Pigmentation is a straightforward coloration: the color is the same from every viewing angle. In contrast, structural coloration is in the eye of the beholder: the colors are dependent on the viewing angle.
We previously covered the knack of various bacteria for synthesizing true blue pigments, which frequently serve defensive purposes. Flavobacteria turn out to have a penchant for structural coloration: they can assemble into 2D photonic crystals, which are periodic assemblages that reflect light only at certain angles. Flavobacterium strain Iridescent 1 (IR1) cells, isolated from a harbor in the Netherlands, arrange into dense hexagons that together interfere with light to appear a flashy green. Figure 1 confirms that they owe their vivid pizzazz to structural coloration, since the apparent color of each colony changes with the viewing angle!
This organization is no easy feat. Self-assembling into these highly regular structures consumes both genetic coding capacity and metabolic resources – so why do it?
A study from 2018 by Johansen et al. discovered a link between the pili-dependent gliding motility characteristic of Flavobacteria and structural coloration. IR1 mutants with defective motility were unable to self-organize into periodic hexagonal structures and were consequently dull-colored. The authors then sifted out genes involved in structural coloration with a transposon mutagenesis screen. The majority of these hits were involved in gliding motility, plus some with no previously assigned function.
The presence of macroalgae also influences the bacteria, which naturally degrade polymers from algae in their estuarine environments. In the 2018 study, the IR1 colonies adjusted their periodic structure – and hence, their apparent coloration – depending on what algal polymer was available. The presence of fucoidan, carrageenan, or starch resulted in red, blue, and purple coloration in addition to the normal green. The removal or addition of powdered fucoidan caused the loss or restoration, respectively, of structural coloration. Fucoidan again plays a curious role (we previously covered other fucoidan-degrading bacteria here).
A more recent study from 2020 by Hamidjaja et al. linked structural coloration to another trait of the IR1 Flavobacteria : predation. Like their relative Flavobacterium johnsoniae, the IR1 cells can invade adjacent colonies and prey upon the cells in a contact-dependent manner. When the researchers spotted IR1 and prey bacteria onto agar, IR1 surrounded and invaded the prey colonies, and the purple and red coloration typical of growth on media containing fucoidan shifted to a bright green. This indicated that IR1 was self-organizing into 2D photonic crystals during predation.
As expected, mutants that could not glide were unable to penetrate prey colonies. More surprisingly, mutants that could glide but were unable to produce structural coloration were also unable to penetrate prey colonies. So, the ability of the cells to organize into photonic crystals is required for the ability to engage in predatory behavior. This 2020 study marks the first clear-cut link between such structural coloration and biological relevance in bacteria.
IR1 also appears to prefer to glide along edges – like those of a glass slide provided by researchers, or the edges of prey colonies – rather than a flat expanse of agar. Tracking along edges in this way may be a strategy for quickly surrounding prey. IR1 isn't picky about its food; the range of prey includes both Gram-positive and Gram-negative bacteria as well as the yeast Candida albicans, but its appetite also isn't indiscriminate. While IR1 approached and surrounded other flavobacterial colonies as quickly as it did prey colonies, it did not prey on its kin, indicating that it prefers to act (surround other colonies) before thinking (discern between self and other).
What of the flipped situation, in which the hunter becomes the hunted? The authors isolated Rhodococcus spp PIR4, which preys on IR1. When spotted on agar, PIR4 invaded and degraded both wild-type IR1 and IR1 deficient in structural coloration. Organization of IR1 into photonic crystals offered no protection against predation – evidently, a good offense isn't always the best defense.
IR1 isn't the only case of structural coloration in bacteria. This trickery of light has also been observed in other Flavobacteria such as Cellulophaga lytica, isolated from anemone. Structural coloration in all of these bacteria is an intrinsically multicellular behavior, as it is only possible at the colony level and not in individual cells. Evidently coloration has snuck into the multicellular predator's toolkit, whether in eukaryotes like tigers and vipers or in bacteria.