Diversity, temporal distribution and physiology of the centric diatom Leptocylindrus Cleve (Bacillariophyta) from a southern hemisphere upwelling system

The marine diatom Leptocylindrus is a major component of phytoplankton blooms in coastal ecosystems and upwelling regions worldwide, however, little is known about this genus in the southern hemisphere. Whilst Leptocylindrus danicus has been reported from south-eastern (SE) Australia since the 1930s, there has been neither unequivocal species identification nor focused examination of the temporal abundance of Leptocylindrus in this region. Such investigations are crucial in the context of climate change and the strengthening of the East Australian Current, which is expected to result in alterations to the seasonal abundance and distribution of Leptocylindrus along the east Australian coast. Thus we also describe the temporal distribution of Leptocylindrus based on 50 years of records, revealing that this diatom is a key component of the seasonal phytoplankton cycle, with greatest abundance in the austral spring and summer. Using light and transmission electron microscopy and molecular phylogenetics based on the nuclear-encoded ITS1-5.8S-ITS2 rDNA region, our study unambiguously revealed three species, L. danicus, Leptocylindrus convexus and Leptocylindrus aporus from 34 clonal isolates from SE Australia, with the majority (82%) of strains identified as L. danicus. Furthermore, we investigated the growth, auxospore and resting spore formation of the most commonly occurring species, L. danicus, under four temperature and irradiance scenarios. The diatom reached maximum growth rates (mu Max, 1.71 divisions day(-1)) under relatively high temperatures (25 degrees C) and light conditions (100 mu mol photons m(-2)s(-1)) between days 2 and 7 of the experiment. When temperature and light regimes were reduced (18 degrees C, 50 mu mol photonsm(-2)s(-1)) auxospores and resting spores were formed. The rapid growth rate and potential of L. danicus to form auxospores are important survival mechanisms in coastal upwelling systems and likely to result in the continued success of this species in Eastern Australia. The ecological, physiological and evolutionary response of this significant diatom group to further ocean warming should be the focus of future investigations.