Monday, January 15, 2018

BacterioFiles 324 - Prokaryote Prefers Poorer Power

Green Dragon Spring, Yellowstone
Credit: National Park Service
This episode: Hot spring archaea prefer to use elements that give them less energy even when more energetic options are available!

Download Episode (8.1 MB, 8.8 minutes)

Show notes:
Microbe of the episode: Vitreoscilla beggiatoides

News item

Journal Paper:
Amenabar MJ, Shock EL, Roden EE, Peters JW, Boyd ES. 2017. Microbial substrate preference dictated by energy demand rather than supply. Nat Geosci 10:577–581.

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    Episode outline:

    • Background: Microbes crazy good at using various energy sources
      • Metals, minerals, gases, various organic compounds
      • Almost anything can either donate or accept electrons to generate energy
    • Would expect each microbe to use best sources of energy it can
      • But expectations don’t always fit reality
    • What’s new: Here, scientists publishing in Nature Geoscience have discovered a species of archaea that prefers to use a weaker source of energy even when a stronger one is available!
    • Acidianus DS80, thermoacidophile from Green Dragon Spring in Yellowstone National Park
      • 3.1 pH, 78C
    • Can take electrons from sulfur or hydrogen gas and transfer to sulfur or ferric iron
      • Then use energy to fix CO2
      • Chemically, most energy from H2-Fe, then S-Fe, and least from H2-S
      • Like eating lettuce vs. steak or pasta
      • Or nuclear power vs. burning coal; much higher energy density
    • Methods: Doubling time was similar for each energy source
      • CO2 taken up also similar with H2+S or S+Fe, but 2x less with H2+Fe
      • And cells larger with H2+S
      • So used up more potential energy for same CO2 gain with Fe vs. H2+S, 8x more
        • Different efficiencies in pathways
    • When cells given all 3 elements at once, still seemed to use only H2+S
      • Didn’t see excess increase in sulfate ions (produced from sulfur ox)
      • Fe reduced but abiotically by H2S from S reduction
    • So why preference for lower energy pairing?
      • Might depend on machinery required for each pairing
      • H2+S has hydrogenase and sulfur reductase with some electron transfer proteins between 
      • Happens around membrane
      • Fe not well known, but cells growing with it make hairlike projections: nanowires
        • Used by others to transport electrons far away from cells
      • Energy lost in making these nanowires
        • Also as heat from resistance in electrical conductance
      • So more expensive to use Fe
    • Summary: Archaea in hot springs prefer to get their energy by transferring electrons from hydrogen to sulfur, even though more energy is available from a transfer between hydrogen or sulfur to iron, because it seems that using iron is more expensive to the cell
    • Applications and implications: More to metabolism than simple theoretical energy in food
      • Also consider genetic and machinery requirement costs
      • If hard to afford facility to use nuclear power, can’t use it
      • Or if can’t digest steak, less-energetic food is better
    • Clarifications if necessary: Same in nature/community as in lab?
      • Often pairing with microbes with other metabolisms enhances both kinds
      • Might make higher-energy pairings more favorable for Acidianus
    • What do I think: Wonder how competition affects metabolism preferences of this species
      • Maybe other species can utilize iron more effectively, hard for Acidianus to compete
      • So it doesn’t try very hard, goes for other energy source
    • Need more study to find out if this is the case, or what
    • Human takeaway: Sometimes what seems like the most profitable or powerful approach is not always the best

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