Centrifugation was a really good idea to get out those protozoans from sea water. So I centrifugated the sea water and used the denser parts at bottom of the tubes for observation. I added two rice grains to the centrifugate and observed after a day.
Some observations :

Added a drop of 1% Methylene Blue to the sample just for fun. Saw that it turned entirely blue. However within a few hours it went back to being colorless. Not just that, protozoans were flourishing in the sample. Some even took up the dye within them…

The ones in last video were alive for more than three weeks!

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To be continued…