How can green algae be both multicellular and unicellular? How can green algae be both multicellular and unicellular? What are the cases in which it can be both uni and multi cellular?

Because _Algae_ are represented across four kingdoms, namely _Bacteria_ (unicellular prokaryotes), _Plantae_ (multicellular eukaryotes), Chromista (multicellular eukaryotes), and Protozoa (single-celled eukaryotes).

For example, _I. Chlorophyta_ is unicellular:

!Unicellular
_I. Chlorophyta_. Source: University of Wisconsin

While kelp can become huge and can form underwater forests:

!Kelp
Kelp. Source: Nature

**Reference**
Guiry, _J Phycol_ 2012; **48** , 1057–63