If plants were created on the third day but sea life only came about on day five, what day was sea algae created on? Algae is a shehakol. It is not an earth based plant, but the Torah does not discuss when such life was created. It only refers to animal life in the sea on day 5.
When were microscopic life forms like amoebas and protozoa created? Were they created on the 5th day as well and part of sheratzay hamayim? Or are these in fact not considered life according to the Torah in spite of their being able to reproduce because they are not nikar le’eynayim?
It is difficult to say something definitive on this , but my assumption is that being that the posuk says ישרצו המים שרץ נפש חיה, that the sea should sprout out creatures that have life. This would lead us to understand that it was only the fish etc. that were created on the fifth day, not all sea life. Plant life was created on the third day.
Regarding amoebas it would depend on the level of life that it has, if it is considered part of the plant or animal kingdom.
Again this is all just an educated guess, as I haven’t seen anything on this.