Algae are a diverse group of photosynthetic eukaryotic protists Algae may be unicellular or multicellular Large, multicellular algae are called seaweeds but are not plants and lack plant-like tissues ...Algae are a diverse group of photosynthetic eukaryotic protists Algae may be unicellular or multicellular Large, multicellular algae are called seaweeds but are not plants and lack plant-like tissues and organs Although algae have little pathogenicity, they may be associated with toxic algal blooms that can and aquatic wildlife and contaminate seafood with toxins that cause paralysis.