Effects of seawater viscosity and temperature on the movement of the marine dinoflagellate Prorocentrum minimum

It is important to understand how planktonic dinoflagellate movements may be affected by environmental conditions, including those potentially influenced by climate change. Because of their small size, dinoflagellates can be expected to be highly sensitive to changes in viscosity; however, there is currently little understanding of how these organisms and other algae may be regulated by seawater viscosity. Previous work that has addressed the effects of seawater viscosity on single-celled plankton considered unnaturally large viscosity changes from a biomechanical perspective, sometimes without considering temperature effects. We studied the swimming of the dinoflagellate Prorocentrum minimum, a common coastal species, when exposed to environmentally relevant temperature and viscosity changes. P. minimum showed an additive response to seawater viscosity and temperature: cold temperature and high viscosity both slowed swimming speeds. However, seawater temperature and viscosity did not affect the movement direction or linearity of swimming of the dinoflagellates. We argue that temperature-related changes in movement may be partially regulated by a mechanical response to viscosity, which increases at cold temperature. We also propose possible future directions for laboratory and modelling studies.