Underwater Photosynthesis in Cyanobacteria: Challenges and Adaptations

Within the class of photosynthetic organisms, cyanobacteria are among the most prevalent and diverse groups. They can multiply under a wide range of conditions which can be attributed to the adaptability of their light-harvesting system. Compared to terrestrial ecosystems, oceans receive different wavelengths of solar energy that are needed for photosynthesis. Red light cannot pass below ~15 m because water quickly attenuates longer light wavelengths. Blue and green wavelengths dominate the illumination of near-surface waters (those above 30 m) in the open ocean, but blue light (400–500 nm) is more abundant in deeper waters (100–200 m). Since seawater in coastal places contains higher amounts of humic compounds and phytoplankton, green wavelengths typically prevail in these environments. Via light-harvesting antennae, oxygen-consuming photosynthetic organisms transform light energy into electron flow within the reaction center of their photosystems. They handle low-gas exchange underwater circumstances in addition to light. Gas penetration, which has a 104-fold lower diffusion coefficient in water than in air, is a major barrier for cyanobacteria. As a result, aquatic organisms are far more likely than terrestrial ones to have inorganic carbon limitation in photosynthesis. Underwater, limited gas exchange has led to the evolution of a variety of adaptive traits. The ability to absorb and utilize the particular wavelengths that enrich a particular area and depth of the ocean would provide a photosynthetic organism an advantage over other organisms in that habitat. Many pigment-protein complexes have evolved in photosynthetic organisms to absorb light and transfer excited-state energy to reaction centers where photochemical and electron transfer activities occur. These light-harvesting antennas boost the effective absorption cross section of the reaction centers. Hence, the present chapter provides insight into the adaptive features of cyanobacteria to improve light consumption underwater, as these acclimations affect underwater photosynthesis.