Gas-filled floats help keep kelp fronds near the water surface to enhance photosynthesis.

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Kelp (brown algae seaweed) possess gas-filled floats known as pneumatocysts that enable fronds to float on the water surface, maximizing exposure to sunlight and enhancing photosynthesis. The gas content of the pneumatocysts can vary, but are usually filled with a combination of oxygen, nitrogen, and carbon dioxide. Depending on the species, the kelp may contain one large pneumatocyst or several smaller pneumatocysts distributed throughout the kelp.

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“…gas-filled floats keep [kelp] fronds at or near the sea surface…” (Koehl and Wainwright 1977:1067)

Journal article
Mechanical adaptations of a giant kelpLimnology and Oceanography, 22(6): 1067–1071November 1, 1977
Koehl MAR; Wainwright SA

“[The floats] contain a mixture of oxygen, nitrogen, and carbon dioxide in varying relative proportions depending on the physiological status of the plant and the partial pressure of the particular gas in the surrounding medium. These gas-filled pneumatocysts provide buoyancy and let entire plants float to the sea surface” (Thiel et al. 2005:186).

Journal article
The ecology of rafting in the marine environmentOceanography and Marine Biology. 43:181-264.January 1, 2005
Thiel M, Gutow L.

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Phaeophyceae (brown algae)

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