Algal pigments and living textile coatings

Two quite different ideas travel under the same heading. One is colouring textiles with material from algae instead of synthetic dyes. The other is keeping photosynthetic cells alive on a fabric so that the garment continues to fix carbon dioxide. They fail for different reasons, and it is worth separating them.

Algal colour is protein colour

The brilliant colours of cyanobacteria and red algae come from phycobiliproteins: phycocyanin, allophycocyanin and phycoerythrin. Their chromophores are bilins — open-chain tetrapyrroles, chemically related to the products of haem breakdown — attached covalently through thioether bonds to cysteine residues. On their own, free bilins are floppy and only weakly coloured. The intense absorption and, in these proteins, strong fluorescence arise because the protein scaffold holds the tetrapyrrole in an extended, rigid conformation.

That is the constraint in a sentence. The colour is a property of a folded protein, so heat, acid, alkali and prolonged light all destroy it — denature the scaffold and the chromophore relaxes, absorption broadens and fades. Phycocyanin is well known for poor thermal and photostability, and textile processing plus a garment’s service life supply exactly the conditions it cannot survive.

Pigment is not dye

A dye works by having affinity for the fibre: reactive dyes form covalent bonds with cellulose hydroxyls, acid dyes bind ionically to protonated amines in wool and polyamide, disperse dyes partition into polyester above its glass transition. Most algal colourants have no such affinity. They are pigments, applied with a binder that glues them to the surface, and surface-bound colour is judged by different tests — rubbing fastness under ISO 105-X12 and wash fastness under ISO 105-C06 — which it typically passes less well than a fibre-reactive dye. The environmental case for algal colour is real but sits mostly in what is avoided: no heavy-metal mordant, no aromatic amine precursors, lower-temperature application. It is not a claim of better fastness.

Algae also enter textiles a third way, as milled biomass dispersed into a regenerated cellulose spinning dope and spun into lyocell-type fibre. Here the algae is a low-percentage filler embedded in a cellulose matrix; that is why it survives laundering, and also why any bioactivity attributed to it is largely locked inside the fibre.

A living coating has a maintenance budget

Photosynthesis needs light of the right wavelengths, liquid water, dissolved inorganic carbon and mineral nutrients. Cyanobacteria and green algae on a fabric surface can genuinely fix carbon while all four are supplied, but a worn garment supplies none of them reliably. Desiccation is the binding limit: most of these organisms lose photosynthetic competence within hours of drying, and while some desiccation-tolerant strains recover on rewetting, tolerance costs growth rate and pigment content. The quantity of carbon fixed by a thin film of cells over a garment’s life is also small next to the carbon embodied in the fabric itself, and any honest account has to state both numbers.

Colour change in these systems is usually a symptom rather than a function. As cells senesce or bleach, pigment composition shifts and the fabric changes hue — legible as an indicator of the coating’s condition, but not a controllable colour-changing mechanism.