Morphological and molecular characteristics of selected species ofMinidiscus(Bacillariophyta, Thalassiosiraceae)

Abstract

We report on taxonomically distinguishing morphological features of six diatom taxa from the marine, nanoplanktonic genus, Minidiscus, found in Eastern Canada and neighbouring waters. Four of the six were also investigated using genetic approaches. We found that the culture deposited in the Provasoli-Guillard National Center for Culture of Marine Phytoplankton (CCMP) as CCMP 496 was morphologically closest to the published type image of M. trioculatus, whereas other isolates were morphologically and/or genetically divergent. Consequently, we propose three new taxa. One monoclonal line isolated from the Bay of Fundy is proposed as a new variety, M. trioculatus var. monoculatus. The well-studied nanoplanktonic diatom strain, CCMP 495, currently considered to be 'M. trioculatus' was morphologically and genetically divergent from CCMP 496 and we propose specific status for it, M. variabilis. We isolated a new strain from the Gulf of St Lawrence that was nearly genetically identical in 18S and ITS regions to CCMP 495, but showed distinct morphological differences and we propose a new form for this strain. Culture-based data were supplemented by those from natural seawater whenever possible. Two species (M. comicus, M. chilensis) known from our region, but unavailable in culture, are presented for comparison.

Morphological adaptations to small size in the marine diatom <em>Minidiscus comicus</em>

“…One was the exterior fluted tube-structures of the fultoportulae. Kaczmarska et al 2009 showed that these structures produce chitin filaments. Similar chitin filaments are found in other small Thalassiosira (Martin-Cereceda and Cox 2011) and Cyclotella , and they have been linked to reducing sinking rate (e.g.…”

“…An alternative possibility is that the chitin filaments could interfere with grazing by flagellates and ciliates, because M. comicus lives in a nanno-habitat along with other small species, such as Parmales. Other possible functions might come from an observation by Kaczmarska et al (2009) that cells of a close relative, Minidiscus chilensis, which were collected from surface sediments, had projecting chitin filaments. It is possible that the filaments, which can be over 30 μm long, could help in the resuspension of cells back into the water column by bridging across the boundary layer at the sediment surface.…”

“…It is widespread along the Pacific and Atlantic coasts, as well as in the Adriatic and Mediterranean Seas (Takano 1981, Lange 1985, Hasle and Syvertsen 1997, Tomas 1997, Bérard-Therriault et al 1999, Shevchenko and Orlova 2002, Kaczmarska et al 2009, Percopo et al 2011, Lee et al 2012. Miniduscus comicus belongs to a small marine genus of seven species (Aké-Castillo et al 2001, Kang et al 2003, Quiroga and Chrétiennot-Dinet 2004, Kaczmarska et al 2009). The first report of M. comicus was in large aggregated flocks in Tokyo Harbour (Takano 1981), but since then it has usually been found as solitary cells or in short chains of 2 to 3 cells (Kaczmarska et al 2009).…”

“…Miniduscus comicus belongs to a small marine genus of seven species (Aké-Castillo et al 2001, Kang et al 2003, Quiroga and Chrétiennot-Dinet 2004, Kaczmarska et al 2009). The first report of M. comicus was in large aggregated flocks in Tokyo Harbour (Takano 1981), but since then it has usually been found as solitary cells or in short chains of 2 to 3 cells (Kaczmarska et al 2009). Kaczmarska et al (2009) Quiroga and Chrétiennot-Dinet (2004) found broader diameter ranges of 1.9 to 7.0 μm and Takano (1981) 2.0 to 7.0 μm.…”

“…The first report of M. comicus was in large aggregated flocks in Tokyo Harbour (Takano 1981), but since then it has usually been found as solitary cells or in short chains of 2 to 3 cells (Kaczmarska et al 2009). Kaczmarska et al (2009) Quiroga and Chrétiennot-Dinet (2004) found broader diameter ranges of 1.9 to 7.0 μm and Takano (1981) 2.0 to 7.0 μm. In this study, we used samples of M. comicus collected from the western Mediterranean (see Estrada et al 2014), to investigate how a change from discoid to spherical cell shape in small cells can help offset loss of volume when diameter decreases as part of the life cycle.…”