Auxiliary pigments

Auxiliary pigments are light - absorbing compounds of photosynthetic organisms that work in combination with chlorophyll a , performing light-harvesting or light-protective functions. These include various forms of chlorophyll, such as chlorophyll b in green algae and light-harvesting antennas of higher plants, or chlorophyll c or d in other algae. In addition, there are many other auxiliary pigments such as carotenoids or phycobiliproteins , which also absorb light and transfer its energy to the main pigment of the photosystem. Some of these auxiliary pigments, in particular carotenoids, are used to absorb and dissipate excess light energy and are antioxidants .

Chlorophylls and pigments associated with photosystems have different absorption spectra , since the spectra of different chlorophylls are modified by their local protein environment or because auxiliary pigments have significant structural differences. As a result, under natural conditions , the joint absorption spectrum of all pigments turns out to be wider than the absorption spectrum of each of them separately. This allows plants and algae to absorb a greater range of visible and infrared radiation. Most photosynthetic organisms do not absorb green light, so under a leaf canopy in the forest or under a layer of water rich in plankton, green light predominates. This spectral effect is called the “green window”. Some organisms, such as cyanobacteria and red algae , contain auxiliary pigments phycobiliproteins , which enable them to absorb green light passing to the depth of their habitat.

In aquatic ecosystems, the main influence on the spectrum is exerted by the absorption of water along with the organic matter dissolved in it. This defines the main ecological niches for phototrophic organisms. The six arms (absorption decays) in the water absorption spectrum at wavelengths between 400 and 1100 nm correspond to at least twenty different types of phototrophic bacteria. Another effect is due to the general tendency of water to absorb light at a low frequency , while organics absorb light at high frequencies. This is why the open ocean appears blue and rich in yellow-colored phototrophic species such as Prochlorococcus , which contains divinyl chlorophyll a and b . Synechococcus is colored red due to phycoerythrin and adapted to coastal conditions, while phycocyanin allows these cyanobacteria to thrive in darker waters ^[1] .