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Morphology and molecular phylogeny constitute the structural elements of diatom taxonomy. These approaches do not, however, give information on the functioning of taxa. Additional methods to serve a more integrated and wide-ranging taxonomy have therefore been called for. Metabolic fingerprinting is one approach used within the field of metabolomics, often applied in classification of samples. Here we apply metabolic fingerprinting in a taxonomic study of a cryptic diatom species. Strains of the cosmopolitan diatom Chaetoceros socialis from two geographical areas; the north-east Atlantic and Arctic and the Gulf of Naples, were cultivated at three different temperatures; 2.5, 8 and 13°C. The strains from the two different geographical areas exhibited different growth rates as well as different photosynthetic efficiencies. Algal extracts, collected at the end of the growth experiments, were analysed by Ultra-Performance Liquid Chromatography High Resolution Mass Spectrometry. The two groups of strains were separated by principal component analysis of their metabolic fingerprints. Analysis of the data revealed both qualitative and quantitative differences in metabolite markers. These phenotypic differences reinforce differences also found for morphology, phylogenetic markers and growth rates, and point at different adaptive characteristics in organisms living under different temperature regimes. [ABSTRACT FROM PUBLISHER]

The cosmopolitan distribution of the marine planktonic diatom Chaetoceros socialis Lauder is generally interpreted as a case of wide ecological plasticity. However, cryptic diversity has recently been observed within this taxon and this genetic diversity could extend to phenotypic differences between allopatric populations. Monoclonal strains of C. socialis isolated from NE Atlantic/Arctic and Tyrrhenian Sea waters were cultivated at 2.5, 8 and 13°C, and analysed for morphological, phylogenetic (LSU rRNA gene) and physiological (growth rate, photosynthetic yield) traits. The molecular analysis indicated an unequivocal divergence between the southern and the northern strains. Clear differences in spore morphology between the two groups were also observed, despite their morphological similarity in vegetative form. The physiological data demonstrated a functional partition between the northern and southern strains. The northern strains showed significantly higher growth rates than the southern ones at the lower temperatures, and vice versa at 13°C. Differences between the groups were also observed in the photosynthetic yields. These results reveal the expression of allopatric speciation in several phenotypic characters, providing a functional dimension to the cryptic, or rather pseudo-cryptic, diversity observed within C. socialis. We believe that such an integrated approach is useful for robust taxonomic and functional delimitations of presumed cosmopolitan microorganisms. [ABSTRACT FROM AUTHOR]