Diversity of Autotrophy: Radioautotrophic Organisms, Rhul.ac.uk Presentation

Learning Objectives

By the end of this session, you should be able to:

• Evaluate what autotrophy is
• Understand the difference between different forms of autotrophy which have been discovered
• Evaluate what different forms of autotrophy could mean for life on other planets.

What is Autotrophy?

• Autotrophy is the ability for an organism to convert abiotic energy into storable and usable chemical energy.
• Photosynthesis is the commonly used example for autotrophy as: 6CO2 + 6H20 -> C6H1206 + 602 Fuelled by the absorption of light

Energy Utilization in Autotrophy

• This energy can be used to:
  • Generate energy currency molecules (such as ATP)
  • Generate energy storage molecules (such as glucose and starch)
  • Generate structural molecules (such as proteins and lipids)
  • Fuel biochemical processes
• Therefore, autotrophy is associated with organic primary production with autotrophic organisms being the base of food chains.

Autotrophy and Plants

• Terrestrial plant species are described as: Photoautotrophic Using photons Sourcing nutritional molecules By itself
• So, they produce their own biological molecules sourcing energy from the absorption of photons of light.

Photosynthesis Reminder

• Plants absorb photons via their photosynthetic chlorophyll
• These photons provide a form of chemical energy known as excitation energy. This energy is used to break down water and to actively pump H+ ions across the chloroplast membrane and produce NADPH.
• The H+ ion formed chemiosmotic gradient fuels ATP synthase

Photoautotrophic Food Chain

• Source of Energy
• Primary Producer
• Primary Consumer
• Secondary Consumer
• Third Level Consumer
• Apex Predator

Prokaryotic Photoautotrophs

• Prokaryotic Photoautotrophs can be split into two groups: Aerobic Uses the hydrolysis of H2O as the source of H+ ions and electrons to form the chemiosmotic gradient Produces oxygen as a byproduct of hydrolysis Examples: Cyanobacteria Anaerobic Uses the breakdown of a range of different compounds as the source of H+ ions and electrons to form the chemiosmotic gradient Produces elemental sulphur and Iron as a byproduct of hydrolysis Examples: Pseudomonadota, Chloroflexota and Acidobacteriota

Cyanobacterial Photosynthesis

• Cyanobacteria have similar photoautotrophic capacities as eukaryotic plants and algae, sharing general structural, metabolomic and genetic traits.
• This due to modern cyanobacteria and chloroplasts, sharing a common ancestor, with eukaryotic plants undergoing an endosymbiotic event ~ 2 billion years ago.
• 6 CO2 + 6 H2O = C6H1206 + 6 02

Green Sulphur Bacteria

• Green Sulphur Bacteria (GSBs) utilise a type I photosynthetic reaction centre (similar to the P700 with aerobic photoautotrophs).
• They do not have a PSII homologue, thus have a much shorter ETC.
• GSBs absorb photons at the Far Red of the spectrum which is used to reduce NAD+, which in turn fixes CO2 via a reverse TCA cycle -> synthesising pyruvate

Green Sulphur Bacteria Electron Sources

• Electrons can be accepted from a range of different sources, including H2S, S2O32-, and Fe2+ ions.
• As GSB have a much shorter ETC, they use the cytochrome bc1 complex to pump the H+ ions across the inner membrane to form the electrochemical gradient
• 12H2S + 6CO2 -> C6H1206 + 12S + 6H2O

Chemoautotrophs

• Chemoautotrophs are organisms which obtains the energy for their overall function via the oxidation of electron donor molecule which are present within their environment.
• Chemoautotrophs species can derive their biological energy from a range of different organic or inorganic sources, including but not limited to the oxidation of:
  • H2S
  • Ferrous Iron
  • Ammonia
  • Methane

Types of Chemoautotrophs

• Chemoautotrophs are commonly described as prokaryotic extremophiles, living in highly hostile environment such as deep-sea volcanic vents.
• These environments are high-temperature, and low pH. These provide sources of H2S and other electron sources and high temperature to aid in speeding up reactions

Nitrifying Bacteria Research

• Growing research has discovered a range of nitrifying bacteria (such as Comammox) which utilise the breakdown of ammonia as their source of electrons.
• Nitrifying bacteria are fundamental source of nitrogen for soil, providing usable nitrogen to aid in plant development and growth

Chemoautotrophic Environments

• Chemoautotrophs within and around deep-sea vents provide the basis for light-independent environments which are ~10,000 to 100,000 times more organism dense than none deep-sea vent adjacent sea-floor environments.
• As chemoautotrophic organism grow, this attracts and sustains amphipods and copepods which graze on the bacteria. This sustains snails, shrimps, crabs, tube worms etc.

Thermal Vent Chemoautotrophs Food Chain

• T
• 0
• Source of Energy
• Primary Producer
• Primary Consumer
• Secondary Consumer
• Third Level Consumer
• Apex Predator

Ammonia Chemoautotrophs

1/2 O2 NO2 NO3 NH + 11/2 O2 2 H+ + H20 NO2 Electron transfer + proton motive force ATP CO2 Biosynthesis Macromolecules

The Nitrogen Cycle

Nitrogen gas N2 Denitrification Organic N in proteins, etc. Assimilation Nitrogen fixation Anammox Nitrate NO3 Decomposition Bacteria and Archaea Nitrification Assimilation Nitrite NO2 Nitrification Ammonia NH3

Radioautotrophic Organisms

• Radiosynthesis, or the process of using ionizing radiation such as alpha, beta, and gamma radiation as a sources of energy to drive metabolic reactions and the synthesis of biological molecule has been hypothesised since the 1950s.
• Scientists had previously concluded that radiosynthesis was not viable because:
  • Radioactive material was too spread out to be a sufficient and reliable food source
  • Ionizing radiation is very damaging to organic material such as DNA, increasing the risk of uncontrolled raditation

Chernobyl Discovery

• However, in 1986, Chernobyl Nuclear Power Station in Ukraine had a violent meltdown, releasing radioactive material into the surrounding area.
• By the mid 1990s, researchers aiding in the clean-up of the area discovered over 200 species of fungi growing rapidly in areas of high radioactive material.

Fungi and Radiation

• These species of fungi were shown to be able to use the excess ionizing radiation to fuel metabolic processes by use high concentrations of melanin within their tissues.
• Not only was melanin the site of radiolysis of water to provide electrons for the electron transport chain, but it was capable of preventing DNA damage.
• Three different species from Chernobyl were shown to have growth rates ~3-fold higher when introduced to radiation levels ~500x that of the general background radiation.

Autotrophs and the Hunt for Life

• Autotrophs are fundamental in the formation of complex life, as without autotrophs there is no way to convert abiotic compounds and energy into biotically active molecules and energy.
• These biotically active molecules act as sources of energy for predator species allowing for more complex organisms to survive.

Extraterrestrial Life Perspective

• Autotrophic life has been shown to much more diverse than first thought, with abiotic energy being provided by a wide range of different sources.
• This alters the perspective of where extraterrestrial lifeforms may exist within our universe.

Europa: A Candidate for Life

• Europa, one of the moons of Jupiter, has been hypothesised as having the greatest chance of extraterrestrial life in our solar system due to:
  • The presence of liquid water (under a protective multi-kilometre thick ice sheet)
  • Geological Activity with deep-ocean thermal vents
  • A purely oxygen atmosphere (although very thin)

Learning Objectives Review

Now you should be able to:

• Evaluate what autotrophy is
• Understand the difference between different forms of autotrophy which have been discovered
• Evaluate what different forms of autotrophy could mean for life on other planets.