Organic spirulina, dried and milled to a deep blue-green powder. Spirulina is Arthrospira platensis — a cyanobacterium rather than a true alga, despite the common name "blue-green algae" — and it is grown in warm, strongly alkaline fresh water, not in the sea. (It is often listed as a marine algae. It is not.)
What it tastes like
There is no polite way to put this: it tastes of pond. Deep, savoury, mineral and unmistakably green. Nobody drinks spirulina for pleasure, and anyone telling you it is delicious in water is being kind. It works because you can hide it.
What is genuinely in it
Spirulina is around 55–70% protein by dry weight, which is remarkable for a plant-type food, and it is a real source of iron, and of the blue pigment phycocyanin that gives it the colour. Those are facts about its composition. What we will not do is turn them into health claims: no health claim for spirulina is authorised in the EU, and the honest position is that its composition is impressive while the clinical evidence for taking it is limited.
Three things you should know before buying any spirulina
It is not a source of vitamin B12. This matters enormously and is still widely misrepresented. Spirulina contains corrinoids that look like B12 in a standard assay but are biologically inactive in humans — pseudo-B12. It cannot be relied on to cover a vegan diet's B12 requirement, and there is some evidence the analogues may compete with real B12. If you are vegan, take an actual B12 supplement. This is not a detail.
Source matters more than with almost any other food. Spirulina is grown in open ponds, and ponds that are not properly controlled can be colonised by other cyanobacteria that produce microcystins — liver toxins. Certified, tested product from a controlled facility is the whole point of paying for a good one.
Phenylketonuria. Spirulina is very high in phenylalanine. Anyone with PKU must avoid it.
Who else should check first
People with autoimmune conditions, people taking immunosuppressants, and people with haemochromatosis or another iron-overload disorder should speak to a doctor before taking spirulina regularly. Pregnant and breastfeeding women should choose a certified product and ask their midwife or doctor.