# DBT JRF 2020 — Question 5

Exam: DBT JRF 2020 · Section: B · Topic: Microbiology · Difficulty: Hard

## Question

Which of the following marine phototrophs can utilise carbon-dioxide as the carbon source?

## Options

- **A.** Heliobacteria
- **B.** Green sulfur bacteria
- **C.** Chloroacidobacteria
- **D.** Halobacteria

## Answer

Option B

## Explanation

Green sulfur bacteria (family Chlorobiaceae, e.g. Chlorobium tepidum) are obligate anaerobic photoautotrophs. They use sunlight as energy source, inorganic sulfur compounds ( $\text{H}_2\text{S}$ , $\text{S}^0$ , $\text{S}_2\text{O}_3^{2-}$ ) as electron donors, and $\text{CO}_2$ as their sole carbon source (autotrophs). They fix $\text{CO}_2$ via the reverse TCA (reductive citric acid) cycle, not the Calvin cycle. They contain bacteriochlorophyll c, d, or e in chlorosomes (light-harvesting antenna). Heliobacteria use organic carbon; Halobacteria (Archaea) use bacteriorhodopsin and are heterotrophic; Chloroacidobacteria are a recently discovered minor group.

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