Division time (td) for in situ growth measurements demonstrates thermal ecotypes of Karlodinium veneficum

Abstract

The toxic dinoflagellate Karlodinium veneficum forms fish killing blooms in temperate estuaries worldwide. These blooms have variable toxicity which may be related to bloom stage and in situ growth rates of the constituent K. veneficum cells. Measurement of in situ growth rates is challenging and methods such as the mitotic index technique require knowledge of the dynamics of cell division. In order to better understand these dynamics, we determined the duration of cell division (t_d) in four geographically distinct laboratory strains of K. veneficum at three different environmentally relevant temperatures. The results demonstrated that the t_d value for each strain, growing at strain-specific optimal temperatures, was 1.6 ± 0.1 h. This value corresponded to a range of growth rates from 0.17 ± 0.08 d^-1 to 0.62 ± 0.07 d^-1. Equivalent values of t_d spread across four geographically distinct laboratory strains and a nearly fourfold range of growth rates implies that 1.6 h represents the t_d value of K. veneficum. Additionally, temperature conditions yielding this value for t_d and the highest growth rates varied among strains, indicating cold-adapted (Norway), warm-adapted (Florida, USA), and eurythermally-adapted (Maryland, USA) strains. These differences have been apparently retained in culture over many years, indicating a conserved genetic basis that suggests distinct thermal ecotypes of the morphospecies K. veneficum. This knowledge together with the first estimate of t_d for K. veneficum will be useful in future field studies aimed at correlating bloom toxicity with in situ growth rate using the mitotic index technique.

Keywords: Cell cycle; Cytokinesis; Dinoflagellate; Duration of mitosis; Ecotype; Harmful algal bloom; In situ growth rate; Karlodinium veneficum; Mitotic index.