Powerful Pigments: Carotenoids and PNS Bacteria

Updated: Oct 6

Pigments are organic molecules of unbelievable power. Bacteria, archaea, algae, plants and other photoautotrophs create pigments to capture photons, thereby deriving metabolic energy from light. Without these miraculous compounds, light would remain forever elusive to biology. There would be no photosynthesis and very little life as we recognize it.

Different pigments absorb different spectra of light. Some pigments are common, such as the near-universal chlorophyll a which makes most land plants green. Other pigments are more rare and are of profound physiological value to a wide variety of organisms. Carotenoids are a set of pigments that not only enable photosynthesis, but also (1) protect cells from UV radiation, (2) reduce cell damage by free radicals, (3) assist in cell-to-cell signaling and (4) manifest coloration. They are crucial to the long-term health and coloration of nearly all aquatic life. However, fish and invertebrates cannot synthesize carotenoids from base molecules and thus must assimilate them through their diet. Reef aquaria in particular are ravenous consumers of carotenoids, which is why a constant and robust supply is necessary to realize their greatest potential impact on animal health and coloration.

Widely used in aquaculture, Rhodopseudomonas palustris is a novel biofiltration agent. In order to collect light, it synthesizes the carotenoid pigments lycopene, canthaxanthin, beta-carotene and astaxanthin. R. palustris systematically eliminates excess deposits of nitrogenous wastes, phosphates and waste carbons, converting them to less and less toxic forms through the processes of nitrification, denitrification, anammox, photosynthesis and heterotrophy. Robust colonies of R. palustris continuously release daughter cells into the reef display and refugium. These cells, along with their carotenoid pigments, can and will be consumed by any of the reef’s filter-feeders, micrograzers and/or substrate sifters. Thus, reef aquarists have the opportunity to craft the ecology of their biofilter to improve their aquarium’s water quality while boosting animal nutrition and coloration.

How exactly do carotenoids capture light energy? And why are carotenoids so valuable in the nutritional health of fish, humans and invertebrates alike? The answers to these mysteries are hidden within the unique chemical structure of carotenoids.

The bonds that tie

Carotenoids are composed of polyene hydrocarbon chains which are sometimes terminated by functional rings. Carotenoids are chained together by double bonds. These double bonds can energetically interact with each other through atomic conjugation—basically, allowing them to bounce electrons back and forth at a highly efficient