Genetic, physiological and ecological diversity of the diatom genus Leptocylindrus

Abstract

Diatoms are among the most productive photoautotrophic organisms on Earth. Within the diatom genus Leptocylindrus, two species, L. danicus and L. minimus, are reported as abundant in coastal waters worldwide and in the Gulf of Naples (GoN). This thesis aimed at a closer characterisation of Leptocylindrus species through different approaches, including the study of their morphology, molecular phylogeny, metagenomics and biochemistry. Eighty-three strains from the GoN and one strain (CCMP 1856) from the Atlantic US coast were analysed. Based on the morphological, molecular and life cycle differences, the taxonomy of the genus was revised. The GoN species previously identified as L. minimus was in fact identified as L. belgicus Meunier and was placed in a new genus, Tenuicylindrus Nanjappa and Zingone which, along with Leptocylindrus Cleve, belongs to the family Leptocylindraceae. Five species in all were included in the genus Leptocylindrus: L. danicus Cleve and L. minimus Gran, two novel species L. hargravesii and L. convexus, and L. aporus (Hargraves) Nanjappa & Zingone, which was raised from the variety to the species status. The real Leptocylindrus minimus was not found in the GoN. To address the distribution of the 6 species outside the GoN, two metagenomic databases, BioMarKs (Europe) and Tara Oceans (worldwide) were explored. Sequences of L. aporus, L. convexus and L. danicus were recovered at many sites in European waters and across the world's seas, while those of L. minimus were retrieved only in the Oslo fjord and those of T. belgicus were only found in the GoN and Oslo fjord. Additional diversity was observed in the Tara Oceans dataset but, in lack of morphological information, whether this diversity is real remains to be clarified. All species except L. minimus were also categorised based on the diversity in their oxylipins pathways. Leptocylindrus danicus and L. hargravesii shared common lipoxygenase pathways, different from the ones shared by the species L. aporus and L. convexus. Tenuicylindrus belgicus exhibits a pathway distinct from that of Leptocylindrus species. Species-specific compounds produced in minor quantities were also observed. Physiological experiments show that L. aporus can withstand higher (26 C) temperature but not lower (12 CC) temperature, while L. danicus can withstand low temperature but not high temperature. This corresponds to the species occurrence in the natural environment, where L. aporus blooms during summer and L. danicus is found in all seasons except summer. Altogether, through an interdisciplinary approach, the studies described in this thesis provide substantial information that may have important implications in the field of ecology, evolution, conservation biology and biotechnology.