Which of the following describes planktonic algae?

Planktonic algae are primarily described as single-celled organisms that float freely in the water column. They play a crucial role in aquatic ecosystems, serving as a fundamental food source for a wide range of marine and freshwater organisms. When conditions are favorable, such as with ample sunlight and nutrients, planktonic algae can proliferate rapidly, leading to blooms that can turn the water green or brown. This phenomenon is primarily due to the dense population of these microscopic algae.

The other descriptions do not accurately capture the nature of planktonic algae. They are generally not visible to the naked eye, as they are primarily microscopic. While some algae can contribute to the formation of reefs, this characteristic is more applicable to multicellular algae or corals rather than planktonic forms. Additionally, while some species of planktonic algae can be harmful, particularly when they produce toxins, not all planktonic algae are detrimental; many are vital for ecosystem health and food webs.