Humans can eat algae. Traditionally it is mostly used in soups or sushi. Dried nori is a popular wrap for sushi. There have been more recent efforts to bring macro algae consumption into the mainstream, hailed as a “super food” (Warning this is not a reliable source: Seaweed grown in pristine unpolluted seawater can still be dangerous, keep reading.)
Humans can be harmed by eating too much of it, though. It’s not a staple or main calorie source anywhere. Marine algae will give you an iodine overdose in large enough quantities [2], can have high levels of arsenic [5], and other heavy metals like cadmium. [1] High levels of cadmium may be related to pollution in the oceans where it is grown but iodine is always going to be there. The ocean is naturally full of iodine and it is absorbed by the algae.
Iodine does not appear to be necessary for algae growth. In a fully recirculating algae growth system no iodine is added except in the initial fill with seawater. We use an iodine free fertilizer (F/2 solution) and have seen rapid growth. Likewise for other heavy metals, since we do not have any seawater inputs we have no problem with heavy metal contamination. This is of course good news for producing feed for fish and shrimp but it also means that it should be safe for direct human consumption. It is a complete protein source, has omega-3 fatty acids and dominated by Ulva, a species known to be edible.[4]
There is one caveat. If humans are going to be eating it, we need to avoid contamination with inedible species. Right now it’s mostly Ulva (which are edible) but cyanobacteria are toxic. Particular care would have to be taken to exclude these species, it would probably have to be run more as a monoculture. But why would you want to eat algal turfs even if we can? Are they particularly tasty? ( I personally have not tried and am not particularly eager to
do so.) It’s possible that some other types of macroalgae are tastier in which case the same method could be used to cultivate them for safe consumption. Perhaps the best reason is that humans are omnivorous and it’s more energy and space efficient for them to eat algae directly. The high growth rates mean that for a closed life support system (spaceship, moonbase) they can be smaller and lighter. If you can convince people to eat an all Ulva diet the life support system can be very small. If you can’t convince them, you could make the
system larger and they can have a mixed diet of fish, shrimp, and Ulva.
To the stars through algae! Ad astra per algae!
References
[1] Victoria Besada, José Manuel Andrade, Fernando Schultze, and Juan José González. Heavy metals in edible seaweeds commercialised for human consumption. Journal of Marine Systems, 75(1-2):305–313, 2009.
[2] K Markou, N Georgopoulos, V Kyriazopoulou, and AG Vagenakis. Iodine-
induced hypothyroidism. Thyroid, 11(5):501–510, 2001.
[4] Jaime Ortiz, Nalda Romero, P Robert, J Araya, J Lopez-Hernández, C Bozzo, E Navarrete, A Osorio, and A Rios. Dietary fiber, amino acid, fatty acid and tocopherol contents of the edible seaweeds ulva lactuca and durvillaea antarctica. Food chemistry, 99(1):98–104, 2006.
[5] Vivien F Taylor and Brian P Jackson. Concentrations and speciation of arsenic in new england seaweed species harvested for food and agriculture. Chemosphere, 163:6–13, 2016.
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