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Valonia ventricosa are single-celled algae that range between one and few centimetres. In rare cases, they can reach sizes exceeding 5cm. They range from grass-green to dark green, and some are even a blackish colour.

Picture of large speherical Valonia ventricosa.

Weirdly, a lot of the literature covering these organisms seems to be pre-1950. This X-ray crystallography project in 1937 identified key structures in the cell wall of V. ventricosa:

It is found to consist of layers in which the cellulose chains in any one layer are inclined to those in the preceding and subsequent layers at an angle which is on the average rather less than a right angle.

The two sets of striations on the layers of the wall correspond closely to the meridian and spiral directions of cellulose chains, while the extinction directions, being defined both by the directions and by the relative proportions of the two sets of cellulose chains, lie in variable positions between. The development of the rhizoids has been investigated and found to be associated with regions of the wall adjacent to the poles of the spiral.

Although the authors hesitate to speculate on their function, has any progress in the field suggested the fibres identified in this 1948 Nature paper be involved in sustaining such a massive cell (Figure below)?

Fibrous surface of cell.

So it seems like the cell wall structure has been categorised long ago. But what allows these single-celled organisms to get so big? Is it the plant-like cell wall, or something else? I was under the impression that cells would burst or collapse well before reaching this size.

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  • $\begingroup$ the problem is diffusion: 1) maybe cytoplasmic streaming via cytoskeleton? You'd need to somehow add bulk flow in order to correct for the limits of diffusion. 2) Also I'm guessing an immensely large vacuole in the center, thus keeping the layer of cytoplasm really thin in order to permit diffusion to be the dominant form of transport... all guesses here... I'm too lazy to actually read about this algae... $\endgroup$ – Jasand Pruski Feb 9 '15 at 18:32
  • $\begingroup$ From what I could find, it seems like people aren't interested in publishing things about why its big. jmicro.oxfordjournals.org/content/early/2014/01/23/… Is the most recent relevent paper I can find and again, it just discusses the cell surface strcture as analagous to plants. Nothing about how it aids the cell to achieve such sizes. $\endgroup$ – James Feb 9 '15 at 20:05
  • $\begingroup$ weird gaps in the literature... perhaps to motivate you to find your own and examine it under the microscope... $\endgroup$ – Jasand Pruski Feb 9 '15 at 21:41
  • $\begingroup$ Its a bit expensive for me to get to a reef where these things grow :P But also it seems like thats all anyone has done and it doesn't tell them much. They have used crystallography, electron microscopy, atomic microscopy etc. From what I can tell all they have learned is that it looks pretty normal for an algae. My guess is that there needs to be something special in the cytoskeletal structure, but hopefully a specialist will come along and tell me why thats a silly idea! $\endgroup$ – James Feb 10 '15 at 14:54
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Ventricaria ventricosa (previously called Valonia ventricosa) is not exactly a single cell. It has a coenocytic structure with multiple nuclei and chloroplasts. As Jasand Pruski correctly guessed the organism possesses a large central vacuole which is multilobular in structure (lobules radiating from a central spheroid region).

The entire cell contains several "cytoplasmic domains" with each domain having a nucleus and a few chloroplasts (See the figure below). Cytoplasmic domains are interconnected by cytoplasmic "bridges" that are supported by microtubules (like in axons). The peripheral cytoplasm (whose membrane is overlaid by the cell wall), is only about 40nm thick.

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Fluorescent confocal micrograph of a cytoplasmic domain. From Shepherd et al. (2004) Protoplasma 223: 79–91. Here, n is nucleus c is chloroplast and v is vacuole.

So these are not huge cellular balloons that can burst easily. Nonetheless a huge structure like this is susceptible to injuries by shearing forces of water currents and by other organisms like fish which may try to feed on them. These organisms have some mechanisms other than the cytoplasmic organization that allows them to cope up with injuries:

  • When the outer membrane ruptures, then the cytoplasm contracts with the help of actin filaments that are attached to the former. Subsequently the membrane fuses, protoplasts form and then cell wall is regenerated. This movement is likely to be triggered by extracellular calcium.

  • A single cytoplasmic domain with nucleus and other essential organelles can regenerate an entire organism.

  • A muclilagenous layer made up of sulphated polysaccharides coats both the vacuolar and extracellular surfaces. During injury it impedes the movement of water and ions (by retaining them in the form of a gel) and therefore acts like a plug.

Other organisms like this (siphonous algae) also have similar mechanisms. Some even employ calcium carbonate to maintain rigidity.

REFERENCES:

  1. Shepherd et al. (2004) Protoplasma 223: 79–91
  2. Menzel (1988) Protoplasma 144: 73-91
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  • $\begingroup$ Ah, fascinating! I'm familiar with coenocytic in terms of fungal hyphae structures. I'm still impressed at the shape of it, most of these other structures are still thin in one plane or another. Just to clarify anecdotally, these organisms are "solid" like a golf ball rather than hollow like a football? I also read that crabs like to munch on them! $\endgroup$ – James Feb 10 '15 at 17:44
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    $\begingroup$ @GoodGravy They are not really solid because there is a central cavity i.e. the vacuole. See figure 2b-e in the first paper. From the optical sections it seems that the "cellular zone" is ~7µm thick. So they are indeed like footballs, with an outer layer (cell wall) the bladder with a central cavity. However unlike the bladder the cellular layer is relatively thicker and organized. $\endgroup$ – WYSIWYG Feb 10 '15 at 17:49

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