Page 48 - Algae

General Overview 31

appearance of these organisms, all photosynthetic organisms were anaerobic bacteria that used light
to couple the reduction of carbon dioxide to the oxidation of low free energy molecules, such as H 2 S
or preformed organics. Cyanobacteria developed a metabolic process, the photosynthesis, which
exploits the energy of visible light to oxidize water and simultaneously reduces CO 2 to organic
carbon represented by (CH 2 O)n using light energy as a substrate and chlorophyll a as a requisite
catalytic agent. Formally oxygenic photosynthesis can be summarized as:

Chlorophyll a
CO 2 þ H 2 O þ light ! (CH 2 O)n þ O 2

All other oxygen producing algae are eukaryotic, that is, they contain internal organelles,
including a nucleus, one or more chloroplasts, one or more mitochondria, and, most importantly,
in many cases they contain a membrane-bound storage compartment or vacuole. The three
major algal lineages of primary plastids are the Glaucophyta lineage, the Chlorophyta lineage,
and the Rhodopyta lineage (Figure 1.48).
Glaucophyta lineage occupies a key position in the evolution of plastids. Unlike other plastids,
the plastids of glaucophytes retain the remnant of a Gram-negative bacterial cell wall of the type
found in cyanobacteria, with a thin peptidoglycan cell wall and cyanobacterium-like pigmentation
that clearly indicate its cyanobacterial ancestry. In fact, the Cyanophora paradoxa plastid genome
shows the same reduction as other plastids when compared with free-living cyanobacteria (it is
136 kb and contains 191 genes). The peptidoglycan cell wall of the plastid is thus a feature retained
from their free-living cyanobacterial ancestor. In this context, the Glaucophyta are remarkable only
for their retention of an ancestral character present in neither green nor red plastids. No certain case
of a secondary plastid derived from Glaucophyta is known.
Green algae (Chlorophyta) constitute the second lineage of primary plastids. The simple two-
membrane system surrounding the plastid, the congruence of phylogenies based on nuclear and
organellar genes, and the antiquity of the green algae in the fossil record all indicate that the
green algal plastid is of primary origin. In these chloroplasts, chlorophyll b was synthesized as a
secondary pigment and phycobiliproteins were lost. Another hypothesis is that the photosynthetic
ancestor of green lineage was a prochlorophyte that possessed chlorophylls a and b and lacked
phycobiliproteins.
The green lineage played a major role in oceanic food webs and the carbon cycle from about 2.2
billion years ago until the end-Permian extinction, approximately 250 million years ago. It was this
similarity to the pigments of plants that led to the inference that the ancestors of land plants (i.e.,
embryophytes) would be among the green algae, and is clear that phylogenetically plants are a
group of green algae adapted to life on land. Euglenophyta and Chlorarachniophyta are derived
from this primary plastid lineage by secondary endosymbiosis; the green algal plastid present in
Euglenophyta is bounded by three membranes, while the green algal plastid present in the Chlor-
arachniophyta is bound by four membranes.
Since that time, however, a second group of eukaryotes has risen to ecological prominence; that
group is commonly called the “red lineage.” The plastids of the red algae (Rhodophyta) constitute
the third primary plastid lineage. Like the green algae, the red algae are an ancient group in the
fossil record, and some of the oldest fossils interpreted as being of eukaryotic origin are often
referred to the red algae, although clearly these organisms were very different from any extant
alga. Like those of green algae, the plastids of red algae are surrounded by two membranes.
However, they are pigmented with chlorophyll a and phycobiliproteins, which are organized
into phycobilisomes. Phycobilisomes are relatively large light-harvesting pigment/protein
complexes that are water-soluble and attached to the surface of the thylakoid membrane.
Thylakoids with phycobilisomes do not form stacks like those in other plastids, and consequently
the plastids of red algae (and glaucophytes) bear an obvious ultrastructural resemblance to
cyanobacteria.