Scenedesmus is not a filamentous form, it is colony of 4,8 and 16 rarely 32 cells.while Spirulina is considered as SCP but recent studies revelas that it may be filamentous rather than unicellualar.

Okay, Scenedesmus include most of colonial green algae look like small chains of four cells, but there is a thick mesh form of thin filaments originating from the warty layer.

And than, my question is "why does microalgae have form like that?". I want to make sample for my research in nanoscience. Thanks in advance.

Dear Hasan, Still I am unable to follow your question, but I would like to give some clarification on this topic. Algae are diversified forms with unicellular, colonial, filamentous, filamentous branched, filamentous unbranched, foliaceous and some other forms. cell wall is made up of  mucilaginous with pectin and some times deposited with silica and calcium carbonate substances. cell wall is adopted with adverse environmetal parameters, diversified forms in micro algae may be their adoptation for chnaging  conditions.

The views expressed by Dr Rao are good for you understand the filamentaous nature in Spirogyra,Zygnema   and Cladophora,The other taxa are univellular(Spirulina) while Scenedesmus occurs in various groups as colonies as suggested above.

Its more ecological adaptation to habitat in relation to habit.

it still is a strange discussion, but concerning the Scenedesmus example, the very thin mucal threads helps against sinking to forgottedness in the sediment. A lot of algae have that kind of adaptations.

HTH Rene