Seaweed is single celled critters?

[tdn]

The other night I was watching Nova, in which they talked about coulerpa taxifolia, a form of seaweed that’s threatening to eat California. In it, they mentioned that coulerpa taxifolia, like all seaweeds, is a single-celled plant. I find it hard to believe that something that grows so large and complex (It has roots, branches, and leaves) could be just one cell. Is this possible? Are all seaweeds like this?

[ultrafilter]

It’d be easier to believe that what we think of as seaweed is a colony of single-celled organisms, but I’m just guessing here.

[astro]

Some are but not all. The more complex forms appear to be multi cellular.

Green seaweeds are part of a large group called Division Chlorophyta, which includes all green algae. Only about 10% of green algae are marine species, most live in freshwater. Green algae are more closely related to the green vascular land plants than any other group of algae. They have the same photosynthetic system as vascular plants, which is dominated by the pigments chlorophyll a and b. The chlorophyll pigments are what give this group of algae their beautiful green colouration. Chlorophyll a and b absorb red light, which is available in shallow waters, but absent in deeper water. Thus, green seaweeds are most commonly found in the shallow intertidal zone. There are more species of green algae found in warm tropical oceans than in cooler temperate seas. The structure of green seaweeds ranges from single-celled forms to multi-cellular sheets, and branched filaments

[Colibri]

No, that’s incorrect. *Caulerpa * and most other “seaweeds” (which includes a great diversity of marine alga that are only distantly related) are multicellular.

The confusion is in the fact that many other green algae are unicellular, and some of these are very closely related to the multicellular forms. Because of this, all the green algae, both unicellular and multicellular, are often included in the Kingdom Protoctista (or Protista, but this properly only includes unicellular forms). Perhaps you misunderstood what they were getting at?

[Johnny_L.A]

I read about Caulerpa taxifolia in the June issue of Calypso Log. I was curious about the ‘single cell’ claim, as I don’t think it’s true. A very casual google search seems to indicate that it is not a single-cell organism (although the NOVA site says in preview that a single cell can be enough to infest an area).

On a related note (related because it’s also in Calypso Log: ‘Located 2.6 billion light years away in the galaxy cluster known as MS 0735.6_7421, the explosion has lasted more than 100 million years.’ If the explosion is 2.6 billion light years away and it’s only been exploding for 100 million years, how can we see it?

I’m guessing that the ‘single cell’ statement and the ‘100 million years’ statements are just poorly-worded.

Of course, IANAS.

[David_Simmons]

@Johnny L.A.

If the explosion is 2.6 billion light years away and it’s only been exploding for 100 million years, how can we see it?

Let’s see now - if the explosion happened 2.7 billion years ago, the first light from the explosion reached us 100 million years ago and light from the explosion is still reaching us. Doesn’t that work?

[Johnny_L.A]

@David Simmons

Let’s see now - if the explosion happened 2.7 billion years ago, the first light from the explosion reached us 100 million years ago and light from the explosion is still reaching us. Doesn’t that work?

Yes, but I thought it was poorly-worded. So I thought that calling the algae ‘single cell’ might be a similar mis-wording.

[tdn]

Google on “caulerpa taxifolia single cell”, and you’ll get a ton of hits. Every news source that came up flatly states that that’s what it is. If it’s a result of poor phrasing, then a misconception is spreading out of control. Sort of like the weed itself.

[Colibri]

Looks like I was wrong. A quick search suggested Caulerpa was multicellular, but actually if and its relatives have a peculiar structure. They have only a single cell wall surrounding the entire organism, and hence a continuous cytoplasm, but have multiple cell nuclei. This is known as a coencytic structure.

From here

Typically, the Bryopsidales are filamentous and consist of a mass of continuous cytoplasm held inside a hollow, cylindrical cell wall. There are usually no cross-walls and therefore no physical separation between nuclei or other internal structures. In some genera, such as Halimeda, the filaments are highly branched and packed into a dense mass called a thallus.

In Caulerpa the filaments are massively enlarged, strengthened with internal support struts, and shaped into organs resembling the stems, leaves and roots of land plants. Unlike land plants, however, these organs are not made up of many separate cells packed together. Instead they are filled with uninterrupted cytoplasm held in place by a single, continuous cell wall.

In essence, any piece of Bryopsidalean algae could be considered a single, enormous, multinucleate cell. Such a cell is said to be coenocytic. This condition appears elsewhere among living things, but never so strikingly. Caulerpa paspaloides, for example, can form a network of rhizomes filled with a mass of cytoplasm 0.25 cm in diameter and several meters long. In some species of Halimeda there is evidence that whole meadows of natural growth may consist of a single individual connected by fine filaments running through the substrate.2 This suggests that some of the largest and most morphologically complex cells of the living world belong to this algal order.

[Colibri]

Note that this is NOT true of most seaweeds, including green, red, and brown algae. The majority of seaweeds are normal multicellular organisms.

[Johnny_L.A]

That’s… Well, it’s amazing! I had no idea that it was true.

So, if you puncture it will all of the cytoplasm flow out of the hole (assuming you kept the hole open)?

[Colibri]

@Johnny L.A.

So, if you puncture it will all of the cytoplasm flow out of the hole (assuming you kept the hole open)?

While I don’t know the specifics, I’m sure they must have some clotting-type mechanism to block up any breach in the cell wall. They can spread by fragmentation, so they must have some way to seal a break.