Impact of cell shape and chain formation on nutrient acquisition by marine diatoms

Diatoms have evolved a multitude of morphologics, including highly elongated cells and cell chains. Elongation and chain formation have many possible functions, such as grazing protecticn or effects on sinking. Here, a model of diffusive and advective nutrient transport is used to predict impacts of cell shape and chain length on potential nutrient supply and uptake in a turbulent environment. Rigid, contiguous, prolate spheroids thereby represent the shapes of simple chains and solitary cells. At scales larger than a few centimeters, turbulent water motions produce a more or less homogeneous nutrient distribution. At the much smaller stall: of diatom cells, however, turbulence drcates a roughly linear shear and nutrients can locally become strongly dl=pleted because of nutrient uptake by phytoplankton cells. The potential diffusive nutrient supply is greater for elongated than for spherically shaped cells of similar volume but lower for chains than for solitary cells. Although the relative increase in nutrient transport due to turbulence is greater for chains, single cells still enjoy a greater total nutrient supply in turbulent cnvironmerits. Only chains with specialized structures, such as spaces between the cells, can overcome this disadvantage and even obtain a higher nutrient supply than do solitary cells. The mod=1 results are compared to laboratory measurements of nutrient uptake under turbulent conditions and to effects ol’ sinking. Diatoms have developed various cell shapes as well as chains of a variety of different structures and sizes. Of the many possible functions of chain formation, Fryxell and Miller (1978) summarized protection from grazing, increased concentrations of possible growth factors, improved fertilization possibilities, and changes in the sinking behavior. Here we consider impacts on potential diffusive and advective nutrient supply and uptake by differently shaped diatom cells and cell chains in a turbulent environment. Spherical and elongated solitary cells as well as cell chains are represented by prolate spheroids characterized by a small radius c and an aspect ratio T(, (Fig. 1). This restricts direct application of the results to solitary cells and compact, short cell chains of rather simple shape and approximately circular cross section. Expansion toward more complicated shapes and longer chains is possible only in a qualitative manner. All symbols are explained in Table 1. At length scales larger than a few centimeters turbulent water motions produce a more or less homogeneous nutrient concentration distribution; however, at the much smaller scales of most phytoplankton, nutrient uptake can locally deplete the nutrients (Fig. 2) in the vicinity of the cells (Droop 1973). Nutrients are resupplied by diffusive and advective nutrient transport toward the cell surface (Fig. 2). Motions of phytoplankton cells or the water surrounding them can enhance the advective nutrient supply (Mann and Lazier 199 1; Jumars et al. 1993; Ki@rboe 1993; Karp-Boss et al. 1996). The relative contribution of advection to total transport is characterized by the nondimensional P&let number Pe, defined as (Boucher and Alves 1959)