Lecture 10: Anaerobic Marine Ecosystems

Lecture 10: Anaerobic Marine Ecosystems

Anaerobic Respiration (AR)

  • Definition:
    • The process of using an electron acceptor other than molecular oxygen (O2) in an electron transport-based oxidation, leading to the creation of a proton motive force.
  • Types of Acceptors:
    • Metals
    • Oxidized ions of nitrogen and sulfur
    • Chlorinated organic molecules
  • Additional Mechanism:
    • Sometimes includes a sodium motive force.
  • Distinction:
    • The lines between anaerobic respiration and fermentation are often blurred.
    • Typically, anaerobic respiration is not found in eukaryotes, as mitochondria are the sites of respiration.
  • Notable Exception:
    • Some fungi can reduce nitrate (NO3-) to nitrous oxide (N2O) and may conserve energy from this reduction.
  • Diversity of Electron Acceptors:
    • Bacteria and Archaea can utilize an enormous variety of electron acceptors.

Anaerobic Respiration and Nitrogen

  • Forms of Nitrogen:
    • Anaerobic respiration using oxidized forms of nitrogen is common in marine ecosystems.
  • Key Processes:
    • Dissimilatory Nitrate Reduction
    • Sulfur Reduction
    • Dissimilatory Metal Reduction
    • These processes require specific reductases to function.

Energy Utilization Among Electron Donors

  • General Principle:
    • High energy electron donors are utilized before lower energy ones.
  • Typical Sequence in Energy Processes:
    • Aerobic respiration → Fermentation → Anaerobic respiration → Methanogenesis

Microbial Ecology in Oxygen Minimum Zones (OMZs)

  • Role of Microbes:
    • Microorganisms are crucial in marine oxygen minimum zones, where oxygen levels are low or non-existent (hypoxia).
  • Adaptations:
    • Microbes can survive in these conditions by using alternative electron acceptors.
  • Biogeochemical Cycles:
    • Bacteria and Archaea mediate various biogeochemical cycles in OMZs.

Anaerobic Carbon Cycling

  • Main Organisms
    • Anaerobic carbon fixation to biomass is primarily conducted by bacteria and archaea.
  • Types of Anaerobic Metabolisms:
    • Fermentation
    • Respiration
    • Lithotrophy using alternative electron acceptors

Nitrogen Cycling in the Oceans

  • Key Processes:
    • N₂-fixation: Conversion of atmospheric nitrogen (N₂) into ammonia (NH3) by specific microorganisms.
    • Denitrification: The reduction of nitrate (NO3-) to N2, contributing to nitrogen loss from systems.
    • Nitrification: The oxidation of ammonia (NH3) to nitrate (NO3-), including both nitrite (NO2-) oxidation and ammonia oxidation.
    • Ammonia Oxidation: Also known as nitrification, which is a multi-step process involving different microbial communities.
    • Organic Nitrogen Remineralization: The breakdown of organic nitrogen compounds back to inorganic forms.
    • Denitrification: Transformation from NO3- to N2, employing both autotrophic and heterotrophic bacteria.
  • Key N-transformations in OMZs:
    • Nitrification:
    • Involves stepwise reduction of nitrate to nitrous oxide (N2O) or di-nitrogen gas (N2).
    • Denitrification:
    • Involves the conversion of nitrates to nitrites and to N2 gas.
    • Anammox:
    • A unique process where nitrite (NO2-) and ammonia (NH4+) are converted to N2, critical to nitrogen removal in anaerobic systems.

Anammox

  • Full Equation:
    NH4++NO2−→N2+2H2ONH4^+ + NO2^- \rightarrow N2 + 2 H2O
  • Characteristics of Anammox:
    • Anaerobic process combining nitrification and denitrification.
    • Competes with denitrification for substrates.
    • First identified in wastewater treatment in the 1950s, it was initially deemed insignificant in global nitrogen cycles until its discovery in oceanic environments in 2003.
  • Microbial Groups Involved:
    • Conducted by Planctomycetes, which utilize specialized compartments called anammoxosomes to protect from toxic intermediates such as hydrazine.

Environmental Impacts and Eutrophication

  • Eutrophication Effects:
    • New production from nutrient-rich upwellings can lead to hypoxia/anoxia.
    • Example: The Gulf of Mexico hypoxic zone influenced by nutrient runoff from the Mississippi River Basin.
  • Microbial Responses:
    • Increased production in these zones can cause shifts to anaerobic conditions and promote processes that lead to oxygen depletion.
  • Viral Communities:
    • Changes in anaerobic conditions are hypothesized to influence viral communities, favoring those that infect anaerobic organisms.

Conclusions and Future Work

  • Consequences of Ocean Deoxygenation:
    • The ongoing deoxygenation of oceans will lead to more intense oxygen minimum zones, affecting microbial community dynamics and food webs.
    • Further research into how these changes impact ecological interactions at various trophic levels is essential.
  • Future Research Needs:
    • Studies are needed to understand shifts in biodiversity and productivity in response to varying oxygen levels, particularly how anaerobic metabolisms interrelate with greenhouse gas emissions.
    • Special attention should be given to the roles that prokaryotes, microeukaryotes, and viruses will play in adjusting to these changing environments.