EFFECTS OF MULTIPLE SPECIES ON MOTILITY IN DIATOM ASSEMBLAGES

Diatoms are unicellular algae that, like other phototrophs, depend on light in order to survive. Many diatoms are known to have regulated motile responses to light, giving them a competitive advantage in their complex community, often containing many other species of algae. In order for similar diatom species to coexist in the same assemblage it is thought that each species will resource partition; a behavioral response that allows closely related species to be successful and cohabitate by using slightly different resources. Many experiments have demonstrated that other organisms exhibit this behavior, although the effect of co-existing diatom species on motility responses to irradiation have not been studied. Multiple species-specific motility experiments have been previously investigated (Cohn & Weitzell 1996, Cohn 2001) with three freshwater, pennate diatom species, Craticula cuspidata (Kützing) D.G. Mann, Pinnularia viridis (Nitzsch) Ehrenberg, and Stauroneis phoenicenteron (Nitzsch) Ehrenberg. This research, extending the work of previous experiments, explores the effects of the presence of multiple diatom species on motility for these three diatom species in order to determine whether they have behavioral differences that could potentially result in resource partitioning. Experiments were performed using an epi-illumination microscope to irradiate gliding diatoms in the leading or trailing end with blue (470 nm) or red (650 nm) light at high irradiation (ca. 105 µmol m-2s-1). When placed in the presence of other species, only one species, S. phoenicenteron, had statistically significant differences in the average direction change response times to blue irradiation at the leading end in the presence of other species. The presence of C. cuspidata alone resulted in a 2 fold increase in response times for S. phoenicenteron, while the addition of P. viridis alone caused no significant change in response times. However, such changes were particularly significant for S. phoenicenteron in the presence of both C. cuspidata and P. viridis, which resulted in a 3 fold increase in response times. To determine if the altered response of S. phoenicenteron was dose-dependent, new motility experiments were preformed using samples with different percentages of C. cuspidata and S. phoenicenteron and, similar to the earlier motility experiments, diatoms were irradiated at the leading end with high intensity blue light. These experiments resulted in motility differences for S. phoenicenteron in the presence of C. cuspidata, where the average direction change response times increased as a function of increasing percentage of C. cuspidata cells in the sample. Specifically, the response times for single-species samples of S. phoenicenteron were significantly longer from response times of all samples containing percentages of C. cuspidata greater than 10%, and samples that contained 90% of C. cuspidata cells showed a 4 fold increase in response times for S. phoenicenteron. These results suggest that the behavior of some diatom species is altered in the presence of other species, and demonstrates how similar diatom species might differentially respond to resources. The observed effects of multiple species on motility suggest how some species might compete for slightly different resources in an algal community, such as directing cells into different areas of light wavelength or intensity. This research suggests potential areas for future studies, such as analysis of diatom stratification in different natural algal assemblages that could further determine the role of multi-species interactions in resource partitioning. Understanding such interspecies behavior and algal resource partitioning could lead to better management of healthy, stable aquatic ecosystems