Monera: Diversity, Metabolism, and Survival

In the five-kingdom scheme, Monera are unicellular prokaryotes without a membrane-bound nucleus or organelles. Their metabolic range is broad: some are photosynthetic, some chemosynthetic, and many heterotrophic. This diversity helps explain why bacteria occur in so many habitats; photosynthesis or a rigid cell wall is not shared by every member.

Bacteria usually multiply by binary fission in favourable conditions. Some form resistant spores when conditions become unfavourable. Spore formation is a survival strategy, not a way to increase the number of individuals.

Cyanobacteria (blue-green algae) are photosynthetic prokaryotes that contain chlorophyll a; blooms can occur in polluted water, and some forms such as Nostoc fix nitrogen. Chemosynthetic autotrophs obtain energy by oxidising inorganic substances and use it to make organic compounds. Watch the verb: the energy comes from oxidation, not reduction, of those substrates.

Archaea tolerate extreme habitats through distinctive cell-wall and membrane chemistry and specialised enzymes. Double-stranded DNA is not the special adaptation; it is common genetic material. Mycoplasma lack a cell wall; some cause disease, but they are not frequently autotrophic. For a pathogen-count question, classify each example by host: Vibrio cholerae, Clostridium tetani, and Salmonella typhi are human pathogens; Xanthomonas citri attacks plants, Nostoc is a cyanobacterium, and Aspergillus, Microsporum, and Trichophyton are fungi. The human-pathogenic bacterial count is three.

The 16S rRNA gene contains conserved and variable regions. A highly conserved region may match several related bacterial species, so a high-abundance OTU with multiple close database matches is not automatically a sequencing error or novel species.