Final answer:
Pneumatocysts provide adaptive buoyancy for seaweeds, allowing them to stay afloat for optimal photosynthesis and access to light and carbon dioxide.
Step-by-step explanation:
The adaptive significance of the pneumatocysts of some seaweeds is that they allow the algae to float and thereby maintain an advantageous positioning in the water column to maximize photosynthesis. Pneumatocysts are gas-filled bladders that provide buoyancy to the seaweed, helping these organisms to stay closer to the water's surface where sunlight is abundant. Similar to how the pneumatophores of mangroves enable gas exchange for roots in oxygen-poor sediments, pneumatocysts are specialized structures for flotation. This adaptation supports the seaweed’s access to essential resources like light and carbon dioxide, which are critical for their growth and reproduction.