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terrestrial and aquatic ecosystem on the planet and are extraordinarily
prolific; by some estimates they account for over 20% of the Earth's annual
oxygen production ( Pisciotta, Zou, & Baskakov, 2010 ). The 'blue-green'
moniker for cyanobacteria comes from the pigments found in specialized
structures called phycobilisomes that are mostly used to harvest sunlight
and funnel the resulting energy into chlorophyll-based reaction centres
for conversion into chemical energy ( Bryant & Frigaard, 2006; Neilson &
Durnford, 2010 ). Cyanobacteria can also fix atmospheric nitrogen
( Herrero, Muro-Pastor, & Flores, 2001 ), though this is not a trait common
to all such organisms. Cyanobacteria are hypothetically linked to eukaryotic
algae and land plants through an ancient endosymbiotic pairing of a non-
photosynthetic eukaryote with a cyanobacterium that then gave rise to
the plastids (including the chloroplast; Neilson & Durnford, 2010; Reyes-
Prieto, Weber, & Bhattacharya, 2007 ).
In this chapter the term 'algae' will refer solely to green algae, because it is
so far the only type of alga for which any physiological information exists
concerning globins. The green algae are part of the supergroup
Archaeplastida, composed of land plants, red algae, green algae and a class
of organisms known as the glaucophytes ( Yoon, Hackett, Ciniglia,
Pinto, & Bhattacharya, 2004 ). The Archaeplastidae include all the species
we classically consider plants (sometimes, the supergroup is referred to a
Plantae sensu lato or 'plants in the broad sense'). The green algae are a class
of unicellular eukaryotes belonging to the phylum Chlorophyta, and are the
most closely related algae to land plants ( Keeling, 2004; Rodriguez-Ezpeleta
et al., 2005 ). Green algae depend solely on chlorophyll for light capture.
Instead of the phycobilisomes of cyanobacteria, green algae have large
chlorophyll-filled antenna complexes (light-harvesting complexes) attached
to their photosystems. Green algae have dedicated chloroplasts with their
photosynthetic thylakoid membranes structured into highly organized stacks
called granum ( Vothknecht & Westhoff, 2001 ).
As mentioned earlier, green algae acquired their chloroplasts through an
ancient endosymbiotic event. As such these cells have undergone several
rounds of horizontal gene transfer, where genetic information has been
incorporated into the nuclear genome during the evolution of both
mitochondria and the chloroplasts ( Richards & van der Giezen, 2006;
Rodriguez-Ezpeleta et al., 2005 ). In addition, there is evidence for more
than one endosymbiotic event involving the evolution of the chloroplast,
or at least multiple horizontal gene transfer events during its evolution
( Vinogradov, Fernandez, et al., 2011 ). This makes the heritage of algal
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