Award: OCE-1657314

The overarching goal of our collaborative project is to investigate whether horizontal orientation of elongate phytoplankton cells and colonies within typical oceanic shear flows enhances their light harvesting abilities in light limited environments, thus conferring a competitive ecological advantage. Our approach included innovative laboratory experiments, custom-designed instrumentation and analysis of field data. To evaluate our hypothesis in the laboratory, uni-algal isolate of Stephaopyxis turris was grown under controlled conditions simultaneously for three different rotation rates in the Couette chamber. Holographic imaging was used to determine colony orientation, length, and concentration over 25 days. Holograms were reconstructed at 1 mm intervals and an extended depth of field image with all colonies in focus was generated by combining reconstructed planes. Laminar and turbulent cases showed an increase in individual diatom chains across the experiment. Higher growth rates were achieved in the turbulent case compared to the laminar case, consistent with previous studies. Our study was unique in the ability to determine the orientation of the diatom chains in these different flows. Our lab studies clearly showed that strong preferential horizontal orientation was seen in the laminar case. We further sub-sampled the diatoms according to their aspect ratios (ratio of major axis to minor axis). Once diatom chains are segregated according to their aspect ratio, the trends again became clear. In the laminar case, stronger horizontal preferential orientation with increasing aspect ratio was seen. On the other hand, there was no discernable difference in the turbulent case. All orientation results discussed here agreed very well with field observations by our group as well as with theory. Modeling and field studies were conducted that focused on the optical effects of phytoplankton orientation and the potential effects on light harvesting. In situ measurements of diatom colony orientation previously acquired with a HOLOCAM instrument were used to model their light absorption efficiency. A geometric optics based modeling approach was used for its ability to accommodate populations of large particles of various size and aspect ratio. Modeling of phytoplankton absorption was performed for several estimated intracellular chlorophyll concentrations over a continuous profile from the surface to ~25 m depth. Holographic images revealed a thin layer of Ditylum brightwellii with strong horizontal orientation. Orientation of colonies in this thin layer enhanced their absorption by up to 24.5% verifying our study hypothesis. Last Modified: 06/18/2024 Submitted by: MelissaOmand