Characterizing phototactic behavior at the individual filament level for the filamentous cyanobacterium Nostoc punctiforme

The goal of this project is to characterize the phototactic behavior of individual hormogonia of the filamentous cyanobacterium Nostoc punctiforme. Nostoc punctiforme can differentiate into three different types of cells depending on the conditions of the surrounding environment. One cell type is hormogonia, short, motile filaments. Previous studies have shown that N. punctiforme hormogonia perform positive phototaxis, directed movement towards a source of light. These studies have relied on macroscopic examination of entire colonies, rather than the behavior of individual motile filaments in response to changes in light intensity. In this study, time lapse microscopy has been employed to characterize the phototactic response of hormogonia at the level of individual filaments. Individual hormogonia were recorded before and after exposure to dark periods of varying length and the percentage of filaments that reversed direction following the dark period was quantified. There was substantial variation in the light response of individual filaments. Shorter dark periods (15-30 s) triggered reversals in only a small subset of hormogonia, while longer dark periods (105 s or longer) triggered a reversal in the majority of filaments. However, even these longer dark periods were not sufficient to trigger reversals in 100% of the filaments. Now that experimental protocols and parameters have been established to quantify phototactic behavior at the individual filament level, these techniques can be employed in future experiments to answer novel questions about the phototactic behavior of N. punctiforme, including whether a refractory period exists following a reversal where filaments are unresponsive to light, and a comparison of the behavior of individual filaments of wild-type N. punctiforme and a non-phototactic mutant strain