Page 118 - Algae Anatomy, Biochemistry, and Biotechnology

Anatomy 101

concentrated in photoreceptive structures and do not contribute to the color of the organism. This is
an important consideration because a primary, historical criterion for algal taxonomy and phylo-
geny has been their bulk pigment composition. Sensory pigments have played no role at all in
these determinations, because it is generally believed that different algal groups have different
photosensory pigments; furthermore, the association of particular sensory pigments with particular
algal groups has been based largely upon the examination of only one or a few species.
For a long time the behavior of algae in response to light stimuli has attracted the attention of
scientists, who hoped that the simplicity of the material would make this phenomenon easy to
investigate. On the contrary, the undertaking turned out to be more difficult than expected and to
date a clear understanding of the phenomenon has still to be attained, while comprehension of
the visual process in humans is extensive. Initial ideas on the chemical nature of receptors for
photobehavioral responses were gained by measurements of their spectral sensitivity. Action spec-
troscopy results were then screened and supported by other methods such as biochemistry, absorp-
tion spectroscopy, electrophysiology, and molecular biology, which when integrated with different
investigative approaches shed more and more light on nature and functioning of photoreceptive
pigments.
In the following we will briefly describe the characteristics of rhodopsin-like proteins and fla-
voproteins and compare spectroscopic and biochemical techniques that have been used to define the
light-absorbing properties of photoreceptive proteins either in vivo (single cell or cell population) or
in vitro (extracted material). To provide a clear perspective on the efficacy of different techniques as
tools for the study of photoreceptive structures, a discussion of the pros and cons, the advantages
and limitations of each technique are included.

Rhodopsin-Like Proteins
Rhodopsins are photoreceptor proteins, universally used from archeabacteria to humans, consisting
of a proteic part, the opsin, organized in seven transmembrane a-helices, and a light absorbing
group, the retinal (i.e., the chromophore). The retinal is located inside a pocket of the opsin,
approximately in its center.
Why these proteins are so special? First, retinal–opsin complex has an intense absorption band
whose maximum can be shifted into the visible region of the spectrum, over the entire range from
380 to 640 nm. Second, light isomerizes the retinal inside the protein very efficiently and rapidly.
This isomerization, that is, the event initiating the vision reaction cascade, can be triggered almost
exclusively by light; in the dark it occurs only about once in a thousand years. Third, remarkable
structural changes (movements of single a-helix) are produced by isomerization of retinal. Light is
converted into atomic motion of sufficient magnitude to trigger a signal reliably and reproducibly.
Fourth, the photocycle (the photoreceptive protein upon light excitation undergoes a series of con-
formational changes which can be driven back to the original conformational state) is very fast,
hence the intracellular photoreceptive machinery is immediately reset for a new response. Fifth,
retinal is derived from b-carotene, a precursor with a widespread biological distribution.
About 1000 rhodopsin-like protein genes have been so far detected in the microbial world.
Genes were detected in Anabaena (aka Nostoc) sp. (Cyanophyta), Guillarda theta (Cryptophyta),
Pyrocystis lunula (Dynophyta), and Chlamydomonas sp. (Chlorophyta). Biochemical and spectro-
scopical evidences for a retinal-based photoreceptor were reported in one lineage of prokaryotes,
that is, in Leptolyngbya sp., (Cyanophyta), and at least in four lineages of eukaryotic algae, that
is, Ochromonas sp. and Silvetia, sp. (Heterokonthophyta); Euglena sp. (Euglenophyta); Gymnodi-
nium sp. (Dinophyta), and Dunaliella sp., Spermatozopsis sp., and Volvox sp. (Chlorophyta).

Flavoproteins
Flavoproteins, or yellow enzymes, are a diverse group of more than 70 oxidoreductases found in
animal, plants, and microorganisms, which have a flavin as a prosthetic group covalently attached