What characteristic defines algae?

The defining characteristic of algae is that they are nonvascular chlorophyll-containing plants that are primarily aquatic. Algae are a diverse group of photosynthetic organisms found in various water bodies, including oceans, lakes, and rivers. Unlike vascular plants, which have specialized tissues for transporting water and nutrients and typically require soil for growth, algae can thrive entirely in water, absorbing nutrients directly from their environment through their cell membranes.

Algae exhibit a wide range of forms and sizes, from microscopic phytoplankton to larger seaweeds. They play an essential role in aquatic ecosystems as primary producers, synthesizing organic materials from sunlight and carbon dioxide while releasing oxygen, supporting the food web.

The other options describe characteristics that do not pertain to algae. Vascular plants needing soil, organisms adapted to extreme dry conditions, and flowering plants that produce seeds in water pertain to very different categories of plant life not relevant to the characteristics of algae.