AlgaeBase (http://www.algaebase.org) is a web‐searchable store of information on those protists generally considered to be algae. Access is free and some 10,000 browser searches on average now take place each month. The database was established in 1996 and at first only included seaweeds. Its main function at this time was as a catalog of the marine algae of Europe for the European Union‐funded BioMar and European Register of Marine Species (ERMS) projects, and for the recently published Atlas and Check‐list of the Seaweeds of Britain and Ireland. The data are now being extended to cover all algae. Over 50,000 names, of which about half are presently accepted species names, are now included, together with the names of some 3500 genera, about 3000 common names, approximately 700 pictures, and in excess of 28,000 literature references. URL‐based links from a number of other databases including the Species 2000 Annual Check‐list, BIOSIS, GenBank, and Codes for Australian Aquatic Biota have been implemented. It is intended to initiate similar connections from new initiatives such as EuroCat and SPICE, and a number of other global biodiversity databases. As part of a further EU‐funded project, SeaweedAfrica (http://www.seaweedafrica.org), AlgaeBase is being completely rewritten as an SQL database with a browser‐enabled interface, enabling access by a panel of taxonomic experts. AlgaeBase hopes thereby to continue to provide high‐quality access to community‐serviced data in the best traditions of the Internet.

Currently, between one-third and two-thirds of marine species may be undescribed, and previous estimates of there being well over one million marine species appear highly unlikely. More species than ever before are being described annually by an increasing number of authors. If the current trend continues, most species will be discovered this century.

Algae have been estimated to include anything from 30,000 to more than 1 million species. An attempt is made here to arrive at a more accurate estimate using species numbers in phyla and classes included in the on‐line taxonomic database AlgaeBase (http://www.algaebase.org). Despite uncertainties regarding what organisms should be included as algae and what a species is in the context of the various algal phyla and classes, a conservative approach results in an estimate of 72,500 algal species, names for 44,000 of which have probably been published, and 33,248 names have been processed by AlgaeBase to date (June 2012). Some published estimates of diatom numbers are of over 200,000 species, which would result in four to five diatom species for every other algal species. Concern is expressed at the decline and potential extinction of taxonomists worldwide capable of improving and completing the necessary systematic studies.

We present a consensus classification of life to embrace the more than 1.6 million species already provided by more than 3,000 taxonomists’ expert opinions in a unified and coherent, hierarchically ranked system known as the Catalogue of Life (CoL). The intent of this collaborative effort is to provide a hierarchical classification serving not only the needs of the CoL’s database providers but also the diverse public-domain user community, most of whom are familiar with the Linnaean conceptual system of ordering taxon relationships. This classification is neither phylogenetic nor evolutionary but instead represents a consensus view that accommodates taxonomic choices and practical compromises among diverse expert opinions, public usages, and conflicting evidence about the boundaries between taxa and the ranks of major taxa, including kingdoms. Certain key issues, some not fully resolved, are addressed in particular. Beyond its immediate use as a management tool for the CoL and ITIS (Integrated Taxonomic Information System), it is immediately valuable as a reference for taxonomic and biodiversity research, as a tool for societal communication, and as a classificatory “backbone” for biodiversity databases, museum collections, libraries, and textbooks. Such a modern comprehensive hierarchy has not previously existed at this level of specificity.

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