I see doubt in the notes so just wanted to say that YES, this is a unicellular organism! This is a species from a genus of green algae called Caulerpa, which are a siphonous algae. The frond shapes, the “rhizomes” it grows from, and the “roots” it extends into substrate are all extensions of a single multi-nucleate cell. Here’s what a siphonous alga looks like under a microscope, with no divisions in its cytoplasm:
It’s not the only algae like this, either! Caulerpa is a member of order Bryopsidales, which are all siphonous. Here’s some more macroscopic single cell algae:
Codium fragile, or dead man’s fingers. This one is a single long noodle of a cell with swollen growths on the outside called urtricles, packed together to create a firm “skin.”
Halimenia, a calcified algae whose shed growths are responsible for a lot of the beautiful white sand on tropical beaches:
