Clonal diversity, population differentiation and bloom dynamics in the centric diatom Ditylum brightwellii

Marine planktonic diatoms are characterized by their wide distribution throughout the world's oceans, with individual species able to flourish across many environments. In terrestrial and freshwater ecosystems, species with extensive ranges commonly develop discrete populations adapted to localized environments. Few physical boundaries to dispersal exist over vast areas of the ocean, potentially limiting comparable genetic differentiation in phytoplankton. Here, a combination of molecular genetic and physiological studies was used to determine the extent of genetic diversity and population differentiation within a diatom species and to explore how genetic variation influences diatom ecology and evolution. To measure genetic variation within field samples of the centric diatom, Ditylum brightwellii, three microsatellite markers were developed and used to obtain DNA fingerprints of individual cells. Water samples collected from two connected estuaries, Puget Sound and Strait of Juan de Fuca, WA, USA, revealed three genetically and physiologically diverse populations: one Puget Sound population sampled repeatedly over two years and two transient populations sampled from Strait of Juan de Fuca waters. Extensive genetic differentiation between populations indicated restricted gene flow and suggested that cells from the Strait were unable to reproduce in Puget Sound. Apparently, selection can overcome genetic homogenization associated with mixing and can partition even planktonic organisms into distinct populations with unique attributes. The Puget Sound population was also sampled daily during a spring bloom in Dabob Bay, Puget Sound, WA. During blooms, asexual growth rate variation among genetically distinct clones could significantly alter the genetic structure of a population, potentially leading to the numerical dominance of few clones. In Dabob Bay, clonal diversity remained high as D. brightwellii cell number increased over one order of magnitude. The resampling of a subset of clonal lineages revealed that at least 2550 lineages composed the blooming population. A significant shift in resampled clones occurred after a windstorm indicating that environmental heterogeneity may prevent the prolonged conditions necessary for selection of individual clones. Ultimately, the formation of distinct populations that maintain high levels of clonal diversity despite a predominantly asexual mode of propagation may allow individual species to occupy, adapt to and thrive across many environments.