The Genetic Diversity of Mesodinium and Associated Cryptophytes

Abstract

Ciliates from the genus Mesodinium are globally distributed in marine and freshwater ecosystems and may possess either heterotrophic or mixotrophic nutritional modes. Members of the Mesodinium major/rubrum species complex photosynthesize by sequestering and maintaining organelles from cryptophyte prey, and under certain conditions form periodic or recurrent blooms (= red tides). Here, we present an analysis of the genetic diversity of Mesodinium and cryptophyte populations from 10 environmental samples (eight globally dispersed habitats including five Mesodinium blooms), using group-specific primers for Mesodinium partial 18S, ITS, and partial 28S rRNA genes as well as cryptophyte large subunit RuBisCO genes (rbcL). In addition, 22 new cryptophyte and four new M. rubrum cultures were used to extract DNA and sequence rbcL and 18S-ITS-28S genes, respectively, in order to provide a stronger phylogenetic context for our environmental sequences. Bloom samples were analyzed from coastal Brazil, Chile, two Northeastern locations in the United States, and the Pribilof Islands within the Bering Sea. Additionally, samples were also analyzed from the Baltic and Barents Seas and coastal California under non-bloom conditions. Most blooms were dominated by a single Mesodinium genotype, with coastal Brazil and Chile blooms composed of M. major and the Eastern USA blooms dominated by M. rubrum variant B. Sequences from all four blooms were dominated by Teleaulax amphioxeia-like cryptophytes. Non-bloom communities revealed more diverse assemblages of Mesodinium spp., including heterotrophic species and the mixotrophic Mesodinium chamaeleon. Similarly, cryptophyte diversity was also higher in non-bloom samples. Our results confirm that Mesodinium blooms may be caused by M. major, as well as multiple variants of M. rubrum, and further implicate T. amphioxeia as the key cryptophyte species linked to these phenomena in temperate and subtropical regions.

The Importance of Mesodinium rubrum at Post-Spring Bloom Nutrient and Phytoplankton Dynamics in the Vertically Stratified Baltic Sea

“…The analysis is based on high-resolution measurements and sampling in the surface layer along the ferry route Tallinn-Helsinki complemented with vertical profiling and sampling through the water column at one station close to the ferry line. We recognize that M. rubrum belongs to a species complex (Johnson et al, 2016) and that our data may include M. major and/or multiple variants of M. rubrum. However, since we did not measure the diversity of genetic variants, we will refer to all observed Mesodinium ciliates as M. rubrum.…”

Ciliate micrograzer dynamics of the New England Shelf

“…Tintinnids, in particular, because of their distinct, but diverse morphology, are a model group for these 'hybrid' studies (Santoferrara et al, 2012. Mesodinium spp., the autotrophic ciliate, presents cryptic diversity (Garcia-Cuetos et al, 2012a;Johnson et al, 2016) through a species complex of 8 different subclades, but contrastingly also presents various morphologies.…”

“…On all three dates when we observed M. rubrum subclade A, it represented >75% of the sequences, Mesodinium spp. cells were on average >20 µm, and temperatures were <10 C. M. rubrum subclade A has been found in environments, such as the Gulf of Finland and California Current, at temperatures >10 C (17.4 and 12.8 C, respectively) (Johnson et al, 2016), though never dominating samples. This may indicate it only dominates in cold temperatures.…”

“…McMurdo Sound, Antarctica (Johnson et al, 2016) further suggesting its affinity for colder waters. These results suggest that larger sized morphotypes observed at MVCO belong to specific M. rubrum/major subclades (A and D), with subclade A possibly occupying a cold water temperature niche.…”

“…Mesodinium spp. has been noted to select for certain types of cryptophytes (Herfort et al, 2011;Johnson et al, 2016), which may complicate these relationships.…”