Understanding cyst-theca relationships in freshwater dinoflagellates

The resting cysts of several extant species of freshwater dinoflagellate assigned to three genera (Peridinium, Parvodinium, and Gonyaulax) are known to fossilize. They are found as part of the palynofacies in pollen preparations and have been shown to record paleolimnological conditions, e.g. productivity, pH, temperature. The cysts produced by freshwater dinoflagellates also appear to be of taxonomic importance- perhaps even more so than the cysts of their marine counterparts are to the taxonomy or marine dinoflagellate species. Despite these attributes they have received little attention, in large part because cyst-theca relationships remain relatively poorly known (with resting cysts described for only 84 of the ~350 dinoflagellate species known from freshwater environments) (Mertens et al., 2012). Although cyst-theca relationships have commonly been inferred on the basis of co-occurrence in the same water body, the motile thecate population at any given time may bear little resemblance to the long-lived seed bank of resting cysts. In Lake Xiaolongwan, China, for instance, small, brown, unornamented cysts were abundant in recent sediments, but these did not appear to be have been formed by either species of dinoflagellate found in the water column of that lake. Chu et al. (2008) suggested a possible affinity with Parvodinium inconspicuum (Lemmermann) Carty based on gross thecal morphology, but the cysts in the sediment are more similar to cysts of Parvodinium umbonatum (Stein) Carty identified by Tardio et al. (2006) than to those assigned to Parvodinium inconspicuum by McCarthy and Krueger (in press). This latter cyst-theca relationship was established due to the extraordinary preservation of thecae in varved sediments from Crawford Lake as well as inadvertent germination of cysts from a few samples during processing. The germination of cysts in these samples also allowed the assignment of cysts previously identified as “Peridinium willei small” to Peridinium volzii Lemmermann. The most rigorous way to establish cyst-theca relationships in extant dinoflagellates is by controlled germination of cysts together with single-cell PCR. This also allows the establishment of LSU/SSU rDNA-based phylogenies. This approach allowed confirmation that the affinity of large cysts common in sediments from Georgian Bay, Canada, is to Peridinium willei Huitfeldt-Kaas (McCarthy et al., 2011). Recently, LSU and SSU rDNA analysis of single cysts of Peridinium wisconsinense Eddy demonstrated that this species is very different from other species of Peridinium Ehrenberg, and is close to the species Chimonodinium lomnickii Craveiro et al. These very distinctive cysts are restricted to North American lakes, but this species may date back to the Miocene based on nearly identical cysts described from the nonmarine Tertiary of Alaska as Geiselodinium tyonekensis sp. nov. by Engelhardt (1976).

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