Algae which are considered as bacteria have a nucleus enclosed within a membrane and plastids bound in one or more membranes.
Algae are a very large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. Most are photosynthetic like plants, and 'simple' because their tissues are not organized into the many distinct organs found in land plants. The largest and most complex marine forms are called seaweeds. Though the prokaryotic cyanobacteria (commonly referred to as blue-green algae) were traditionally included as 'algae' in older textbooks, many modern sources regard this as outdated as they are now considered to be bacteria. The term algae is now restricted to eukaryotic organisms. All true algae therefore have a nucleus enclosed within a membrane and plastids bound in one or more membranes.
Contributions by 18.104.22.168, 22.214.171.124, and Marshman.