Why can't prokaryotic species be defined as a group of similar organisms that could sexually reproduce together?

The prokaryote divides by asexual mode of reproduction.

Explanation:

The concept of species which defines a species as a group of individuals of a population which can interbred and produce a fertile progeny is applicable to the organism which can reproduce sexually.

Since the bacteria divide through asexual means and not through sexual ways therefore the concept of defining a species becomes complicated and does not apply.However the concept of species in prokaryotes is still a topic of research.  

Thus, the biological concept of species is not applicable to prokaryotes.

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b. the carbon cycle ensures living things have a source of hydrogen