What is the meaning of multicellularity?

Question

I can't understand what multicellularity is. Wikipedia states that any organism having many cells is multicellular. By this definition bacteria can also be multicellular. For example, cyanobacteria can form filaments made up of specialised cells like spore cells and vegetative cells. But aren't bacteria supposed to be unicellular only? I would appreciate it if someone could clear up my confusion.

Answer 1

Bacteria are generally only unicellular organisms. However, your example is unique. Most extant cyanobacteria are descendants of a multicellular ancestor, with reversal back to unicellularity occurring 5 different times. Today, they can form filaments or microbial mats which include your mentioned spores and vegetative cells. These spores and vegetative cells are still cyanobacteria and will behave like all others once growing conditions are ideal. Once growing, the cyanobacteria will live on their own like most other bacterial species.

To clear up the multicellularity issue, think of multicelluar organisms as giant accumulations of different cell types doing different cellular jobs to keep the life form alive. Without one type of cell, the life form most likely couldn't survive. This contrasts unicellular organisms that can live regardless if the cell next to them is alive because they are only worried about themselves.