Algae are mostly eukaryotic organisms, with only a few exceptions.
Being eukaryotic means that they have a nucleus and other organelles enclosed by cell membranes.
Algae cells have nuclei like those of higher plants, their DNA is enclosed within a permeable nuclear membrane, and they all have mitochondria, endoplasmic reticulum, and Golgi bodies inside their cells.
They prefer damp, watery habitats and contain chlorophyll, red algae, and green algae are common examples of eukaryotic algae.
They make their food through photosynthesis, produce oxygen, hold nutrients, bind heavy metals, benefitting the waterbody, and are a good source of nutrition for the fishes.
Blue-green Algae or cyanobacteria are prokaryotes and are therefore not classified as algae (because all algae are considered eukaryotic organisms).