What are the general characteristics of microscopic algae?

Microscopic algae are often characterized by their small size, which generally requires the use of a microscope to observe them effectively. They can vary in color, most commonly appearing as green, blue-green, yellow-green, or even red depending on the type of pigments present. The description provided in the answer highlights the presence of yellow-green or red streaked scum, which can be indicative of certain types of algae blooms. Such blooms can produce visible scum on water surfaces, particularly in nutrient-rich conditions, and can have a significant ecological impact.

This option captures the fundamental aspects of microscopic algae, particularly in the context of water quality and ecology, as opposed to the other choices which describe characteristics that do not apply to microscopic algae. For instance, large plants extending above the water are more representative of macrophytes or larger aquatic vegetation rather than microscopic organisms. Similarly, rooted plants in shallow water refer to established aquatic flora rather than the free-floating nature typical of microscopic algae. The characteristics that involve hair-like structures could mislead one to think of certain protozoa or filamentous algae but do not encapsulate the broader group of microscopic algae as successfully as the answer given.