Which best describes filamentous algae?

Filamentous algae are characterized by their structure, which consists of chains or filaments of cells that are attached end to end. This arrangement allows filamentous algae to form long, thread-like structures that often appear green and can be found in aquatic environments such as ponds, rivers, and lakes. These filaments can vary in length and may create a tangled appearance in the water.

The other options do not accurately represent filamentous algae. Large multi-cellular structures refer more to seaweeds or larger algae rather than the simpler structure of filamentous varieties. Colonial organisms with distinct cell types often refer to more complex forms of organization, such as those seen in some types of green algae, which form colonies with differentiated cells. Floating mats of algae describe a type of algal bloom that often consists of multiple species, including filamentous and other forms, but does not specify the attached end-to-end characteristic that is essential for identifying filamentous algae.