Lecture 13: Lithotrophy
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What is a lithotroph?
1. Organism that uses inorganic material for catabolism 2. "A Lithoautotroph can use inorganic substrates for both catabolism and anabolism
What is an assimilatory pathway? Give examples.
“Assimilatory metabolism refers to metabolic processes wherein elements are incorporated into cellular structures” • Carbon: • CO2 + H+ + ATP + NADH = organic carb. • Nitrogen: • N2 + NADH + ATP + H+ = Amines • Oxygen/hydrogen: • H2, O2, or H2O + ATP • Sulphur/phosphorus/metals: • Sulphates/phosphates dissolved in water (ATP dependent import and NADH used for reduction assimilation)
How is ATP generated in lithotrophy?
- Inorganic substrate level phosphorylation (predominant form) a. E.g., H2, HS-, CH4 - All autotrophs also use chemiosmosis (Na+ or H+) for ATP prod.
What is the role of Quinols in proton motive forces?
“REDOX LOOPS” A. NADH + H+ -> NAD+ + 2H+ b. 2x e- -> Q = QH2 C. QH2 -> Q d. 2H+ -> periplasm f. 2x e- + oxidised mol. = reduced mol. G. H+ into cytoplasm = ADP -> ATP
What are examples of using redox loops to generate proton motive force?
E.g., Nitrate respiration 1. Formate oxidation: HCOOH + Q -> CO2 + QH2 2. Nitrate reduction: NO3- + QH2 -> NO2- + H2O + Q
For bacteria that can respire using nitrate, what are the 3 operons encoding nitrate reductases? What are their purposes?
• NAR: cytoplasm facing a. used to generate PMF to allow growth • NAP: Outer cell facing b. used to remove electrons from quinol without generating PMF • NAS entirely cytoplasmic a. used to generate nitrite for nitrogen assimilation
How could organisms respire on the early Earth?
• Geochemical evidence for Fe(lll) being first external electron acceptor (not S0) • Fe(lll) derived from photochemical oxidation of Fe(ll) from Archaean seas and hydrothermal vent fluids • Demonstrated in deep branching hyperthermophilic archaea and eubacteria • Reduction of Fe(lll) via long-range electron transfer need not require specific enzymes • Magnetite accumulation attributed to Fe(lll)-reducing microorganisms detected in ancient deposits
What are two examples of microorganism’s that use metal to respire?
• Shewanella a. found in sediments • Geobacter a. found in anoxic soils • Require organic carbon - not autotrophic • Insoluble minerals = preferred electron acceptors
What covers the surface of metal reducing bacteria? what do they do?
• Nanowires a. electron transfer systems protruding from the cell surface
How does Shewanella’s anaerobic metabolism work?
• Substrate level phosphorylation AND • Reparation • Lactate is oxidised and produces ATP • Electrons are transported out into the environment • Proton motive force is used for flagellar transport/substrate import
What are iron oxidizing bacteria and how do they work?
• TRUE LITHOTROPHS • Include: a. Acidophiles: (acidothiobacillus, Leptospirillum) b. Neutrophiles: (Sideroxydans, Gallionella) • Use Fe2+ and O2 for respiration • V. slow growing (8hr doubling time)
What is Sideroxydans Lithotrophicus and how does it work?
• Neutrophilic lithoautotroph • Lives in micro-oxic zone in soil • Obligate aerobe • Uses Fe(ll) as electron source • BUT Fe(ll) chemically reacts with oxygen Lives at oxygen concentration where chemical rate of iron oxidation is slower than the biological rate
How does Acidithiobacillus work?
1. Oxidises pyrite (FeS) into Fe3+ and So4^2- using oxygen 2. TRUE LITHOAUTOTROPH a. Proton motive force generate by proton pumping oxidase b. NADH generated from quinol pool c. CO2 fixed by ATP and NADH d. SO4^2- lowers pH of environment to less than 1