Degree

Doctor of Philosophy (PhD)

Department

Biological Sciences

Document Type

Dissertation

Abstract

Analysis of ammonium chemotaxis in Chlamydomonas reinhardtii is largely hindered, compared to that of phototaxis, despite equal importance on flagellated microalgal physiology. A major contribution of this shortfall is the lack of proper assay method. We developed a simple Petri dish assay method in which light is homogenously exposed while patterns of the cellular migration are tracked with a function of time. Using the method, new findings were revealed. First, this research presented that a strain lacking the eyespot organelle required for light gradient-sensing exhibits similar chemotactic behavior compared to a wild-type strain, suggesting Chlamydomonas sense an ammonium gradient not in the eyespot but in other compartments. A strain lacking AMMONIUM TRANSPORTER 4 (AMT4), the major ammonium transporter to transport from the exterior environment, also exhibits similar chemotactic behavior compared to the wild-type strain. This suggests that cellular uptake of ammonium is not involved in ammonium chemotaxis in Chlamydomonas as is the case in bacteria. Second, and most importantly, this research presented the first observations of collective migration in Chlamydomonas. In a Petri dish, Chlamydomonas collectively migrate toward the ammonium source while it does not migrate this way during phototaxis. Migration speed in ammonium chemotaxis is about 100-fold slower than that in phototaxis, suggesting the steering mechanism in ammonium chemotaxis largely differs from that in phototaxis. Collective migration is enhanced by blue light, but phototropin, one of the blue light receptors in Chlamydomonas, is not involved in the enhancement. A strain carrying the AGGREGATE 1 (AGG1) gene mutation (agg1-) exhibits more robust collective migration than strains that carry the wild-type AGG1 gene. This suggests that the AGG1 protein may function as a suppressor of collective migration during ammonium chemotaxis. Lastly, this research presented that ammonium chemotaxis is disrupted by adding a reactive oxygen species (ROS)-generating agent in a Petri dish but not by adding ROS-quenching agents. On the other hand, ROS-generating and quenching agents caused the cells to migrate toward and against light, respectively, in phototaxis. This suggests that maintaining a reduced status of the cells may be required to sense an ammonium gradient while the sign of phototaxis is determined by the redox status of the cells. These research results suggest that significant parts of the signal transduction and steering mechanism in ammonium chemotaxis are independent from that in phototaxis but may synergistically function depending on the cellular redox status.

Date

4-16-2024

Committee Chair

Naohiro Kato

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