Award: OCE-1657332

Diatoms form an important component of the oceanic phytoplankton community and are crucial links in marine ecosystems. They also play an important role in marine biogeochemical cycles and carbon sequestration. Different diatom species have vastly diverse morphologies that affect how they interact with their environment. The origin and function of these diverse forms has long been of interest to biological oceanographers. Many species are colonial, forming long, high aspect ratio chains of cells. These shapes in particular interact with small scale flow fields in ways that may affect their orientation, growth, and sinking rates. Such orientation effects could have important implications for primary production and the biological pump. In this project, the phenomenon of preferential (nonrandom) orientation of elongate, colonial diatoms was characterized through a combination of laboratory experiments, field observations, and optical modeling. A circular Couette flow apparatus was designed and employed to characterize diatom orientation and light limited growth at two different rotation rates, chosen to reflect laminar and turbulent flow regimes. Four different elongate colonial diatoms – Odontella sinensis, Stephanopyxis turris, Skeletonema sp., and Pseudo-nitzschia sp. – with slightly varying morphologies (aspect ratio, cell size/width), were used in the experiments. Several replicates with each species were conducted to generate a significant database. In most cases, growth was higher for colonies oriented perpendicular to the light source, a configuration that should maximize light capture. These results were also compared to Jeffery’s theory which states that a rigid ellipsoidal particle exposed to laminar shear exhibits periodic "flipping" motions in a fixed orbit. Furthermore, with increasing aspect ratios, the particle spends longer time aligned to the horizontal (flow) direction during these rotations. For all different species, strong preferential horizontal orientation was observed at the laminar regime, which increased with increasing aspect ratio (longer) in the diatom chains. These results are consistent with prior observations as well as in excellent agreement with Jeffery’s theory. Laboratory experiment results were also compared to prior field observations. These field datasets, recorded using a biophysical profiling instrumentation suite, consisting of a holographic imaging system and acoustic Doppler velocimeter, were used to characterize diatom orientation and potential associations with local flow structure. Clear evidence of preferential particle orientation, especially for high aspect ratio (long) particles/diatoms, across all stations - always correlated with regions of low shear and turbulence- was obtained, providing the first ever comprehensive observations of this phenomenon in the field. The results agreed well with both the laboratory data and Jeffery’s theory. The modeling study focused on the optical effects of phytoplankton orientation and the potential effects on light harvesting. For this study, in situ measurements of diatom colony orientation from the field data sets were used to model their light absorption efficiency. A geometric optics based modeling approach was used for its computational efficiency and ability to accommodate populations of large particles of various size and aspect ratio. The results indicated that when compared to randomly oriented colonies, horizontally oriented ones resulted in significant increase in absorption of downwelling irradiance. A maximization in the projected colony area when oriented perpendicular to incident light direction leads to this effect. The results highlight the potential effects of diatom orientation on light absorption, and at a broader scale, marine ecosystem structure and function. Broader Impacts: Two graduate students, two postdoctoral researchers and a high school student were trained as part of this project. A special issue (Research Topic) pertaining to small scale spatial and temporal patterns in marine plankton and organisms, conceived and developed over the course of this project, that included 24 peer-reviewed contributions, was published in the journal Frontiers in Marine Science. The large field dataset, consisting of several million holograms, has been processed to create a labelled database of different planktonic species. This database has been used to create novel automated classification routines for rapid identification of different planktonic species from holographic datasets. This database is made available to the general public and scientific community on request. Last Modified: 07/30/2021 Submitted by: Aditya R Nayak