Q.16 A microorganism isolated from a salt–rich (salt concentration –2 M) lake was found to possess diglycerol tetraethers, with
polyisoprenoid alcohol side chains, as the major lipid component of its cell membrane. The isolated organism is
(A) a planctomycete.
(B) a cyanobacteria
(C) a unicellular amocha.
(D) an archaca
The correct answer is (D) an archaea. Diglycerol tetraethers with polyisoprenoid alcohol side chains serve as hallmark membrane lipids unique to archaea, enabling survival in extreme salt-rich environments like 2 M salt lakes.
Option Analysis
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(A) a planctomycete: Planctomycetes are bacteria with ester-linked fatty acid lipids or specialized ladderane lipids in some cases, not ether-linked tetraethers or polyisoprenoids. They lack these archaeal features despite some unique membrane traits.
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(B) a cyanobacteria: Cyanobacteria use ester-linked glycerolipids such as monogalactosyldiacylglycerol and phosphatidylglycerol, without ether bonds or isoprenoid chains typical of archaea.
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(C) a unicellular amoeba: Amoebae, as eukaryotes, have phospholipid bilayers with ester-linked fatty acyl chains, not archaeal tetraether structures.
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(D) an archaea: Archaea feature ether-linked isoprenoid lipids, including glycerol dialkyl glycerol tetraethers (GDGTs, aka diglycerol tetraethers), which form stable monolayers ideal for high salinity (e.g., halophilic archaea in salt lakes).
Introduction to Archaeal Lipids
Microorganisms thriving in salt-rich lakes at 2 M concentration often possess unique adaptations like diglycerol tetraethers with polyisoprenoid alcohol side chains. These lipids define archaea, distinguishing them from bacteria and eukaryotes in extreme environments.
Lipid Structure Explained
Diglycerol tetraethers, or glycerol dialkyl glycerol tetraethers (GDGTs), link two glycerol molecules via ether bonds to polyisoprenoid chains, forming membrane-spanning monolayers. This resists high salt, heat, and acidity, unlike bacterial ester lipids. Halophilic archaea dominate such hypersaline habitats.
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Ether linkages prevent hydrolysis in brine.
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Isoprenoid tails (C40) enhance packing.
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Tetraether form boosts proton impermeability.
Why Not Other Organisms?
Planctomycetes feature ornithine lipids or hopanoids; cyanobacteria have galactolipids; amoebae use standard phospholipids. Only archaea match this lipid profile precisely.
Exam Relevance for IIT JAM
This question tests microbial membrane biochemistry, key for biotechnology entrances. Archaea’s extremophile lipids inspire biotech applications like stable enzymes.
