Final answer:
Dinoflagellates, euglenoids, green algae, and red algae are all photosynthetic organisms with chloroplasts acquired through the secondary endosymbiosis of a red alga, and they contribute to plankton in aquatic environments.
Step-by-step explanation:
The common feature shared by dinoflagellates, euglenoids, green algae, and red algae is that they are all capable of photosynthesis and have chloroplasts that were derived through secondary endosymbiosis, essentially ingesting a red alga. This group of organisms, which are a mix of unicellular and multicellular entities, can be found in a variety of aquatic environments and play important roles in their ecosystems, primarily as components of plankton. Some members also share attributes like the presence of cellulose in cell walls or the capability to reproduce both sexually and asexually.