plant evolution

Emir Khatipov khatipovNO at NOuchicago.edu
Wed Aug 14 14:26:06 EST 2002


In addition to what was so comprehensively discussed here, I would like to
add that in my understanding longer wavelength-absorbing pigments were more
beneficial from the point of view of penetrability of the light through the
water. I believe it is not questionable that bacteria appeared on the
surface of the Earth much earlier than primitive plants. Thus, much of the
evolution of the pigments must have occurred in bacteria. Most of the
phototrophic bacteria are marine or fresh water species that are suspended
in water at different depths, depending on the level of tolerance to oxygen
(that by the way is not produced by bacteria as a result of terminal
oxidation in photosynthesis, with the exception of cyanobacteria that have 2
photosystems), or sediment in bacterial mats on the shallow floors. The
light of longer wavelength can penetrate much deeper into the water, whereas
the short wave light is almost completely absorbed by upper water layer.
Chlorophylls, and especially different bacteriochlorophylls with absorption
maxima at 650-1100nm would be the pigments of choice for water species.
- Emir

"Nobody" <nobody at hgmp.mrc.ac.uk> wrote in message
news:3D466FEB.36FFBCB at mail.uni-mainz.de...
>  Dear Dr. Seager,
>
>  I remember a thread in this newsgroup some years ago induced by an
>  astronomer's question: "why are plants green?"
>  Thanks to GOOGLE's search abilities I could locate this 1997
>  discussion where Joe Berry, Winslow Briggs and Frantisek Vacha
>  presented some ideas about the evolution of photosynthetic pigments
>  which need no further comment. I add these documents at the end of
>  my posting.
>
>  Sincerely,
>
>  W. Ruehle
>  Inst f. General Botany
>  Joh. Gutenberg University
>  D-55099 Mainz
>  Germany
>
>  My comments to some special questions in Dr. Seager's posting:
>
>  seager at ias.edu schrieb:
>  >
>  >  Hello,
>  >
>  >  I am an astrophysicist with a few plant related questions.
>  >  I hope someone will be able to answer these and to provide
>  >  references, or to point me in the right direction.
>  >  These questions are about the red edge reflectance
>  >  signature of chlorophyll producing plants--the order of magnitude
>  >  increase in reflectance just redward of about 700nm.
>
>  Light scattering by intercellulars leads to an increased
>  chlorophyll absorbance. So the reflected red light (<700nm) is
>  diminished but reflectance of >700nm remains nearly 100%.
>
>  >  I have read that the high reflectance redward of 700m is from light
>  >scattering
>  >  in the air gaps between plant cells, a function that has evolved as a
>  >  cooling mechanism to prevent degradation of chlorophyll.
>
>  I do not know your source but possibly you misunderstood something.
>  The high reflection of plant tissue >700nm results from its low
>  absorbance and does not differ in its mechanism from inorganic
>  compounds like silver or MgO. Certainly we are protected against IR
>  radiation beyond the crown of a tree but the reflecting leafs above
>  us are not "cooled" by this mechanism but rather by the evaporation
>  of water. However some leaves have evolved a hairy surface and look
>  white as a protection against too much light and transpiration. These
>  hairs reflect all wavelength about equally and with such a vegetation
>  one would not expect a pronounced red edge.
>
>  >  This red-edge signature has become
>  >  something of interest to astrophysicists as an indicator
>  >  of life--a civilization 100s of light years away from us with a large
>  >  space telescope would be able to detect the red-edge signature
>  >  on the spatially unresolved Earth.
>
>  Could one really resolve a red edge signal in a distance of 100s of
>  light years from the reflectance of the "blue planet" which originates
>  mainly from oceans with very diluted chlorophyll contents, relatively
>  small patches of terrestrial vegetation producing red edge signals,
>  and fluctuating atmospheric signals?
>
>  >  1) Would evolution of a light-harvesting organism
>  >  always lead to a reflectance signature (at a different wavelength
>  >  regime than the harvested one)?
>  >  Or could it also be likely that another method of energy
>  >  dissipation could evolve?
>
>  Every compound that has a 1.singulet absorption band produces an edge
>  between the wavelength of its absorption and light of lower energies
>  which are no longer absorbed because the 1.singulett state could not
>  be reached. The magnitude of the red-edge effect is a function of
>  absorbance. But only a compound generated by living organisms is
>  likely to cover a planet and will evolve in an optical window of this
>  planet's atmosphere. So far your idea is promising.
>
>  >
>  >  2) Do photosynthetic plants absorb at optical wavelengths
>  >  because of the required energy for molecular electronic
>  >  transitions?
>
>  see discussion below
>
>  >  I'm hoping that there are specific examples from light-harvesting
>  >  organism evolution or existing photosynthetic organisms
>  >  (e.g., bacteria that absorbs light in the infrared) that will
>  >  shed light on these questions, if not an answer to them.
>  >
>  >  Please email any responses directly to me at
>  >  seager at dtm.ciw.edu
>  >
>  >  Sincerely,
>  >  Dr. Sara Seager
>  >  Faculty, Carnegie Institution of Washington
>  >  Washington, D.C.
>  >
>
>  +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  Thread from Sept 1997:
>
>
>  =46rom: Joe Berry <joeberry at biosphere.Stanford.EDU>
>  Subject: A Question
>  Date: 1997/09/28
>  Message-ID: <9709282059.AA01736 at biosphere.Stanford.EDU>#1/1
>  Distribution: world
>  Sender: daemon at net.bio.net
>  Organization: BIOSCI International Newsgroups for Molecular Biology
>  Newsgroups: bionet.photosynthesis
>
>
>
>  Dear Photosynthesis Researchers,
>
>  I received an interesting question that might be a useful topic for
>  discussion on the photosynthesis net.  The question comes from an
>  astronomer via Maxine Singer, President of the Carnegie Institution.
>
>        Allan Sandage at the Observatories sent me the following
>  question.      Can you help with an answer?  He doesn't 'do' email, so
>  email me     the answer and I will send it on to him.
>
>        "Why are plants green??  (I suppose this means and not yellow or
>        blue or red)  What evolutionary advantage does green have re
>        photosynthesis?"
>        THanks, Maxine
>
>  I would appreciate hearing the thoughts of other photosynthesis
>  researchers.  I have included my answer and a response from Winslow
>  Briggs below.
>  Thanks,
>  Joe Berry
>  Carnegie Institution of Washington
>  Stanford, CA 94305
>  joeberry at biosphere.stanford.edu
>
>  An Answer:
>  Here is one way to look at it: Chlorophyll's absorption is at
>  wavelenths <700 and >400 nm. This "window" was probably prescribed by
>  the chemistry of the primordial oceans. These are thought to have
>  contained high concentrations of Fe+2 ion (which absorbs strongly at
>  wavelengths >700 nm) and dissolved organic compounds (which absorb in
>  the blue and near UV).  Thus, chlorophyll is a pigment that "fits"
>  into a window of available light energy. In this sense, it is ideally
>  suited for photosynthesis. On the other hand, chlorophyll is green
>  because it dosen't completely fill the window.  This is not an
>  advantage, and plants have evolved a number of accessory pigments to
>  fill the hole in the chorophyll absorption spectrum.  These pigments
>  donate absorbed photon energy to chorophyll.
>  __________________________________________________________________
>  Subject: Re: (Fwd) Question
>  Author:  "Winslow Briggs" <BRIGGS at andrew.stanford.edu> at Internet
>  Date:    9/25/97 11:26 AM
>
>
>  Let me add to Joe's comment:
>
>  There aren't any conjugated double bond pigments that I know that have
>  extremely broad absorption bands. Below 400nm, the increasing energy
>  of the photons raise the spectre of photochemical damage. Beyond 700
>  nm, the energy levels are sufficiently low that except in exceptional
>  cases they are insufficient for effectively driving photochemistry. A
>  compromise: an absorption band safely above the UV, and one
>  sufficiently down in the red that useful photochemistry is still
>  possible. My guess is that a single band in either wavelength region
>  would probably be selected against. The situation in higher plants is
>  not perfect, as Joe points out, and accessory pigments are made in
>  some algae to fill in the gaps. Even higher plants use carotenoids,
>  absorbing in the blue, to enhance energy capture, but these still do
>  not extend too far into the green window left by chlorophyll.
>
>  It seems to me that given the properties of conjugated double bond
>  systems in absorbing light energy, making a molecule with two major
>  bands within the biologically constrained wavelength range is not all
>  that simple, and chlorophyll is an ideal solution.
>
>  (Note the waving of hands!).
>
>
>
>
>  =46rom: Frantisek Vacha <vacha at GENOM.UMBR.CAS.CZ>
>  Subject: Re: A Question
>  Date: 1997/10/03
>  Message-ID: <102310A2B96 at genom.umbr.cas.cz>#1/1
>  Distribution: world
>  Sender: daemon at net.bio.net
>  Organization: BIOSCI International Newsgroups for Molecular Biology
>  Newsgroups: bionet.photosynthesis
>
>
>
>  Another wiev why plants are green
>
>  Two reasons:
>
>  =46irst. We have to ask why plants use chlorophyll or generally
>  porphyrins.
>  According to my opinion nature hadnlt much choices and plants used the
>  most convenient way to develop a useful pigment system. Well before
>  chlorophyll-like organisms there have been heterotrophic organisms with
>  =46e-porphyrins, hems. Hem is suitable for many enzymatic reactions but
>  its
>  absorption properties are not good (main peak at about 400 nm and then
>  some nothing about 550 nm) and having a heavy metal Fe in the centre its
>  properties as a species for energy transfer, energy conservation (longer
>  excitation times) or even charge separation are bad. However, nature had
>  already developed path for synthesis of a potentially good pigment
>  (chlorophyll). Note that the synthesis pathway of hem and chlorophyll is
>  the same to the IX-protoporphyrin. Protoporphyrine and even
>  Mg-protoporphyrin have absorption mainly at about 400 nm and almost
>  nothing in the red region. The advantage of absorption in the red is
>  made
>  by reduction of a 7-8 bond of protochlorophyll. The Mg atom in the
>  centre
>  in not needed for such absorption profile as seen on pheophytin but it
>  is
>  definitely needed for porphyrins to became pigments suitable for
>  photosynthesis.
>
>  However, there is also bacteriorhodopsin in Halobacterium and in
>  Holococcus. Is it photosynthesis? Synthesis of bacteriorhodopsin has
>  different pathways from chlorophyll. Here it is seen that nature had
>  tried
>  more paths to evolve photoautotrophic organisms. And everything could
>  have
>  been orange!
>
>
>  Second. Why isnlt the question aewhy are plants red-brown?" ? There are
>  green sulphur bacteria and purple bacteria. Green sulphur bacteria are
>  actually not very green (depends on the level of carotenoids) and their
>  red
>  absorption maximum (Qy transition) is at 753 nm. So in the middle of the
>  evolution way we are still not green as we are now. Here I have to note
>  that bacteriochlorophyll absorbs far beyond 700 nm and the energy
>  absorbed
>  by bchl is efficient to drive charge separation. Important is that
>  bacteriochlorophyll in its kation state is not able drive an electron
>  from
>  water in any conditions which nature or evolution had tried. The
>  limitation
>  of electron donors, the fact that there is enough water in environkment
>  lead to the evolution of system which started to use water as a donor of
>  electrons. This had to be probably initiated by changes of pigments to
>  chlorophyll a which has, under certain conditions in photosystem II,
>  such
>  redox potential to drive electrons from water. And here the Qy
>  transition
>  (red absorption peak) is moved to shorter wavelengths and the overall
>  colour of chlorophylls to the green.
>
>  I donlt know anything about evolution of photosynthetic pigments and
>  some
>  people say that chl was before bchl but the key things are the
>  similarity
>  of synthetic pathways of porphyrins hem and chlorophyll and the need of
>  chlorophyll a to drive electron from water.
>
>
>  Regards
>
>  =46. Vacha
>
>
>  ---------------------------------------------------
>
>  =46rantisek Vacha
>  Inst. Plant Molec. Biol.
>  Branisovska 31
>  370 05 Ceske Budejovice
>  tel. 00420-38-7775523
>  fax. 00420-38-41475
>

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