Diatom taxonomy: morphology, molecules and barcodes

Abstract

Diatoms are the most species rich algae, with number of species estimated to reside somewhere between 10 000 and 1 000 000 (Mann 1999, Andersen, unpublished). Until recently the basis of the most commonly practiced concept of species has been the discontinuity in expression of one or more morphological characters when compared to the known species (Round et al. 1990, Mann 1999). Indeed, the micro-architecture of the diatom frustule, the valve in particular, provides a richness of structure on which to base such distinctions. However, a great variety of characters from various other stages of the diatom life cycle are also rich in morphological detail but have not been fully explored for their taxonomic significance. Previously unappreciated features are now often being shown to be taxonomically informative (Medlin & Kaczmarska 2004, Edlund & Spaulding 2006, Sims et al. 2006). Following Mann (1999) we argue for expansion of the characters used in species delimitation beyond valve morphology. In this paper we discuss an example of a more holistic approach to diatom taxonomy, one that integrates morphological, molecular and life history characteristics in order to evaluate the congruence between them. DNA extraction, amplification and sequencing are becoming simpler, faster and less expensive tools for examination of diatoms, including species identification. For this to succeed, a library of taxonomically sound reference sequences is needed. The planktonic diatom Pseudo-nitzschia delicatissima (Cleve) Heiden is an example of a species where sequence-based identification should aid in detection of taxa whose valve morphology is too subtle to routinely discriminate between them in natural samples, even with the aid of electron microscopy. Existence of cryptic diversity in the P. delicatissima-species complex was suspected since extensive genetic variability was documented in their nuclear rRNA gene in the LSU (Stehr et al. 2002) and later in the ITS region (Orsini et al. 2004, Amato et al. 2007) which led to description of new species (Lundholm et al. 2006). Our own data indicate also that the diversity of this sub complex has not yet been fully documented.