Plankton, particularly microalgae such as Spirulina and Chlorella, are sometimes used as dietary supplements for their purported antioxidant properties, which may help counteract free radical damage. Scientific interest in plankton stems from their rich content of bioactive compounds, including carotenoids (such as astaxanthin and beta-carotene), phycocyanins, vitamins (notably vitamin E and C), and polyphenols. These compounds have demonstrated antioxidant activity in vitro and, to a lesser extent, in vivo animal and human studies.

Research has shown that extracts from certain microalgae can scavenge free radicals and decrease oxidative stress markers in laboratory settings. For example, astaxanthin, derived from microalgae like Haematococcus pluvialis, has been studied in small clinical trials and shown some promise in reducing oxidative stress in humans. Similarly, Spirulina supplementation has been associated with reduced lipid peroxidation and increased antioxidant enzyme activity in a few human studies, though sample sizes have generally been small and study durations short.

However, while these findings are encouraging, the overall body of clinical research is limited and often of low methodological quality. There is not yet sufficient high-quality evidence to strongly recommend plankton or microalgae supplements specifically for the prevention or treatment of free radical damage in humans, but the existing scientific rationale and preliminary data provide a modest basis for their use.