This class of food belongs in a unique niche.
Edible algae belong to a number of different species. From the sea comes the nori wrapped around your sushi, the kombu in your miso soup. Growing in fresh water come three different algae which may be used as a food supplement and may be made into capsules, tablets or powder and added to drinks, smoothies or nutritious food bars.
All of them are safe to eat, high in chlorophyll and protein and contain omega-3 essential fatty acids. In addition, all contain a blue pigment, phycocyanin, thought by some researchers to be an anti-cancer compound.
But there must be differences. What follows is a brief description of the algae. My personal preference is AFA which my family and I have been eating daily, more or less continuously since 1985.

Aphanizominon flos-aquae (a.k.a. AFA or Super Blue Green Algae™ Please look carefully at the photo below, I took it lying on a dock in Upper klamath Lake in Oregon, the source of AFA. Each of the strands of algae is made up of a colony of about 200 cells. The strands float freely in the water, rising to the top to get a catch the sunlight, or sinking deeper to pick up mineral nutrients. This algae grows wild in huge, spring-fed, Upper Klamath Lake. In the winter, the lake freezes. The algae dies off. In the spring, the algae cells which have been strong enough to survive, bloom and multiply in such quantities that there would be enough algae in this lake to feed everyone on the planet two grams daily. The lake is clean. No chemicals or heavy metals have ever been detected in the water of Upper Klamath Lake nor in the algae from it. AFA is unique because, in addition to being a rich source of chlorophyll and easily digested protein, it is the only algae proven to have a humanly effective vitamin B12. Other algae have a vitamin B12 analog which cannot perform the crucial role which this nutrient performs in our systems. This feature has been a boon to vegans (who don't eat any animal products and who don't want to take synthetic vitamins). AFA is also the richest source of the blue pigment, phycocyanin. AFA, out of the three, also seems to have the most beneficial effect on the immune system. In a study in which immunizations were injected into lab animals which were also fed on one of the three different algae, the effectiveness of the immunization was highest in the group which consumed AFA. In a Harvard University study in which rats were fed AFA as a supplement to their diet, the animals fed AFA had cholesterol levels which were significantly lower than the group fed no algae. Click here for more information about AFA. Chlorella This single cell algae is cultivated in huge ponds, notably, in Japan where it has long been consumed as a part of a healthy diet, In most cases, the water used is pure and the nutrients on which the algae grows are added to the water. It looks like this under a microscope. Before is can be digested, the cells go through an ultrasound process to crack open the tough membrane around the cells. Chlorella is our second choice. It appears to be highly beneficial for large numbers of people. Spirulina This bluegreen algae is also cultivated in huge ponds and fed a man-made concoction of nutrients. It is also a beneficial food, high in protein and chlorophyll. Its cell membrane is soft and requires no processing to become digestible.             Introduction Innate Immunity Acquired Immunity Digestive Immunity Respiratory Immunity Strengthen Your Immunity Why is my Immunity Weak? Cells of the Immune System Home Obtain Products         Chlorella Spirulina             Introduction Innate Immunity Acquired Immunity Digestive Immunity Respiratory Immunity Strengthen Your Immunity Why is my Immunity Weak? Cells of the Immune System Home Obtain Products