Algae, proalgae, and eualgae

As Torres (1991) and others have noted, the term “algae” no longer refers to a taxonomically useful group. It does, though, refer to an easily recognized adaptive facies. This adaptive facies contains both prokaryotes and eukaryotes, as do various others (e.g., nanoplankton or decomposers). The blue-green algae are thus indeed algae in this useful sense. More than terminology is involved, because a concept like that of adaptive facies can be forgotten if we focus solely on taxa. The terms “proalgae” and “eualgae” can be used transparently when we want to distinguish prokaryotic from eukaryotic algae. “Proalgae” has in fact been proposed as a formal taxon (Van Valen, 1986); the terms come from Van Valen and Maiorana (1980). Similarly, one can refer to the nonhomologous kinds of flagellae as proflagellae and euflagellae while retaining a general term.