From: PO2::"Toby.Tyrrell@SOC.SOTON.AC.UK" "Toby Tyrrell" 1-MAR-1996 10:17:54.11 To: Multiple recipients of list DIATOM-L CC: Subj: summary of replies (diatoms and silica (LONG) This is a compilation of replies to a question I asked recently on the mailing lists ALGAE-L and DIATOM_L. The question was "why do diatoms build their cell walls (tests) from silica?". I've edited out the mail headers, etc. Toby Tyrrell email: tt@socnet.soc.soton.ac.uk ---------------------------------- From: John Berges Hi Toby, I guess there are several perspectives from which to answer, here are a couple: a) ecological: diatoms have found a resource that few other groups exploit (but remeber things like the silicoflagellates!) and have taken advantage. Their lifecycles often involve sedimentation when nutrients deline and later resuspension, an so maybe nutrient deficiency isn't such a barrier for them (see e.g. Victor Smetachek's 1985 paper). b) chemical. Things like CaCO3 will dissolve at depth (remember carbonate compensation depth?), whereas silica is more resistant. Note the huge areas of the sea floor covered in diatomaceous material. c) biophysical. Amorphous silica has interesting optical properties, unlike CaCo3 which is largely opaque. Why does a photosynthetic organism like a coccolithophorid choose to bury itself in opaque material? Some belive silica frustules can refract light to actally make it more available for diatoms. Maybe the complex frustule patterns have functional significance. Just some ponderings, I hope are useful. Cheers. _/_/_/_/ _/ _/ _/_/_/ Dr. John A. Berges _/ _/ _/ _/ _/ _/ Biology and Biochemistry _/ _/ _/ _/ _/_/_/ Queen's University, Belfast _/ _/_/ _/ _/ _/ _/ BT9 7BL Northern Ireland _/_/_/_/ _/_/_/_/ _/_/_/ FAX: 44 (0)1232 236 505 _/ From: Charlotte Zaremba According to Raven (JA Raven, Biol. Rev. 1983, 58, 179-207, silica deposition is, on a unit weight basis, 3.7% as energeticaly costly to deposit as lignin, and 6.7% as costly as cell-wall carbohydrate. (See also E. Epstein, Proc. Natl. Acad. Sci. USA, 1994, 91, 11-17. I am replying to the whole list because I agree with many subscribers, that answers to questions such as this one on why diatoms make siliceous shells, should be made available to all, not just the question-asker. I'm not sure the best way to accomplish this, but that's my vote! Charlotte Zaremba, Chemistry UC Santa Barbara Sender: "Gianni P. Felicini" Some considerations about the Tyrrell question: 1. Silicon is the most spread earth element. 2. Diatom ancestors, perhaps, came into the world when silicate was more abundant. 3. Fortunately, silicate can be limitant, otherwise...... (see algal mucilage phenomen in Adriatic s.). On the other hand, all the organisms have an Achilles' heel. 4. Without hard frustule most benthic diatoms should be unable to live where they live; in general, they are winning in competitions for substrate (e.g. on the sand). I hate diatoms growing in my macro-algal cultures. Usually they are too exuberant. I'm gratefull to Lewin for germanium dioxide as French revolutionaries were gratefull to M.eur Guillottin! Bye-bye Gianni F. ================================== G.P. Felicini gfelih@mbox.vol.it (home) -------------------------------------------------------------------- Istituto Botanico, Universita' (CAMPUS) Via Orabona - 70125 BARI, Italy FAX: +39 080 544 2155 & 080 544 2162 ================================== Sender: STEVE WILLIAMS I think you hit the nail on the head. However, your answer brought some more questions to my mind. First, what aspects of diatom biology have led them to become such a ubiqutous and successful group? Second, how much of this success can be attributed to the use of silica in their frustules? And finally what group of algae did the diatoms evolve from? From: Marvin W Fawley Toby Tyrrell posed an interesting question, but one without a simple answer. Whenever we ask "why" about something like silica in diatoms, we start looking for ways that the feature adapts the organism to the environment. Possibly, silica does make diatoms more fit in some niches, giving them the edge over competitors. On the other hand, there may be cases where they would be better off without silica, but they still thrive. The real "reason" they have silica may be evolutionary. The ancestors of diatoms had silica scales and diatoms modified these into frustules (I believe that is a current hypothesis). Many protists use silica. It is possible that a different material could have been better for diatoms, but a metabolism using silica was what they had to work with. Whether silica frustules were an adaptation that triggered the radiation of diatoms, of if some other feature of these organisms was more important for the success of this group, is very difficult to determine. Then again, it might just be chance. Marvin Fawley North Dakota State University Sender: "Gonzalez, Elma Biology" Perhaps its useful to think of these features as a series of solutions to challenges posed by an organisms' changing environment. I like to think of the coccolithophorids as first having solved two metabolic problems 1) high calcium in sea water and 2) high alkalinity of the oceans. Calcification would have addressed both challenges. Subcellular calcification is a neat solution because it takes advantage of the secretory system to push crystals on 'rafts' away from the cell (I think external encrustation as in corals and molluscs might pose additional problems limiting growth/mobility). The coccolith vesicle membranes are conserved and the base plates are primarily carbohydrate which saves on profligate expenditure of scarse nitorgen. Coccoliths could have easily and simply been secreted away from the organism to satisfy the hypothetical problems mentioned above. If thats all that happened we would see naked, subcellulary calcifying cells at present. Clearly, though, that's not all that happened. Coccospheres might have come later, as a subsequent evolutionary stage. Prabably in response to some other environmental challenge where retention of the coccoliths would confer added adaptive value. I suppose the fossil record can not be asked to provide evidence of subcellular calcifiers with no coccospheres? Elma Gonzalez Gonzalez@biology.lifesci.ucla.edu From: Tom Fitzsimmons A good question! I'll be interested in responses because diatoms were very helpful in reducing silica in a water supply used for the cooling tower makeup (to replace evaporated water from the tower) in a big power plant in Wyoming. This suggests an industrial and profitable application of diatom biology. Tom Fitzsimmons. From: Edward Theriot I have a lot of other stuff I should be doing, so I am writing to answer this instead. I thought it would be fun to respond to the whole list, so I took the liberty of doing so. There is really lots of nifty stuff hidden in this simple question. Apologies in advance if this is not appropriate etiquette. "Why do diatoms use silicate to form their tests?" Simplest answer based on my belief that origin of features of organisms is based in no small part on roles of the dice (MAINTENANCE of features may be quite a different story): Why not? Silica is utilized by lots and lots of different animals and plants (presumably due in some part to the fact that both carbon and silica have four valence electrons). Thus, the answer to your question is partly and overly simply put as: They have silica shells because their ancestors had silica shells. Addressing the question of why has the silica shell been _maintained_ over some 100 million plus years of evolution is a bit more complex. Most simple answer: Because diatoms can make silicate walls using about 30% less energy (or so I have read, can't recall the citation, but it makes sense) with minerals such as silicate rather than CO2 and its various forms (which also become limiting in many regions, by the way) which they can better put into sugars and making more little diatoms. As long as silicate supply exceeds or equals demand, diatoms win. More complex, and partly assuming the first answer is reasonably accurate, answer: First, the North Atlantic ain't the only place in the world. Nitrate apparently "runs out" first in Yellowstone Lake and many other volcanic lakes, phosphate in Lake Superior, CO2 in other places. Diatoms could have evolved someplace other than the North Atlantic. In fact, they had to. When diatoms evolved (a common mispractice of ecologists is to assume the world was always as it is and that everything originally adapted to modern conditions - there is a difference between adaptation and ability to function; i.e., a difference between proximate and ultimate causes - but enough diatribe), the North Atlantic wasn't even there. Third, remember, when diatoms evolved, there may well have been no serious competitors for silica and therefore silica limitation was perhaps globally not a big deal early on! Just as humans litter their own world, so diatoms may well have created silica limitation for themselves. I am, of course, ignoring radiolarians. But they are animals. Who cares? :) Seriously, they should be taken into account. Is there any literature on competition between rads and diatoms for silica? I think this might really be important for radiation of marine diatoms into freshwater where diatoms put a serious dent into silica concentrations in many lakes and reservoirs (as the email from Fitzsimmons noted). Fourth and most important to modern systems, Hutchinson's "paradox of the plankton" articulated the fact that places ain't even the same on a day to day basis. Diatoms just have to win some of the time to exploit certain "niches", and phytoplankton niches are transient both in _TIME_ and space. Thus, if using silicate does give a competitive advantage SOME of the time, that is enough. Something else will become limiting at some later time, then mixing occurs and things start all over, there are gradients constantly shifting (viz., resource competition theory), etc. The author of the original email used the words "compete equally". Of course, competing equally is almost impossible. The slightest differences will lead to one winner, given stable enough conditions over time and space. Hutchinson's insight was to see that this doesn't happen in nature; the race is always starting over. A related question is why hasn't a "super" plankter evolved, one that can shift its shell from silica to carbon based? The hypothetical answer is partly phylogenetically based. Test making systems are presumably homologous and have been modified through time. It would take some sort of gene duplication and/or lateral transference to provide the opportunity for evolution to produce two different shell making systems by providing two separate genetic blueprints for selection to act on. It could even be that most of the system stays the same, just that some "homeobox" type developmental controller system is affected. But I am getting in way over my head here. ******************************************************************************* Dr. Edward Theriot Vice President, Division of Biodiversity and Evolution Chair, Diatom Herbarium Academy of Natural Sciences of Philadelphia 1900 Benjamin Franklin Parkway Philadelphia, PA 19103 Voice: 215-299-1078 Email: theriot@say.acnatsci.org FAX: 215: 299-1028 ******************************************************************************* From: HABERYAN <0100730@ACAD.NWMISSOURI.EDU> Hi Diatomeers, I would only add one more benefit to using silica: anti-grazer protection. Some algae use sheaths, others spines; diatoms use silica that can break into dangerous shards. Some preliminary work suggests that long, fragile species (like Nitzschia) break into many pieces, while Aulacoseira and perhaps Stephanodiscus tend to stay intact. These observations are based on species frequencies (and fragmentation) inside fecal pellets versus outside of pellets. For further details, see Limnology & Oceanography 30:1010, (1985). I have, unfortunately, been unable to keep up with more recent advances along these lines. Kurt Haberyan From: Susan Kilham I sent a personal message, but thought I would pass along some of my thoughts on why diatoms have Si (I agree with Theriot, too, so will not discuss again what he did). 1) 20 years ago my late husband, Peter Kilham, speculated (L&O 76, p.409-417) that one of the major adaptive reasons for a Si frustule is to sink relative to stream lines, thus overcoming diffusion limitation. Motile cells can do this as well, but diatoms cleverly use gravity to their benefit. Of course, this means that there has to be a certain amount of turbulence to pull this off successfully- just another reason diatoms can do so well in large lakes and oceans, or during mixing periods in smaller lakes. 2. Increase in surface area. Laying your cell membrane out along a complex structure can greatly increase your SA for absorption. I remember ca 8 yrs ago a paper (I think by Villareal) on a microtubule running the length of the spines of Chaetoceros, suggesting this possibility even for the spines. 3. There was speculation about 20 years ago (again, I forget the provenance-sorry!!) that the frustule was acting as a light pipe, focusing photons onto the chloroplasts. I sort of remember there being a flurry of tests of this idea-all negative. Peter talked to quite a lot of optical physicists about this idea and they all said that theory breaks down at the wavelengths of importance to diatoms, so they couldn't really help (their suggestions included "scaling up" a diatom frustule and using microwaves to check this out!!). 4. About 25 years ago when I was working on calcification, there were some papers written which showed that Si was an essential element for the process of calcification to occur, even though it was not incorporated into the CaCO3 structures. Those papers are in deep storage-sorry again. This is an endlessly fascinating subject because it is so speculative. It takes me back to the mid-70's when everyone was talking about the "adaptive significance of X" . At least this is fun, if not productive! From: Annemarie.Schmid@sbg.ac.at (Annemarie Schmid) Hello from Salzburg, I am Anna Maria Schmid, just recently plugged in into this royalty of Diatom-L: I am intersted in your discussion on "why do diatoms use silicate for their walls"? Let me answer in a modified way to Ed Theriot: "Why do so many other organisms not, if we consider that silicon is the second most element on earth, and the plasmamembrane is essentially permeable to silicon? Although in modern diatoms the silicon for wall formation appears brought into the cell by active pumps in addition (see refs. below by Werner, Volcani...etc)" Well amorphous silica is rigid and inextensible, so growth of tissues would be impossible, except when used as an endoskeleton like in sponges, or sometimes in higher plants to stiffen leaf edges etc. Also for single cells it has to consist of two pieces to allow cell growth and division. Or in the most closed form, in statospore-cysts etc, you have to have at least a hole, for the cytoplasm to crawl in and out..... Then I would like to complement Susan Kilham's letter: (her point 3): the speculation about optical properties of the siliceous wall, proposed especially for centric diatoms with their archaic hexagonal chambers (therefore compared to the ommatidia in the insect eye) was by Johnathan R. Rider (the last adress I have is : Rider and associates Biostratigraphic services, 18455 Augustine Rd; Nevada City, Ca.95959. Recently, Guy Steucek had, together with a collegue, redone some measurements, and he is pretty sure, that Rider is correct. Prof. Steucek, is to be found at Millersville University, in Pennsylvania. S.K. point 4) Si as an essential Element for Calcification: Carlisle E (1978) Essentiality and function of silicon. In: Bendz G & Lindquist I (eds): Biochemistry of Silicon and related Problems (the entire book is interesting!) Carlisle E (1981) Silicon in bone formation. In (Simpson TL & BE Volcani) Silicon and siliceous structures in biological systems (the entire book is interesting!) Philip Linthilac, Univ. Vermont, pers. comm. reported similar results BACK TO Ed Theriots letter: as to the low energy costs : the first to state this was, I think, Marshall Darley in 1973, an then there is a very long paper by John Raven which has lots of refs in the back: Raven JA (1983) The Transport and Function of Silicon in Plants. Biol.Rev. 58, 179-207 Polycondensation of monosilicic acid into opaline silica is according to these authors, just about 15% of that value needed for polycondensation of sugars to form a cellulosic cell wall from the same size. So I would guess that this is more important for a cell(species, clone) to survive, as being fragile and kill, after its own death, their grazers with its broken corpse as I had interpreted the suggestion of somebody out there). As Ed stated, this mechanism may have developed at a time, when everything in the environments looked completely different to the present day situation. So one should, I believe, then ask the question just for the personal benefit of the cell and I am sure, that the grazer- and competition problem comes much later. There is an old theory, and it seems somehow not so weird in light of the calculations and measurements presented by John Raven: The theory is from Klaus Bonik, a Theoretical Biologist, who speculated just based on studies of literature, that the development of the siliceous wall has started as a waste product at a time, when warm, salty ponds, puddles and oceans had plenty of dissolved silicon in their nutrient broth. The plasmamembrane is permeable for silicon (see also Raven 83), and may have been even more permeable at that time, and they may not have possessed exclusion mechanism for too much silicon, so they had to extrude it to the exterior under ATP-consumtion. They finally learned, (through millions of years) that, if they leave it at their cellular surface as an exoskeleton they would be able to reduce their cytoskeleton for shape control and thus save energy (because they need energy just for the formation of the exoskeleton, but not for its maintenance as they would for cytoskeleton). Anybody who ever sectioned diatoms, however, still finds a lot of cytoskeleton around. But nobody can say, whether it would not be more without a rigid cell wall, which has essentially the same function as any cell wall. And I predict, if we ever will be able to clarify the funtions of a cell wall, we can best do it with the diatom wall. Bonik,K (1978): Die Entstehung der Kieselalgen - ein stammesgeschichtliches Modell. I. Die Enwicklung der Schale. Natur & Museum, 108 (9) 267-273 Bonik K (1979): Die Entstehung der Kieselalgen - ein stammesgeschichtliches Modell II. Die Konsequenzen der Schalenbildung. Natur & Museum, 109 (1),1-9 Pickett-Heaps, J, Schmid AM, Edgar L (1990) The cell biology of Diatom valve formation. In: Round & Chapmann (eds) Progress in Phycological Research, vol 7; 1-168. Schmid AM (1994) Aspects of morphogenesis and function of diatom cell walls with implications for taxonomy. Protoplasma 181, 43-60 Then there are the excellent articles on Silicification by Joyce Lewin, M. Darley, and Dietrich Werner : Lewin J (1962) Silicification. In: Physiology and Biochemistry of Algae (R. Lewin ed) Acad. Press NewYork Darley M (1974) Silicification and Calcification In: Algal Physiology and Biochemistry (WDP Stewart ed) Botanical Monographs vol 10; Univ.-California press pp 655-676 Werner D (1977) Silicate Metabolism. in : The biology of diatoms (Werner D.ed) Botanical Monographs 13, Blackwell Scientific publ. Leadbeater, BSC & R.Riding (eds) 1986: Biomineralization in lower plants and animals, The System. Associat. special Vol Nr 30., Clarendon Press Oxford And finally: Gordon, R & Drum RW (1994) The chemical basis of diatom morphogenesis. Internat. Review on Cytology. (This is an excellent article, although our theories diverge somewhat on several topics) Hope this is of some help to you. Best wishes, AMS Anna Maria M. Schmid Univ.-Salzburg, Plantphysiology Hellbrunnerstr. 34 A-5020 Salzburg e-mail: Annemarie.Schmid@sbg.ac.at From: PO2::"kingston@USGS.GOV" "John Kingston" 2-MAR-1996 11:19:56.25 To: Multiple recipients of list DIATOM-L CC: Subj: Re: diatoms and silica Another interesting paper, this one about Si in higher plants, is: Epstein, E. 1994. The anomaly of silicon in plant biology. Review = Article. Proc. Natl. Acad. Sci. USA 91: 11-17. Epstein points out that higher plants take up and metabolize Si except = in the rare cases where plants are grown in defined media excluding Si. = He gives various lines of evidence that Si provides diverse benefits to = higher plants, even though it generally has not been considered a plant = nutrient. Reading this paper caused me to think of the similarities among diatoms, = chrysophytes, and higher plants, though this is not Epstein's main = point. --John From: PO2::"curtp@BINGHAMTON.EDU" "Curt Pueschel" 12-MAR-1996 13:48:47.99 To: Multiple recipients of list ALGAE-L CC: Subj: Silica and diatom walls Many of the responses to the question of "why do diatoms have silica walls?" cited ways in which silica walls have proven adaptive: anti-grazer defenses, increased surface area, energetically economical to produce, etc. However, these properties of diatom walls are secondary adaptations that may explain the success of diatoms relative to other organisms, but they offer less insight into understanding the origin of silicification. Anna Maria Schmid discussed exclusion of silicon from the cytoplasm and the possibility that regulation of silicon led to exclusion and sequestration, which presumably are critical steps in creating silica structures. This argument is similar to those which suggest that calcification arose from the need to control intracellular calcium concentrations at levels well below that of surrounding waters: mineralization as a means of excretion. However, calcium is essential for the regulation of numerous cellular activities, and fluctuations in calcium concentration have profound effects. For a parallel argument to be made regarding silicon exclusion and silicification requires that silicon concentrations have important effect on cellular metabolism. Could someone enlighten us regarding the importance of intracellular silicon regulation for non-silicifying cells? Do all cells have silicon exclusion mechanisms? What is the effect of too much or too little silicon on eukaryotic cells? Curt Pueschel Department of Biological Sciences State University of New York at Binghamton Binghamton, NY 13902-6000 E-mail: curtp@binghamton.edu Telephone: 607-777-2602 FAX: 607-777-6521