Phytoplankton Orientation in a Turbulent Ocean: A Microscale Perspective

Abstract

Phytoplankton are by definition autotrophic microorganisms that passively drift with fluid motion. Accordingly, the traditional view of a turbulence-homogenized phytoplankton distribution in the ocean, where cells randomly organize and interact, is deeply rooted in biological oceanography studies. However, increasing understanding of microscopic processes in the ocean is revealing a world of microscale patterns resulting from cell behaviors and fluid-cell interactions that challenges this vision. Autotrophic cells have developed active (i.e., flagella) and passive (i.e., morphological structures and vesicles) motility mechanisms that allow them different degrees of spatial control. Their complex interaction with the ocean physicochemical landscape commonly results in small-scale spatial heterogeneities and non-isotropic orientations that can strongly influence ecosystem level processes. Cell orientation, in particular, is fundamental for key biological functions such as sensing, metabolism, locomotion, chain formation, or sexual reproduction. Moreover, preferential alignment of elongated cells can modulate the propagation of light through the ocean and is fundamental for accurate interpretation of remote sensing data. Innovative observational and experimental techniques (e.g., in situ holography, laser diffractometry, etc.) allowing the subtle analysis of cell-fluid interactions are revealing that, at the microscopic level, organisms present well defined orientation and interaction patterns under prevalent conditions in the sea. Thus, the interplay of biology, fluid dynamics, and optics may shape, by means of anisotropic cell distributions, pivotal cross-scale aspects of phytoplankton ecology.

Automated plankton classification from holographic imagery with deep convolutional neural networks

“…Particularly, the biological pump, a complex mechanism by which anthropogenic carbon gets sequestered to the ocean's interior, is driven by particle/plankton aggregation and sinking (Turner 2015; Briggs et al 2020). Recent studies have shown that plankton orientation in the water column fundamentally alters light propagation in the water and has significant consequences to remote sensing and optical modeling (Marcos et al 2011; Nayak et al 2018 a ; Basterretxea et al 2020). Sudden uncontrolled growth of plankton can lead to the formation of harmful algal blooms, which can be devastating to both aquatic ecosystems and human health (Paerl et al 2011; Visser et al 2016).…”

Method for the determination of preferential orientation of marine particles from laser diffraction measurements

“…Indeed, full quantification of the errors associated with scattering from complex particles requires information on particle orientation. Likewise, cell orientation intervenes in key biological processes such as the formation of thin layers or cell pairing [38,39,43].…”

“…Likewise, in sufficiently quiescent environments, the force of gravity can vertically orient cell chains and particle aggregates [13]. Although evidences of these phenomena in the natural environment are still scarce, recent studies show that preferential orientation of marine particles may be more prevalent than previously believed [36][37][38][39]. Here, we describe a procedure for inferring preferential particle orientation in the sea from the PSD spectrum obtained using in-situ LD.…”

Enhanced Light Absorption by Horizontally Oriented Diatom Colonies

“…Large phytoplankton (>20 µm) contribute substantially to the downward flux of carbon in the ocean, and large size classes are often dominated by diatoms that form cells or colonies with high aspect ratios (length/width > 10) Newton, 1995, 1999;Smetacek, 1999;Tréguer et al, 2018). It has generally been assumed that these non-spherical phytoplankton are randomly oriented throughout the water column due to turbulence (Basterretxea et al, 2020). However, 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).…”

“…In the ocean, shear fields capable of orienting particles can occur within the pycnocline, at the base of the surface mixed layer where phytoplankton biomass is often enhanced (e.g., thin layers or deep chlorophyll maxima, Sullivan et al, 2010b). The resulting orientation can influence light absorption by large phytoplankton and may have important ecological consequences for speciesspecific growth rates, community structure, and the efficiency of the biological pump (Basterretxea et al, 2020).…”