What characterizes filamentous algae?

Filamentous algae are characterized by their multicellular structure that typically forms long, thread-like formations, which can be seen floating in aquatic environments or attached to substrates. These long threads, made up of numerous interconnected cells, allow filamentous algae to thrive in a variety of water conditions and can create dense mats in certain situations. This structure enables them to maximize surface area for photosynthesis, which is crucial in nutrient-rich waters.

The other options describe characteristics that do not accurately represent filamentous algae. For example, the description of unicellular organisms forming balls pertains more to different groups of algae, such as certain types of green algae. Similarly, many filamentous algae can grow throughout the water column, not just at the surface, making the statement about surface-only growth misleading. Additionally, the reference to single-layered structures and not forming mats contradicts the common growth patterns of filamentous algae, as these organisms can form extensive mats that affect aquatic ecosystems. Thus, the correct characterization of filamentous algae is that they are multicellular and form long threads.