All life needs vitamins - some were needed by the first life on earth. Vitamins provide cofactors that many enzymes need to work.
Humans need vitamins for a healthy life - we can get this from eating lots of fresh fruit and vegetables.
Mineral supplements One vitamin not made by plants is vitamin B12 (cobalamin), which many algae contain in high levels, but they need an external supply like humans!
Bacteria to the rescue Algae rely on bacteria in their growth media to provide vitamin B12.
With a little help from my friends... This association or "symbiosis" was found in our laboratories, and shows how we rely on both algae and bacteria for vitamins.