Microbiology- ICAR JRF/SRF 2024

• Abundance of Chemical Elements:
• Universe: The four most abundant elements in the universe by mass are:

• Hydrogen ($H, \sim 73-75\%$)
• Helium ($He, \sim 23-25\%$)
• Oxygen ($O, \sim 1\%$)
• Carbon ($C, \sim 0.5\%$)
  • Earth's Crust: In the Earth's crust, the most abundant elements are Oxygen (46.6%), Silicon (27.7%), Aluminum (8.1%), Iron (5.0%), Calcium (3.6%), Sodium (2.8%), Potassium (2.6%), Magnesium (2.1%), followed by Titanium, Hydrogen, Phosphorus, Manganese, Fluorine, Barium, and Carbon ($15^{\text{th}}$ most abundant at $\sim 200\text{ ppm}$ or $0.02\%$).

• Conclusion: The element is Carbon (Option 3).

• Types of Microbial Interspecies Interactions:
• Pair-by-Pair Matching:
• Tetrahymena and Klebsiella $\rightarrow$ (II) Predation: Ciliated protozoan Tetrahymena acts as an active predator grazing on bacterial prey Klebsiella.
• Desulfovibrio and Methanobacterium $\rightarrow$ (V) Commensalism / Syntrophism: Often involved in interspecies metabolic transfers; here paired with (V).
• Enterococcus faecalis and E. coli $\rightarrow$ (IV) Syntrophism: Cross-feeding and mutual assistance in metabolic degradation pathways.
• Bdellovibrio and E. coli $\rightarrow$ (I) Parasitism / Predation: Bdellovibrio bacteriovorus is an obligate intraperiplasmic parasite/predator of Gram-negative bacteria like E. coli.
• Paramecium and Chlorella $\rightarrow$ (III) Mutualism: Endosymbiotic relationship where the alga Chlorella lives inside Paramecium bursaria, providing photosynthetic sugars while receiving nitrogenous waste and mobility.
• Correct Choice: Option 3 maps (A)-(II), (B)-(V), (C)-(IV), (D)-(I), (E)-(III).

Correct option: (3) (A) — (I), (B) - (VY), (C) - (IV), (D) = (1), (E) = (I)

• Sulfur Transformations in Soil and Aquatic Environments:
• Statement (B) is correct: In aerobic agricultural soils, heterotrophic sulfur oxidation carried out by heterotrophic bacteria and fungi accounts for a substantial or predominant portion of sulfur oxidation compared to obligate chemolithoautotrophs (Acidithiobacillus).
• Statement (C) is correct: The $aprA$ gene encodes the $\alpha$-subunit of adenosine-$5'$-phosphosulfate (APS) reductase, which operates reversibly in both sulfur-oxidizing bacteria (SOB) and sulfate-reducing bacteria (SRB); PCR profiling of $aprA$ diversity provides functional insights into both pathways.
• Statement (D) is correct: Sulfate-reducing bacteria (SRB, e.g., Desulfovibrio) are metabolically versatile, utilizing alternative electron acceptors or entering syntrophic partnerships (e.g., interspecies $\text{H}_2$ transfer with methanogens) when sulfate is depleted.
• Statement (A) is incorrect: The most abundant biogenic volatile sulfur compound in nature is dimethyl sulfide (DMS, $(\text{CH}_3)_2\text{S}$), produced in vast quantities from dimethylsulfoniopropionate (DMSP) in marine systems, not carbon disulfide.
• Statement (E) is incorrect: Heterotrophic sulfur oxidizers typically carry out incomplete oxidation, releasing intermediate thiosulfate or tetrathionate rather than completely oxidizing sulfur compounds to sulfate.
• Conclusion: Statements (B), (C), and (D) are correct (Option 2).