Algal Photosynthesis

Abstract

Algae are a very diverse group of predominantly aquatic photosynthetic organisms that account for almost 50% of the photosynthesis that takes place on Earth. Algae have a wide range of antenna pigments to harvest light energy for photosynthesis giving different types of algae their characteristic colour. Early work done with algae contributed much to what is presently known about the carbon dioxide fixation pathway and the light‐harvesting reactions. The processes of photosynthesis in algae and higher plants are very similar. From among the three types of carbon dioxide‐concentrating mechanisms known in photosynthetic organisms, two types are found in different types of algae. Algae are proposed to play a role in the global carbon cycle by helping remove excess carbon dioxide from the environment. Owing to their efficient absorption and conversion of solar energy into chemical energy, algae have great potential for biotechnological and biofuel applications.

Key Concepts

  • Algal photosynthesis account for almost half of the photosynthetic carbon fixed every year.
  • Scientists have utilised different algal species, including algal mutants, to study different aspects of photosynthesis.
  • Algae's efficiency at pulling inorganic carbon out of the environment is dependent on growth condition, which implies the presence of an inducible carbon dioxide‐concentrating mechanism in algal cells.
  • The proposed mechanism for carbon dioxide acquisition in algae includes active HCO3 accumulation, Rubisco packaging in specialised structures and the presence of a carbonic anhydrase near the location of Rubisco.
  • Carbonic anhydrase is an enzyme that interconverts carbon dioxide and bicarbonate, which supply Rubisco with carbon dioxide from the pool of HCO3.
  • The major antenna pigments in algae include chlorophylls, phycobiliproteins and carotenoids, and the variation in the composition and relative abundance of these pigments give algae their distinctive colour.
  • Antenna complexes are proteins with many bound antenna pigments, which are important in absorbing light energy.
  • Algal photosynthesis is thought to increase when key nutrients such as N, P and Fe are available. This can sometimes lead to algal blooms.
  • If algal photosynthesis increased more carbon dioxide might be removed from the environment.
  • Photosynthetic biofuel production still is a promising alternative form of energy supply, and algal photosynthesis is the system of choice for energy applications.

Keywords: algae; photosynthesis; Calvin cycle; phytoplankton; biotechnology

Figure 1. General scheme of algal photosynthesis showing the separation of the electron transport chain and the Calvin cycle.
Figure 2. General arrangement of antenna complexes in algae. In green algae as well as in higher plants (a) the antenna complex is in the membrane and associates with the photosystem in the membrane. In red algae and cyanobacteria (b) the antenna complex is a phycobilisome and is a soluble protein. The phycobilisome attaches to the photosystem where the photosystem extends from the membrane.
Figure 3. SeaWiFS satellite image showing chlorophyll content in the ocean. This image was an average of images taken in June 2018. The bright green and yellow colours indicate high levels of chlorophyll. Source: Feldman GC (2018) NASA's OceanColor Web. Greenbelt, MD: NASA Goddard Space Flight Center. [https://oceancolor.gsfc.nasa.gov].
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Salisbury FB and Ross CW (1992) Plant Physiology, 4th edn. Belmont, CA: Wadsworth Publishing.

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Ynalvez, Ruby A, Dinamarca, Jorge, and Moroney, James V(Nov 2018) Algal Photosynthesis. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000322.pub3]