Hi Tarak, put the sample in nitrogen depleted condition (low conc than optimum). by nature if non planktonic they will tend to stick to the bottom as benthic forms naturally tend to attach to substratum for nutrients. If truly planktonic, then even under stress it will float and not stick to the bottom. Try this and I hope it will be useful for you.

Planktonic cyanobacteria have a characteristic property of possesing gas vacuoles which provide them buoyancy but the imbalance in the carbohydrate and protein ratio may some time leads to loss of the propety so what you can do is either go for the genome analysis looking for the vacuole forming genes. Another simple thing you can do is take some culture which is aroud 5-7 days old centrifuge it down if you can see it floating still its for sure a planktonic form. hope it works for you.

@Avik Kumar Choudhury, Disagree by your answer. Let us take the example of all Chlorella sp. or even Chlorella minutissima in particular. Chlorella minutissima is planktonic but still in stick to bottom in a flask. It does not prove its benthic nature. Actually sea, rivers and even lakes also provide some force them to swim. but in lab 95% of the planktonic will behave as benthic.

@Avik Kumar Choudhury, in still water for long time all planktonic will began to settle. water movement is also necessary for planktonic to float. We have to standardize or search in literature about the time difference range of settling to bottom of planktonic algae. Or else you can ask question on research gate by naming the algae and someone will certainly tell you the answer whether it is planktonic or not?

I was already mentioned for Cyanobacteria not for green algae or other algae

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@Manojit Debnath: I was trying to explain the fundamental behind this. I you want that I should quote some examples from cyanobacteria e.g. Chroococcus minutes, Microcystis sp. etc are planktonic. principle is same dear friend. cyanobacteria having mucilage and live in clusters are surely benthic e.g. Merismopedia sp., Nostoc sp. etc.

benthic or planktonic none of them has relationship with water movement to show there property. its all about the presence or abscence of the gas vacuole in youger culture gas vacuoles are prominent but as it gets old n ratio of protein to carbohdrate changes its buoyancy reduces. Some times it might be the genetical rearrangements that might lead to loss of buoyancy. Please check the attached papers.