Solution:

A heterocyst is a differentiated cyanobacterial cell that carries out nitrogen fixation. The heterocysts function as the sites for nitrogen fixation under aerobic conditions. They are formed in response to a lack of fixed nitrogen (NH4 or NO3).

Nitrogen fixation is the process by which nitrogen is taken from its stable gas form (N2) in the air and changes into other nitrogen compounds (such as ammonia, nitrate and nitrogen dioxide) useful for other chemical processes. It is an important part of the nitrogen cycle.

A zygospore is a diploid reproductive stage in the life cycle of many fungi and protists. Zygospores are created by the nuclear fusion of haploid cells.

An akinete is an enveloped, thick-walled, non-motile, dormant cell formed by filamentous, heterocyst-forming cyanobacteria under the order Nostocales and Stigonematales. Akinetes are resistant to cold and desiccation.

Hormogonia are motile filaments of cells formed by some cyanobacteria in the order Nostocales and Stigonematales. They are formed during asexual reproduction in unicellular, filamentous cyanobacteria some contain heterocysts and akinetes.