How can algae be microscopic or grow into large multicellular forms?

Algae exhibit a vast range of sizes, spanning seven orders of magnitude from microscopic single cells to massive, complex organisms. Their ability to exist in these diverse forms is a result of different evolutionary strategies regarding cellular organization and growth.12

Microscopic Algae

Many algae are unicellular and microscopic, existing as individual cells that may be spherical, rod-shaped, or spindle-shaped. Some of these microscopic forms are motile, utilizing flagella to swim through aquatic environments. Others exist as simple colonies, which are aggregations of cells that may be produced through rapid cell division until a specific number is reached.13

Multicellular and Large Forms

Algae grow into large, macroscopic forms through several structural arrangements:
* Filamentous forms: Cells are arranged in chains, which can be unbranched or branched and bushlike. In some species, these filaments intertwine to create complex, plantlike bodies.13
* Parenchymatous forms: These are tissue-like structures where groups of cells specialize for specific functions, such as anchorage, transport, and photosynthesis. This specialization allows organisms like giant kelp to reach lengths of many meters.1
* Coenocytic forms: These algae grow to large sizes without forming distinct cells; they are essentially multinucleated organisms where the protoplasm is not subdivided by cell walls.1