The species concept in diatoms

Diatoms are the most species-rich group of algae. They are ecologically widespread and have global significance in the carbon and silicon cycles, and are used increasingly in ecological monitoring, paleoecological reconstruction, and stratigraphic correlation. Despite this, the species taxonomy of diatoms is messy and lacks a satisfactory practical or conceptual basis, hindering further advances in all aspects of diatom biology. Several model systems have provided valuable insights into the nature of diatom species. A consilience of evidence (the ‘Waltonian species concept’) from morphology, genetic data, mating systems, physiology, ecology, and crossing behavior suggests that species boundaries have traditionally been drawn too broadly; many species probably contain several reproductively isolated entities that are worth taxonomic recognition at species level. Phenotypic plasticity, although present, is not a serious problem for diatom taxonomy. However, although good data are now available for demes living in sympatry, we have barely begun to extend studies to take into account variation between allopatric demes, which is necessary if a global taxonomy is to be built. Endemism has been seriously underestimated among diatoms, but biogeographical and stratigraphic patterns are difficult to discern, because of a lack of trustworthy data and because the taxonomic concepts of many authors are undocumented. Morphological diversity may often be a largely accidental consequence of physiological differentiation, as a result of the peculiarities of diatom cell division and the life cycle.