Quantifying diatom aspirations: Mechanical properties of chain‐forming species

Abstract

Diatoms developed a variety of mechanisms to form chain‐like colonies, resulting in diverse morphologies and bulk mechanical properties. These properties affect translation, rotation, and deformation of colonies in ambient flows as well as their susceptibility to breakage by flow‐ and grazer‐induced forces. Morphological characteristics of diatom chains have been extensively studied, yet no studies have examined their mechanical properties. We studied the flexibility of four morphologically distinct species (Lithodesmium undulatum, Stephanopyxis turris, Lauderia annulata, and Guinardia delicatula) by measuring their deflections when held across a capillary tip in developing pipe flow and applying simple beam theory and a finite‐difference analysis of curvature to calculate flexural stiffness. Flexural stiffness varies greatly, with at least four orders of magnitude difference among the examined species (from 1.7 × 10−13 N m2, the most rigid, to 1.3 × 10−17 N m2, the most flexible), but two other species (Melosira nummuloides and a Thalassiosira sp.) were too flexible to measure with our apparatus. Vulnerability to breakage by flow also varied between species and, for species with heavily silicified joints between cells, was enhanced under nutrient depletion. These results highlight yet another attribute underlying the biodiversity of diatoms and their potential for utilizing highly differentiated ecological niches. Quantitative information from this study can now be used in the design of more mechanically realistic models that capture the dynamic coupling between elastic particles and flow to study diatom–flow interactions and their effects on nutrient acquisition, encounter with grazers, aggregate formation, and settling.

Hydrodynamics of diatom chains and semiflexible fibres

“…A least-squares fit is used to determine the macroscopic bending rigidity A. Alternatively, one may use the above procedure to determine spring stiffness constants t kl that will result in a prescribed macroscopic bending rigidity A. This is the approach that we will take in building the computational fibres to match the bending rigidities measured for diatom chains in [8]. For simplicity, in the model diatom chains discussed herein, we choose the same spring stiffness t kl ¼ S 0 for each of the linkages in the structure.…”

“…Moreover, we argue that the dynamics of complex elastic fibres cannot always be predicted by assuming that they are homogeneous, linear beams. In the following sections, we will describe the structure of the computational diatom chain and the computational experiments that were performed to mirror the laboratory experiments in [8]. We will then examine how the orbits of these semiflexible fibres in shear flow depend upon their elastic properties and the connectivity properties of the network of points and springs that build them.…”

“…The local flow environment around phytoplankton affects processes such as nutrient uptake and predator -prey interactions [6,7]. While diatoms are unicellular, some types have evolved the ability to form colonies in the form of chains using a variety of linking structures that hold the cells together (figure 1) [8]. Figure 1a,b shows micrographs of Lithodesmium undulatum and Guinardia delicatula.…”

“…The correspondence between the mechanical properties of the model diatom chains in [9] and real diatom chains could not be directly assessed because, at that time, no laboratory measurements of bending rigidity or flexural stiffness of diatom chains were available. However, recently, Young et al [8] performed experiments that quantified the flexural stiffness of different types of diatom chains by measuring their deflections under flow when held across a capillary tip. A simple beam theory was applied to approximate the flexural stiffness given the measured deflections and imposed volumetric flow rate.…”

“…A simple beam theory was applied to approximate the flexural stiffness given the measured deflections and imposed volumetric flow rate. Here, we describe the calibration of these laboratory experiments [8] with the development of a full three-dimensional computational model of an inhomogeneous elastic chain. Having verified that the computational elastic structure has captured elastic properties of true diatom chains, we examine the dynamics of these chains in shear flow.…”