Abstract
Through \textit{in situ} holographic imaging of undisturbed water, diatom colonies with high aspect ratios have been found to exhibit preferential horizontal orientation within high biomass subsurface layers. We analyzed holographic video to determine the abundance, size, and orientation of colonies over several vertical profiles from the surface to $\sim$25 m depth. A geometric optics model based on these measurements was then used to estimate light absorption by phytoplankton throughout the water column. Results show a substantial increase in absorption of downwelling irradiance (up to 24.5\%) for horizontally oriented colonies when compared to randomized orientations. The effect can be attributed to maximization of the projected area of colonies when oriented perpendicular to the direction of incident light. Formation of high aspect ratio cells and colonies may represent an adaptation to maximize light harvesting in low light environments through interaction with the low velocity shear fields commonly found along pycnoclines at the base of the surface mixed layer. The effect of orientation on light absorption by large chain forming diatoms could influence their abundance and distribution in the ocean as well as the broader structure and function of marine ecosystems.
Highlights
Marine phytoplankton, responsible for approximately half of global primary production, are fundamental components of marine food webs and play a critical role in biogeochemical cycles linked to climate change (Field et al, 1998)
Preferential horizontal orientation of long diatom chains was clearly visible in extended depth of field (EDF) images acquired within the thin layer (Figure 5)
The optical model used in this study showed an increase in absorption of 4.5–24.5% for populations of horizontally oriented diatom colonies relative to random orientations
Summary
Responsible for approximately half of global primary production, are fundamental components of marine food webs and play a critical role in biogeochemical cycles linked to climate change (Field et al, 1998). It has generally been assumed that these non-spherical phytoplankton are randomly oriented throughout the water column due to turbulence (Basterretxea et al, 2020). Recent observations using in situ holographic imaging suggest that preferential horizontal orientation may be a common occurrence in the ocean (Talapatra et al, 2013; Nayak et al, 2018). Theoretical and laboratory based experimental studies have shown that non-random orientations result from interaction of particles with low velocity shear fields (Jeffery, 1922; Karp-Boss and Jumars, 1998). Shear fields capable of orienting particles can occur within the pycnocline, at the base of the surface mixed layer where
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