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Under pressure : biomechanics of buoyancy in Bull Kelp (Nereocystis leutkeana) Liggan, Lauran

Abstract

Maintaining buoyancy with pneumatocysts is essential for subtidal seaweeds with long flexible thalli, such as Nereocystis luetkeana (Nereocystis), to achieve an upright stature and compete for light. However, as Nereocystis grows, pneumatocysts are exposed to significant changes in hydrostatic pressure. Exposure to changing hydrostatic pressure could cause complications since the pneumatocyst is filled with gases that may expand or contract, potentially causing pneumatocysts to break, flood, and no longer be buoyant. This study explored how Nereocystis pneumatocysts resist biomechanical stress and keep the developing sporophyte upright in the water. Throughout development, pneumatocysts had an internal pressure consistently less than atmospheric pressure (3 – 100 kPa), indicating pneumatocysts always experience compressional loads. The structural integrity and design of the pneumatocyst to resist buckling was assessed by measuring compressional modulus (material stiffness), calculating material stress, analyzing critical geometry, and estimating critical buckling pressure. Small pneumatocysts found at depth (inner radius = 0.8 - 0.9 cm; wall thickness = 0.2 cm) were demonstrated to have reached a critical size in development and are at greatest risk of buckling. Pneumatocysts do not adjust material properties or geometry to reduce wall stress, but they are naturally resistant to hydrostatic loads. Critically small pneumatocysts are estimated to buckle at 35 m depth, which was observed to be sporophytes’ lower limit in the field. Data suggest that hydrostatic pressure, not just light limitation, might explain the maximum depth to which Nereocystis is capable of growing. Pneumatocyst gas composition did not change throughout development, and contrary to previous studies, internal gas concentrations were different from the atmosphere with O₂, N₂, CO, and CO₂ concentrations of 59%, 40%, 1.6%, and 0.6% respectively. Furthermore, pneumatocyst surface area to volume ratio did not correlate with the exchange of gases produced from photosynthesis and respiration. As sporophytes grow, total buoyant force is steadily outpaced by the weight of growing thalli, and the risk of the pneumatocyst sinking increases. Adult sporophytes are estimated to sink when pneumatocysts volume reaches 1.3 L, close to the maximum observed size in the field.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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