Microbial Ecology of Oxygen Minimum Zones Amidst Ocean Deoxygenation

Abstract

Oxygen minimum zones (OMZs) have substantial effects on the global ecology and biogeochemical processes of marine microbes. However, the diversity and activity of OMZ microbes and their trophic interactions are only starting to be documented, especially in regard to the potential roles of viruses and protists. OMZs have expanded over the past 60 years and are predicted to expand due to anthropogenic climate change, furthering the need to understand these regions. This review summarizes the current knowledge of OMZ formation, the biotic and abiotic factors involved in OMZ expansion, and the microbial ecology of OMZs, emphasizing the importance of bacteria, archaea, viruses, and protists. We describe the recognized roles of OMZ microbes in carbon, nitrogen, and sulfur cycling, the potential of viruses in altering host metabolisms involved in these cycles, and the control of microbial populations by grazers and viruses. Further, we highlight the microbial community composition and roles of these organisms in oxic and anoxic depths within the water column and how these differences potentially inform how microbial communities will respond to deoxygenation. Additionally, the current literature on the alteration of microbial communities by other key climate change parameters such as temperature and pH are considered regarding how OMZ microbes might respond to these pressures. Finally, we discuss what knowledge gaps are present in understanding OMZ microbial communities and propose directions that will begin to close these gaps.

Keywords: global climate change; microbial ecology; microeukaryotes; nutrient cycles; oxygen minimum zone (OMZ); viruses.

FIGURE 1

Global map of oxygen concentrations at 300 m depth. Data from NOAA World Ocean Database using all O₂ data from 2009–2018. Data was plotted using Ocean Data View.

FIGURE 2

Simplified schematic of carbon, nitrogen, sulfur cycles in oxygen minimum zones, underlying sediments and overlying oxic waters. Purple cocci shapes represent processes that archaea perform, green rods represent bacteria, teal pennate diatoms represent microeukaryotes, and maroon phage particles represent processes viruses potential augment. Full details in text.

FIGURE 3

Schematic of how future deoxygenation may affect the ecology of OMZs and their adjacent areas. Upward pointing arrows indicate the process or organisms whose abundance/rate is expected to increase, not considering biotic interactions and feedback loops by climate change and downward pointing arrows indicate negative effects.