Question

Heterocysts are found in certain
A) Viruses
B) Bacteria
C) Cyanobacteria
D) Mycoplasma

Answer 1

Heterocysts are special types of cells which are involved in nitrogen fixation.They express a gene nif which synthesizes enzyme nitrogenase that helps in fixing nitrogen when there is a shortage of fixed Nitrogen.

Complete answer:
Heterocysts are present in filamentous organisms and carry nitrogen fixing machinery with them.Some organisms require nitrogen for biosynthesis and Heterocysts help in it by providing the enzymatic machinery and a safe micro anaerobic environment as nitrogenase is sensitive to oxygen.
Let us analyse the options to find the correct answer.

Option A: Viruses are microscopic parasites which are incapable of surviving outside a host.It only consists of viral RNA or DNA enclosed in a protein coat and only uses the host's cell machinery.Hence, this is an incorrect option.
Option B: Bacteria are unicellular prokaryotic organisms.Bacteria like Azotobacter are found within the root nodules of plants like Legumes in a symbiotic association.This helps in the biological fixation of nitrogen in the nitrogen cycle.Hence, this is an incorrect option.
Option C: Cyanobacteria are gram negative bacteria and are also known as Blue-green algae.They have an outer peptidoglycan membrane.Filamentous cyanobacteria are multicellular organisms and are found in terrestrial habitats like geothermal springs, hot deserts and polar regions.In the filamentous cyanobacteria outer membrane remains continuous with the filamentous structure.In conditions where there is shortage of nitrogen Heterocysts perform nitrogen fixation along with vegtative cells that do aerobic photosynthesis.Examples include Anabaena and Nostoc.Hence, this is the correct option.
Option D: Mycoplasma are parasitic or saprotrophic bacteria which lack a cell wall. They are very small and do not perform nitrogen fixation.Hence, this is an incorrect option.

Additional information:
(i) Heterocysts have a thick cell wall.
(ii) They are present in Cyanobacteria after every 9-15 cells in the filament.
(iii) A One-dimensional pattern is formed.
(iv) Heterocysts have two envelopes; one made of glycolipids and other of polysaccharides.
(v) They have a distinct neck structure at their poles.
Thus, the correct answer is option (C) Cyanobacteria.

Note: Some of the cyanobacterial strains show multicellularity by the presence of filaments that are composed of almost 100 cells. Two cell types; vegetative cells that fix carbon dioxide and heterocysts that fix nitrogen develop during Nitrogen shortage. They are dependent on each other and communicate by signalling. This feature is a great event for the generation of multicellularity in evolution.