They are young, and they are many: dating freshwater lineages in unicellular dinophytes

Dinophytes are one of few protist groups that have an extensive fossil record and are therefore appropriate for time estimations. However, insufficient sequence data and strong rate heterogeneity have been hindering to put dinophyte evolution into a time frame until now. Marine‐to‐freshwater transitions within this group are considered geologically old and evolutionarily exceptional due to strong physiological constraints that prevent such processes. Phylogenies based on concatenated rRNA sequences (including 19 new GenBank entries) of two major dinophyte lineages, Gymnodiniaceae and Peridiniales, were carried out using an uncorrelated molecular clock and five calibration points based on fossils. Contrarily to previous assumptions, marine‐to‐freshwater transitions are more frequent in dinophytes (i.e. five marine‐freshwater transitions in Gymnodiniaceae, up to ten but seven strongly supported transitions in Peridiniales), and none of them occurred as early as 140 MYA. Furthermore, most marine‐to‐freshwater transitions, and the followed diversification, took place after the Cretaceous–Paleogene boundary. Not older than 40 MYA, the youngest transitions within Gymnodiniaceae and Peridiniales occurred under the influence of the Eocene climate shift. Our evolutionary scenario indicates a gradual diversification of dinophytes without noticeable impact of catastrophic events, and their freshwater lineages have originated several times independently at different points in time

Late Holocene sea-level rise in Tampa Bay: Integrated reconstruction using biomarkers, pollen, organic-walled dinoflagellate cysts, and diatoms

A suite of organic geochemical, micropaleontological and palynological proxies was applied to sediments from Southwest Florida, to study the Holocene environmental changes associated with sea-level rise. Sediments were recovered from Hillsborough Bay, part of Tampa Bay, and studied using biomarkers, pollen, organic-walled dinoflagellate cysts and diatoms. Analyses show that the site flooded around 7.5 ka as a consequence of Holocene transgression, progressively turning a fresh/brackish marl-marsh into a shallow, restricted marine environment. Immediately after the marine transgression started, limited water circulation and high amounts of runoff caused stratification of the water column. A shift in dinocysts and diatom assemblages to more marine species, increasing concentrations of marine biomarkers and a shift in the Diol Index indicate increasing salinity between 7.5 ka and the present, which is likely a consequence of progressing sea-level rise. Reconstructed sea surface temperatures for the past 4 kyrs are between 25 and 26 ° C, and indicate stable temperatures during the Late Holocene. A sharp increase in sedimentation rate in the top ∼50 cm of the core is attributed to human impact. The results are in agreement with parallel studies from the area, but this study further refines the environmental reconstructions having the advantage of simultaneously investigating changes in the terrestrial and marine environment. © 2009 Elsevier Ltd