline, suggesting that carbon fixation is occurring.

BATS Oxycline Students
David Picton: BATS and U.K. Associates of BIOS (University of Newcastle in 2014 and University of Durham in 2019)
Gabriel Schuler: National Science Foundation (NSF) Research Experiences for Undergraduates (REU) 2014 (St. Francis University)
Sarah Amiri:  NSF REU 2015 (University of California, Santa Barbara)
Katelyn McLeod: CABIOS 2015 and 2016 (University of Guelph)
Miguel Desmarais: CABIOS 2016 and 2017 (University of British Columbia)