Phytoplankton Pigments
(^) Photosynthesis underlies most biological energy and material conversions on Earth,
so understanding of it is central to ecology and biological oceanography. Light-
absorbing pigments make photosynthesis possible. Biological pigments generally are
molecules carrying substantial systems of conjugated double bonds among carbon
atoms. The resonant electrons of those unsaturated carbon chains can absorb a photon,
shift to a new energy state, then pass the acquired energy into enzyme-regulated
reactions. Chlorophyll-a (Fig. 2.18) acts as the key pigment in all photosynthetic
organisms, except in Prochlorococcus, which has the very similar divinyl-chlorophyll.
Functional chlorophyll-a includes a large protein matrix that presents to the light a
porphyrin ring with a magnesium atom held by central ligands. The ring has a tail, a
linear carbon chain called phytol, by which it attaches to the protein portion of the
system. Chlorophyll-a is associated with other protein-bound pigments (carotenoids,
xanthophylls and several chlorophyll variants) in one of two types of “photosystem”:
Photosystem I, or P700 for its wavelength of maximum absorption and Photosystem
II, or P680. P680 applies absorbed energy (Fig. 2.19) to capture electrons from water,
releasing protons (4H+) that are actively transported into the thylakoid lumen, and
oxygen (O 2 ) that diffuses out of the thylakoids. The electrons move in a complex
series of transfers to P700. More photic energy is absorbed there, raising the energy
level of the electrons, which then reduce NADP+ (nicotinamide adenine dinucleotide
phosphate) to NADPH, a moderately stable, diffusible molecule that carries reducing
capacity (energy available for biosyntheses) into the cytoplasm. The protons pumped
into the thylakoid generate an energetic gradient (up to 3.5 pH units) across the