Contribution to Taxonomy and Biogeography of Mastogloia (Diatomeae, Bacillariophyceae): A Pantropical Species and a Potential Regional Endemic

Benthic marine diatoms are speciose but vastly underexplored eukaryotic microbes. Diatoms are identified by their intricately ornamented silica cell walls known as frustules, following removal of all organic matter with acid or strong oxidants. When living samples of diatom communities are examined, it is impossible to detect all the species present, as rare ones are easily obscured among the other materials present, and taxonomic identification of living diatoms can be uncertain or impossible, even with isolated cells. These features of diatom taxonomy have important consequences for biogeography, which we illustrate and discuss using new observations from two species. Despite being the mainstay for diatom descriptions, species described by light microscopy (LM) alone may conflate two species or (as in the case presented) lead to spurious new species; both need ultrastructural study to ascertain taxonomic and geographical boundaries. The species studied with scanning electron microscopy (SEM) over the last 45 years by Stephens & Gibson, Pennesi et al., and Lobban under the name of Mastogloia hustedtii is shown to be synonymous with M. grunovii. The former became known in the SEM era to bear both pseudoconopea (longitudinal flaps parallel to the sternum, invisible in LM) and silica plaques on the inner margins of the partecta (chambers on the valvocopulae), with the latter supposedly bearing neither, but there is a single, pantropical/Mediterranean species encompassed in the original description of M. grunovii. A new ultrastructural feature for the genus is reported from this species: marginal chambers formed by laminae over the mantle areolae and the first 2–3 areolae on the valve face. The second species studied, M. meisteri, had been reported a few times from one region based on very rare frustules, which do not meet the first criterion for biogeography: where did they live? Although we, too, did not observe living cells, the number of specimens present is evidence for a living population epiphytic on a Virgin Islands coral reef. The ultrastructure of this species is also shown for the first time. Because absence of evidence is overwhelming in microbial biogeography, the best we can say is that this species is potentially a regional endemic.