From the Katia product page:

“SeaCell Cotton is a yarn composed of 100% biodegradable matter and seaweed which are collected in a sustainable manner thanks to an innovative and totally ecological process. With SeaCell Cotton you can knit or crochet soft and comfortable garments. In addition, the yarn is beneficial for your skin because it contains high levels of antioxidants, vitamins, amino acids and minerals in the seaweed.”