symbiotic silicalemma?

Richard Gordon gordonr at cc.UManitoba.CA
Sun Jun 22 13:12:35 EST 1997


Dear Debbie,
The best way to get conversation going is to reply on the list. Let's list
some points for and against the idea that the silicalemma, the membrane bag
within which a diatom preciptates a silica valves or girdle bands, is a
symbiotic organelle:

For:
1) Like the chloroplast and mitochondrion, the silicalemma is membrane bound.
2) It has a mysterious object in it, of unknown origin and ontogeny, the
nucleating center for silica precipitation (Gordon, R. & R.W. Drum (1994).
The chemical basis for diatom morphogenesis. Int. Rev. Cytol.  150,
243-372, 421-422). This invites speculation on whether that object contains
DNA.
3) We are similarly lacking information on how the silicalemma itself is
constructed. No earlier stage than a nucleating structure wrapped in a
bilayer membrane has been found.
4) The relationship between microtubule organizing centers and silicalemmas
suggests a correlation with possibly self-reproducing centrioles.
5) The numbers of silicalemmas in a diatom are generally closely controlled.
6) The variety of structures produced by silicalemmas is consistent with a
symbiotic origin. (Across phyla there is quite a variety in chloroplasts
and mitochondria, so that your argument against symbiosis based on variety
does not hold.)
7) Similar structures are found across a number of phyla, suggesting
lateral transfer.

Against:

1) Silicalemmas are too big. But chloroplasts or mitochondria can
individually just about fill some other kinds of cells. In any case, they
all start small, extending as the silica preciptates.
2) The silacalemma's product is a large structure (the silica valve or
girdle band). (But this forms by precipitation of small units of silica.)
3) The silica shell can vary so much between diatom species. (But this may
be due to simple variations in the shape and positioning of the nulceating
center, and in the physicochemical conditions set up for silica
precipitation.)
4) The numbers in auxospores seem huge and uncontrolled.
5) Silicalemmas are recent in terms of evolution, having appeared recently
in the fossil record, compared to the symbiotic chloroplasts and
mitochondria, which originated from bacteria.
6) No free living, procaryotic organism exists that precipitates silica
within itself.

Perhaps others could extend these arguments? While neither case is strong,
there are matters here worth investigating.
Yours, -Dick Gordon
ps: To tempt the algae and molecular evolution people to jump in, I'm cross
posting this once. I've also included the small bit of prior
correspondence. (Please reply on the diatom list. Instructions below.)

>Dick,
>I must say that at first glance, I don't like the idea.  But I have to think
>through it to determine why it bothers me so much.  One quick thought is
>that a chloroplast is a chloroplast, while there is such variety among
>diatom frustules.  I'm not up on mitchondrial or chloroplast literature, but
>is there any actual product from either of these organelles other than
>relatively small molecules (ATP or sugars).  Is there any putative symbiotic
>organelle that manufactures something on the scale of the diatom frustule?
>As you probably know, sea urchins and some other calcareous secreting
>organisms possess a similar structure, sometimes associated with cells from
>the organism (sea urchins) sometimes just as an separate sac
>(coccolithophorids, forams etc.).  This may not hold water on further
>deliberation, but the whole concept strikes a sour note.  I will give it
>much more thought, and hope to see a lot of conversation about it.
>Debbie Swift

Let me propose the outlandish hypothesis that the silicalemma in diatoms is
a symbiotic organelle, like chloroplasts, mitochondria, and perhaps
centrosomes (see: Margulis, L. & M.F. Dolan (1997). Swimming against the
current. The Sciences  37(1), 20-25, for the latter). Does anyone have
disproof? Need to check for DNA, self-replication, continuity of existence
of at least some part of it through the cell cycle, transmission through
sexual phases, homology of this organelle between sponge spicules, diatoms,
radiolarians, and other algae with silica scales, etc.
Yours, -Dick Gordon

From: "SCHWANDES, LARRY P" <LPSCH at GNV.IFAS.UFL.EDU>
>Dear List Service,
>     Does anyone have any ideas how diatoms and sponge spicules relate to
>each other?  Some deposits of diatomaceous earth are dominated by spicules
>and others are dominated by diatoms.  Why does that happen?

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--------------------------------------------------------------------
Dr. Richard Gordon, Department of Radiology
University of Manitoba, Health Sciences Centre
820 Sherbrook Street, Winnipeg, MB R3A 1R9 Canada
Phone: (204) 789-3828,  Fax: (204) 787-2080, Home: (204) 589-0411
E-mail: GordonR at cc.UManitoba.ca





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