Form and Function of the Pneumatocysts of Marine Algae. I. Variations in the Pressure and Composition of Internal Gases

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Abstract
The factors which affect the pressure and composition of internal gases and the consequent effects on buoyancy of pneumatocysts were examined in 8 spp. of brown algae [Cystophora retroflexa, C. torulosa, Carpophyllum maschalocarpum, Ca. flexuosum. Ca. plumosum, Ca. angustifolium, Sargassum sinclairii and S. undulatum] that occur in New Zealand waters. A variable proportion of O2 (17-35%), slight CO2, (usually less that 1%) and a high N content (65-83%) was characteristic of freshly collected pneumatocysts. Fluctuations in pressure reflected changes in gas composition and are dependent primarily on the O2 exchange activity of the pneumatocyst wall tissues although permeability and thickness of the wall tissues and partial pressure of dissolved O2 in the external medium contribute. Wall tissues are highly permeable to CO2 and consequently metabolic exchanges of this gas do not play an important role in composition-pressure relationships. N while changing in percentage volume does not alter markedly in partial pressure from the level at which it occurs in the surrounding water and plays a passive role. Water vapor in the pneumatocyst gases may condense following compression but is apparently absorbed by medullary filaments as intact pneumatocysts did not contain free moisture. The pneumatocysts have a special respiratory or metabolic function. Diumal changes in pressure and composition that occur in situ have only a slight effect on the buoyancy of pneumatocysts. The time dependent losses in intermal gas volume occurring in the dark and under compression lead to a lowered internal gas pressure and in turn may render these organs more susceptible to implosion with a consequent loss of their buoyant function.