Though traditionally grouped together — usually as plants — fungi and algae actually have no direct evolutionary connection. Freshwater algae may be the common ancestors of all living plants, but most scientists believe that fungi share ancestry with members of the animal kingdom. Although algae and fungi do share some similarities — both can be multicellular, consist of eukaryotic or nucleus-possessing cells, and have cell walls — these organisms are distinct life forms.
Algae can be unicellular or multicellular, but among fungi, only yeast can be unicellular. Common algae forms include filaments, colonies, single cells or multiple cells. Algae ranges in size from microscopic bacterial algae to brown algae species such as sea kelp or bladderwrack, which can grow up to several hundred feet long. Fungi generally comprise groupings of filaments, or hyphae, joined together to form dense structures called mycelia. Fungi such as mushrooms are “filamentous forms that provide a high surface-to-volume ratio that greatly enhances their mode of nutrition,” according to University of California, Berkeley.
The majority of algae live in water, though some species live in symbiotic relationships with terrestrial species such as lichen, protozoa or invertebrates. The majority of fungi live on land, although some fungi live symbiotically with aquatic species such as the giant clam.
Fungi fall into four groups, or phyla, depending on their mode of reproduction, which can occur through production of sexual spores, asexual fragmentation, or production of asexual spores. In contrast, algae produce only asexually through fragmentation or cell division.
Biologists divide fungi into two groups based on feeding practices. A few species are parasitic, and live off of other plants or animals. Examples include a parasitic fungus called Cryphonectria parasitica, which has infected and killed many stands of the American chestnut tree across the United States. However, most fungi are saprobes, or saprophytes, which feed on decaying organic matter. According to Midlands Technical College, saprophyte fungi are "extremely important as decomposers which recycle nutrients back into the biosphere." Algae, on the other hand, obtain nutrition through photosynthesis. Algae synthesize sugars using two types of chlorophyll. Algae photosynthesis plays an extremely important role in the earth's atmosphere, as planktonic algae create most of the molecular oxygen in the atmosphere.