Figure 6.

Overview of hypoxic or apparently anoxic ecosystems that have been reported to contain isotopically light dissolved oxygen, comprise strictly aerobic microorganisms, or O₂-producing or -consuming metabolic genes and pathways. Traces of O₂ were found in groundwaters (e.g. Winograd and Robertson 1982, Ruff et al. 2023) and fracture fluids (e.g. Nisson et al. 2023), while aerobic microorganisms and O₂-dependent enzymes were reported from a variety of anoxic systems, such as bedrock, groundwater, marine and freshwater sediments, and hydrocarbon reservoirs. These microbes were associated with oxidation of methane, other hydrocarbons, and ammonia (e.g. Hayashi et al. 2007, Lösekann et al. 2007, Aburto et al. 2009, Mills et al. 2010, Ruff et al. 2013, 2019, Stępniewska et al. 2013, 2014, Tavormina et al. 2013, Pytlak et al. 2014, Tiano et al. 2014, Kalvelage et al. 2015, Martinez-Cruz et al. 2017, Padilla et al. 2017, Bhattacharya et al. 2020, He et al. 2022, Mosley et al. 2022, Almog et al. 2024, Schorn et al. 2024). Expression of nod and cld genes was detected in O₂-deficient waters, sediments, groundwater, and wastewater treatment plants (e.g. Bhattacharjee et al. 2016, Padilla et al. 2016, Cheng et al. 2021, Ruff et al. 2023, 2024, Elbon et al. 2024, Sarkar et al. 2024), and DOP was confirmed in P. aeruginosa, Ca. M. oxyfera, and N. maritimus using stable-isotope labeling approaches (Ettwig et al. 2010, Lichtenberg et al. 2021, Kraft et al. 2022). The figure and legend features select examples, details are provided in the respective section of this review.