Cyanobacteria and heterotrophic bacteria have been clubbed together in eubacteria of kingdom-Monera as per the "five kingdom classification", even though the two are vastly different from each other. Is this grouping of the two types of taxa in the same kingdom justified? If so, why? Discuss the cellular composition of cyanobacteria and heterotrophic bacteria that make them classified as eubacteria.

(A) $1$. According to the "five kingdom classification" proposed by $R$.$H$. Whittaker, all prokaryotic organisms are placed under the kingdom Monera.
$2$. Both cyanobacteria and heterotrophic bacteria are prokaryotic in nature, meaning they lack a membrane-bound nucleus and other membrane-bound organelles.
$3$. The classification is justified because the primary criterion for placing organisms in kingdom Monera is their prokaryotic cellular organization.
$4$. Cyanobacteria (blue-green algae) are photosynthetic autotrophs containing chlorophyll-$a$, while heterotrophic bacteria obtain nutrients from organic matter. Despite this difference in the mode of nutrition, their fundamental cellular structure (prokaryotic) remains the same, which is the defining characteristic of eubacteria.