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Background

In North of Scotland there are many seaweed types. One is a green algae (chlorophyta).

They are full of 'bubbles'. According to web searches one of those bubbles (there are two types you can notice by sight) are spores; the other ones are called floaters.

According to searches, Kelp (or brown algae) floaters are full of gases. But we find most of the Chlorophyta full of water, some of them contain bubbles but tiny ones.

Conjecture

The water inside could be of a lower density than the sea water, instead of air bubbles.

Question

Any information on this respect?

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One consideration is the energetic costs to the organism. It's going to be energetically more favorable for an organism to be in osmotic balanced (isotonic) with its environment than to constantly work to maintain lower internal osmolarity through active transport of water or solutes. Second thing to consider is cell size and density differentials. Air at sealevel is several hundred times less dense than water, so even tiny gas bubbles would create significant buoyancy for a cell. On the other hand, pure water is only about 2.5% less dense than sea water, which means a much (much) larger volume would be needed to generate equivalent buoyancy.

All that said, there seems to be some plausibility to your conjecture, as there are some large marine phytoplankton that use active water transport to maintain vacuoules at lower density than that of seawater (Source).

This paper discusses some other competing mechanisms as well, but as the author of the article states:

"... most of the cases of positive buoyancy in phytoplankton not generated by gas vesicles concern diatoms..."

That's not an incontrovertable refutation of your conjecture, but it seems to me that gas vesicles are a much more efficient means of generating buoyancy, and would probably be energetically favored over other biological means of generating buoyancy.

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  • $\begingroup$ I really appreciate your knowledge, thanks. (I can't yet upvote answers.) Where do you take the energetic considerations from? $\endgroup$ – mister nobody May 22 at 21:10
  • $\begingroup$ My reasoning is this. For the water in a cell compartment to be less dense, it would need to have a lower solute concentration than the surrounding seawater. Maintaining that disequilibrium between the cell and its environment requires constant energy expenditure to overcome the constant osmotic diffusion of water across the cell membrane. Full disclosure, I'm a microbiologist, so there may be some aspect of algae cell physiology I'm overlooking. I tend to think of every problem from a bacterial POV. $\endgroup$ – MikeyC May 23 at 23:07

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