Shaping Up: Toward Considering the Shape and Form of Pollutants.

H manufacture a dizzying array of chemicals and products, many of which enter the environment, and the sum of their effects can be regarded as a powerful driver of global change. Research on pollutants often centers on their chemical nature, for example, their composition, solubility, toxicity, environmental availability, or persistence. An aspect that is comparatively less well appreciated is the physical shape (2-dimensional) or form (in three dimensions) of particulate items released into the environment. Nevertheless, when studying environmental effects of different materials, shape and form are important factors. The fact that shape plays an important role is powerfully illustrated by microplastic particles in the environment. In soil, naturally occurring primary particles, like sand, silt, and clay, are irregularly shaped or roughly spherical, bacteria are spherical or rod-shaped, fungal hyphae are branched linear objects. When adding microplastics of roughly similar shape to the particles already present, for example spherical particles or fragments, they might behave like primary particles. However, there are other shapes, such as films and fibers that diverge in form from existing particles. There is no equivalent to films (thin, planar objects), and also foams (porous, expanded cellular plastics). Fibers are superficially reminiscent of fine roots or hyphae, but are typically much coarser, not branched, and more rigid. When comparing the effects of these different types of forms, fibers have repeatable effects on a number of soil physical properties, whereas more spherical particles have a less clear effect irrespective of chemical composition. The mechanisms are not entirely clear. Perhaps fibers, when incorporated into soil aggregates, introduce an instability that makes soil aggregates more prone to disintegration. Also, films were shown to produce pathways for water flow that are unparalleled in any natural structures and could represent predetermined breaking points in the soil. Shape may also influence environmental transport; for example, it can be expected that fibers are less rapidly transported down a soil profile than beads; and shape may signal if an object is likely a food item or not in aquatic systems. The issue with shape is likely much broader, and occurs at different spatial scales. At a smaller scale, nanoparticles differ in their toxicity based on their shape (e.g., carbon nanotubes, -fibers, and -particles;). Asbestos is another example, where this naturally occurring mineral has toxic properties when in fiber form, and manufactured mineral or vitreous fibers are also a concern because of their shape. At a larger scale, six pack holders, straws, bags, long lines, or nets in the aquatic environment differ to different degrees from shapes that naturally occur; these items largely function as traps for animals based on their form or cause obstructions. Even though many shapes also occur in nature (e.g., nets by spiders, films as microbial biofilms, and tubes by vascular plants), engineered materials have different sizes and properties. Scale matters when defining the shape of a material. For instance, a brick that is made of mortar and used for construction has a rectangular shape at the scale of >10 m. At the same time, it can be regarded as flat planar object at a smaller scale (10−2 m), for example for a small insect moving on its surface. At a far smaller microbial scale (10−4 m), the shape is neither rectangular nor planar, but would present a very complex heterogeneous 3D structure because of pores. Shape is a scale-dependent feature, and thus the effects of object shape and interactions with other objects depend largely on the scale of interest. Are there thus general properties of shape and form of materials released into the environment that should be examined in their own right? We offer the shape dissimilarity hypothesis: the more dissimilar in shape to the population mean shape of naturally occurring items at a given scale, the stronger any effects of any human-made objects can be (Figure 1). In the case of microplastics, this could be specifically tested by using objects with identical chemical properties and equivalent sizes, while systematically varying shapes. Other properties of items (e.g., rigidity, surface charge, texture) may attain significance specifically in combination with shape, for example rigidity in combination with linear