Just about every saltwater aquarium owner is familiar with a macroalgae known as "bubble algae". This form of algae attaches tightly to a rock and grows a green, reflective bubble, reaching several centimeters in diameter before bursting and releasing its contents. I had two "bubbles" growing on a rock in my aquarium, so I decided to remove them and find out just what was inside. In this case, the species was Ventricaria ventricosa, which is identified by its oblong shape. My aquarium does not appear to be afflicted by any other species.
Physical Description: After I detached the larger of the bubbles, it surprisingly started floating towards the top of the water column. The liquid inside the bubble was clearly less dense than the surrounding saltwater. After removing it from the water, I noted that it was rather "squishy", but still very strong and did not burst despite my squeezing it. The texture was smooth for the most part, but the area where it had attached to the rock was very rough and had surface growths. I also noticed the presence of a small air bubble on the inside of it, which was interesting, considering that it had grown entirely underwater. Perhaps it was oxygen produced by photosynthesis? Additionally, I removed a smaller bubble algae from the rock and this one was much stronger and did not deform when I squeezed it, an advantage that its size apparently lent to it.
Extracting the Contents: Since the exterior skin of the algae was quite tough, I decided to use a scalpel to slice it, rather than spray the contents all over after squeezing it to the point of rupture. I noticed that after slicing the skin, the liquid inside was still reluctant to come out and required considerable external pressure to coax it onto my glass microscope slide. The liquid had a hint of green to it, but looked mostly like normal water. I put some of it on a refractometer and it gave me a specific gravity value of about 1.035, which would suggest that the salinity of the contents was higher than the saltwater (1.026), but I wasn't sure since other things in the liquid could have impacted the refraction. The liquid did not really have an odor to it.
Microscope Examination: After preparing my slide, I put it under my microscope and gave it the 400x treatment. Sure enough, the liquid was filled with green cells. Due to the abundance of free-floating cells, I believe that the liquid normally contains such cells, which are released and propagate after the skin bursts underwater. The cells in the skin and the free-floating cells appeared to be very similar in structure, but my microscope's limited power did not allow me to examine the contents of the cells in detail. I approximated the diameter of these cells to be about 5 micrometers, a bit smaller than the human red blood cells I had examined earlier.