Development of a Microfluidic Device to Quantify Algal Chemotaxis

The directed migration of algae in response to external chemical cues, or chemotaxis, can play an important role in the development of harmful algal blooms (HABs). These blooms pose a significant concern to human health and the environment due to the decrease in oxygen levels and increase in toxin production. It is well established that many environmental factors influence the formation and development of a bloom; however the behavior of algae within the bloom has not been thoroughly studied. One area of interest is the diel vertical migration and recruitment of algae to a bloom, both of which are governed by chemotaxis. Due to the limitations of current technology and assay techniques, direct measurement of algal chemotaxis has not been completely studied. Microfluidic devices, which allow for the precise control of the cellular microenvironment have been used in several studies to quantify the chemotactic response of other organisms. However, to date, this technology has rarely been adapted to the study of algal chemotaxis. Here, we present the development and optimization of a â??flow-freeâ?? microfluidic gradient generator for the study of algal chemotaxis. The device consists of an agarose hydrogel bottom layer and a PDMS top layer imprinted with three parallel microchannels. A chemical gradient is developed by flowing media containing a source compound (ammonia for algae) in an outer channel and a sink solution in the opposing outer channel, providing two constant-concentration boundary conditions. An important feature of this device is that the center is a flow free channel, which exposes cells to a stable, linear gradient in the absence of direct flow. As a proof-of-concept, we demonstrate the utility of the device by exposing Chlamydomonas reinhardtii, a model unicellular green alga, to a linear nitrogen gradient. Preliminary findings indicate that both gradient intensity and steepness influence the tactic response of the cells. Additionally, the migratory behavior of C. reinhardtii was found to be strongly influenced by the presence of ambient light. The direct observation of algal chemotaxis has not been possible to date, thus the results presented here provide new insight into the dynamic migratory behavior of algae.