(601ah) Harmful Algal Bloom (HAB)-on-a-Chip: Development of a Microfluidic Device to Characterize Algal Chemotaxis

Harmful algal blooms (HABs) are one of the most predictable, yet poorly understood, natural phenomena that cause significant health, environmental, and economic issues for coastal states, including Louisiana. HABs, also known as red tides, are characterized by an overabundance of naturally occurring algae; however, the mechanisms utilized by algae to initiate and sustain a bloom are not well known. Previously, it was hypothesized that the rapid accumulation of algae was due to the uncontrolled growth of certain species found in a bloom (e.g., Karenia brevis) as a result of increased nitrogen content in the water column. Recently, it was proposed that the increase in algal mass found in a bloom is more than what can be attributed to algal growth alone, suggesting that the accumulation can partially be explained by the recruitment of algae via tactic migration in response to nutrients and light. The directed migration of cells towards chemical cues, or chemotaxis, is well documented in mammalian cells, but not adequately characterized in algae. These limitations can be attributed to limited technologies available to investigators, mainly large, homogeneous tanks incapable of recapitulating dynamic, HAB-like conditions. Alternatively, microfluidic devices, such as chemical gradient generators, are an increasingly popular method to study chemotaxis due to their low reagent cost, relative ease-of-use, optical transparency, and dynamic control over the environment.

This presentation highlights recent work developing a microfluidic device capable of generate stable, well-characterized chemical gradients without the need to expose the cells to direct flow. This is achieved in a device comprised of three separate channels fabricated into PDMS layered on top of an agarose matrix, analogous to the traditional under-agarose assay. Further, gradients of varying steepness and intensity were generated by altering easily controlled variables such as inlet flow rates, source and sink concentrations, and the composition of the bottom agarose layer. The performance of the device was characterized using fluorescent tracers supplemented with numerical simulations in COMSOL.  Finally, this device was used to track the migration of a model algae species (Chlamydomonas reinhardtii) in response to several external nutrients (e.g., nitrates, ammonia, and phosphates) under various environmental conditions (e.g., light intensity, temperature, and salinity) to ascertain how external nutrient gradients influence the direction and persistence of chemotaxing algae. Such controlled studies are not possible in situ, resulting in a new method to assess how individual and combinations of environmental factors contribute to HAB formation.