The major producers in marine and freshwater ecosystems

Explanation

Diatoms are single-celled algae with silica cell walls, and they are the most prolific producers in aquatic environments. They perform a significant amount of the oxygen-producing photosynthesis on Earth. In contrast, while euglenoids and dinoflagellates contribute to aquatic ecosystems, they do not match the abundance or ecological impact of diatoms. Red algae are more specialized and less prevalent in terms of primary production compared to diatoms.