What are algae primarily characterized by?

Algae are primarily characterized by resembling plant cells in sunlit water because they are photosynthetic eukaryotic organisms that play a crucial role in aquatic ecosystems. Like plants, many algae contain chlorophyll and other pigments that allow them to capture sunlight and convert it into energy through the process of photosynthesis. This trait is fundamental to their classification and defines their role as primary producers in marine and freshwater environments, contributing to the food web by providing energy and organic material.

In addition, algae can vary greatly in form and size, from single-celled microalgae to large multicellular forms like seaweeds, yet their defining characteristic is their photosynthetic capability in environments where sunlight is available. This aspect helps distinguish them from other life forms like fungi, which do not perform photosynthesis and instead absorb nutrients from their surroundings.