Known to children swimming in the local watering hole as pond scum, these algae became a fast growing segment of the health food industry in the late 60's. They do have some health values ref ref , but let's get right to the main point: DO NOT CONSUME BLUE GREEN ALGAE FROM OPEN WATER SOURCES. No lakes, etc. unless an 
independent lab has verified purity. There is a very high likelihood of heavy metal contamination. Consider only if grown in special controlled manmade ponds from such brands as Hawaiian Pacifica and Earthrise Spirulina. ref General values ref
ANOTHER POSSIBLE CONTAMINATE
In open water situations, there is also a higher probability of contamination from a certain cyanobacteria, microcystis aeruginosa, which is hepatoxic. It is associated with liver cancer as a tumor promoter. Oregon Health officials tested 87 supplements and drinking water samples some time ago and 85 had traces of this contaminate. 63 samples were over the safe limit set by the health department. Another area of the Country was tested and 50 of 50 samples showed this cyanobacteria, especially in lakes with farm runoff since fertilizers provide a good growth medium.
The safe level for this contaminate was set at 1 milligram per gram of supplement. Drinking water was set at 1 PPB, parts per billion. One supplement had over 20 mg/G. These cyanobacteria produce microcystin which has been associated with numerous cases of deaths to humans and livestock. A tragic case of contamination a few years ago caused quite a stir in Brazil when over 50 kidney dialysis patients died. In China, areas where the drinking water contains increased levels, there are higher rates of liver cancer. Over time, even safe levels may accumulate and cause minor symptoms. An association might not be made to this contamination source. And people differ in abilities to fight off the effects of this cyanobacteria.
MAN-MADE PONDS HELP PREVENT CONTAMINATES
While lake sources now have stricter testing procedures, at this point, it is wise to only use blue green algae from the manmade controlled sources to minimize possible risk. Plus, if this is not reason enough, there is always the heavy metal contamination issue, i.e. lead, cadmium. Some characteristics of blue green algae actually help protect against heavy metal contamination, possibly from detox methods, but best not to add more from unclean algae.
CHLORELLA
Another algae, Chlorella, was investigated by an Oncologist, Dr David Steenblock. He discovered that Chlorella given to his cancer patients undergoing chemotherapy reduced the degree of their adverse side effects. He assumed this benefit might be due to Chlorella's high chlorophyll content and some of the other phytonutrients with Liver detox functions or blood building aspects. He wrote a book about his discoveries entitled, CHLORELLA. More research and testing is ongoing. ref
Caution: Due to the concentrations achieved in algae supplements and green cereal grasses, a little goes a long way in supplying nutrients. Higher amounts could pose dangers from heavy metals and also this toxic cyanobacteria unless purity is consistently checked. Make sure the Brand you choose verifies by testing products. ref Not an endorsement for this product, just the info. At 60% protein, algae supplements are not able to be super concentrated unless part of the protein is also removed. Protein cannot be concentrated. Algae mainly just have water removed in the drying process to get to a powder.
Of Interest: Algae is being studied for use as a bio-fuel source. An Oregon coal plant is using collected waste air pollutant CO2 to increase algae growth for this purpose. ref Remember plants take in CO2 and give off oxygen. Algae is a floating plant that has numerous health Benefits when purity is maintained. One advantage of algae is that it produces a low molecular weight protein that has great ability to cross the blood brain barrier and help protect brain nerve cells. ref ref ref
NOTE: Elements found in some algaes look like vitamin B12. They are mostly puesdo-vitamin B12 forms that are not biologically active in humans. These forms are found along side active B12 in humans, foods, and supplements. ref
Team RightWay