Green alga. Differential interference contrast micrograph of the desmid green alga Xanthidium antilopaeum. Desmids are a common group of freshwater single-celled algae that have intricate cell walls. Xanthidiums are placoderm desmids, which have the cell wall divided into two symmetrical halves, joined by a thin central region called the isthmus. Xanthidiums are highly adapted to nutrient poor, acidic water. Magnification: x120 when printed at 10 centimetres wide.