This is a star algae, Micrasterias truncata, as photographed very recently and uploaded to the Flickr Commons collection, “Encyclopedia of Life images.”
The description for this series of images is in Spanish — they’re supplied by “Proyecto Agua” — but the Google Translate renders one bit of it as:
Micrasterias truncata is one of the few species of star algae that live among the humid mosses of peat bogs, from the valley areas to the summits, and within the genus one of the smallest and most widely distributed.
Its symmetrical and flat body, its discreetly cut contours and its almost straight apexes with hardly any ornamentation, make it an unmistakably beautiful species, although it may present a notable morphological variation.
Micrasterias truncata almost always lives in clean waters that are very poor in nutrients, and it can also appear in oligotrophic or slightly mesotrophic lakes and reservoirs. In some countries in central Europe, such as Germany, it is included in the Red Book as it is considered an endangered species, but in others, such as Holland, it is widespread and well represented.