Light harvesting by long-wavelength Chlorophyll forms (red forms) in algae: focus on their presence, distribution and function

The efficiency by which oxygenic photosynthetic organisms, and particularly microalgae, utilise near infrared radiation for sustaining metabolic processes has attracted attention since the pioneering studies of Emerson and coworkers. In the vast majority of photosynthetic organisms, which uses Chlorophyll (Chl) a as their main light harvesting as well as photochemically active pigment, the capacity of absorbing incident photons at wavelength longer than 700 nm is associated to the presence of specific Chl a spectral forms, known as "red forms". These have been considered to be almost exclusively, and rather ubiquitously, associated to either the core or the external light harvesting apparatus of Photosystem I (PSI). Therefore a large body of information has been gathered, concerning red forms associated with either the core antenna of cyanobacteria or the external light harvesting complexes of green algae as well as those of higher plants which share a common structural architecture. On the other hand, recent ecophysiological in field measurements, together with studies performed in the laboratory on model red clade organisms, challenged this general consensus. In field measurements put in evidence that the presence of PSI red forms, particularly in oceanic waters, is probably less diffused than generally assumed on the basis of model organisms analysis. Moreover, the study of red clade algae demonstrated the presence of red spectral forms associated also to Photosystem II (PSII), particularly under conditions of culture self-shading or growth under far-red illumination. Therefore, in this review chapter the nature and photophysical/photophysiological role of red forms associated to both PSI and PSII will be surveyed and discussed. Moreover a general discussion of the impact of antenna forms absorbing at lower energies than the respective reaction centres, therefore being in competition for excited state localisation with photochemical processes and productive photon energy utilisation, will be discussed in a simplified, although generalised, framework.