Flap, flutter, and flow: morphological adaptation of marine macroalgae under hydrodynamic stress

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The intertidal zone of wave-swept rocky shores is among the most physically stressful habitats on the planet. At low tide, seaweeds are exposed to terrestrial conditions and must resist temperature and desiccation stresses. At high tide, crashing waves impose hydrodynamic forces that threaten to break or dislodge algae from the rocks. Despite this adversity, rocky shores support rich and morphologically diverse communities of macroalgae. * My research program explores the biomechanical, morphological and physiological adaptations of seaweeds to physical stressors, such as wave-induced drag force. Living in a hydrodynamically stressful habitat, most macroalgae develop flexible thalli that "go with the flow" and many species can adjust thallus morphology to suit environmental conditions. Experiments in the current proposal test adaptations to hydrodynamic stress at multiple scales of analysis, from cell wall chemistry, through tissue composition, to whole organism performance. Experiments will explore the evolution of novel structures in wave-swept macroalgae. For example, we will investigate the repeated evolution of erect segmented fronds from calcified crustose ancestors, allowing otherwise calcified thalli to be flexible, and the repeated evolution of buoyant structures in brown macroalgae to compensate for tissue degradation and dislodgement. Through chemical, structural, and biomechanical comparisons, we will determine the mechanism of parallel evolution in these ecologically important features. Additional experiments will explore trade-offs between morphology and physiology during macroalgal evolution. For example, we have shown that branched seaweeds experience more drag and must attach more securely to the substratum than unbranched seaweeds to avoid dislodgement. By cutting different shapes out of real seaweeds, but holding photosynthetic surface area constant, we will test the hypothesis that branching also increases photosynthesis and nutrient exchange in flow, compensating for biomechanical costs. Further experiments will investigate the drivers of morphological plasticity in flow and the limits to biomechanical adaptation under future climate stress. Using our state-of-the-art growth flumes, we will first grow kelps at different flow speeds to determine how precisely they are able to adjust blade morphology to limit drag. We will then increase temperature and dissolved CO2 concentration to test the impact of climate stress on the morphological response to flow. These data will lend insight into the current and future survival of these important forage- and habitat-forming seaweeds in wave-swept habitats. In sum, proposed experiments examine the biomechanical adaptations and functional morphologies of marine macroalgae that have been shaped by selective pressures imposed by the physical environment. Exploring a broadly diverse set of macroalgae at multiple scales of analysis, my lab's integrative and innovative approach seeks to explain how evolution has orchestrated the striking morphological diversity of seaweeds we see today.