Mussel Attachment in a Dynamic Ocean: an Ecomechanical Perspective
George, Matthew Nicholas
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Marine mussels are masters of underwater adhesion, attaching to a wide variety of substrates using an array of collagen-like fibers (byssal threads) that are each tipped with a powerful protein-based adhesive (plaque). The molecular mechanisms that underlay plaque adhesion are an integral part of a worldwide mussel aquaculture industry, while also inspiring the development of anti-fouling coatings for use in the maritime industry, and the design of medical adhesives for use as sutures in the human body. In this dissertation, I seek to understand the mechanisms underlying plaque adhesion using an ‘ecomechanical’ perspective, by exploring the direct interaction between the local environment and the adhesive plaque after it is deposited on a surface. To accomplish this, in Chapter 1 I perform a series of laboratory experiments wherein mussels make attachments to surfaces under standard ‘open-ocean’ conditions, and attachments are separated from the animal and matured in different seawater treatments. When sampled overtime, plaque attachment strength increased over time, nearly doubling in adhesion strength (+94%) after 12 days in seawater at pH 8.0. However, holding plaques in low pH conditions (<7.0) prevented this strengthening, causing the material to tear more frequently under tension. These results point to the role of seawater pH as a ‘molecular trigger’ during the adhesive curing process, necessitating that plaques have access to a basic pH after initial substrate adhesion is achieved. Chapter 2 expands on this finding by investigating the influence of other seawater parameters, such as seawater temperature, salinity, and dissolved oxygen concentration on the plaque curing process. In contrast to pH, high temperature (30ᵒC) and low salinity (1 PSU) had no effect on adhesion strength, while incubation in hypoxia (0.9 mg L-1) for 12 days left plaques with a mottled coloration. Oxygen deprived plaques were more likely to peel from substrates before the thread could be loaded, leading to a 51% decrease in adhesion strength. Atomic force microscopy (AFM) imaging of the plaque cuticle found that plaques cured in hypoxia had regions of lower stiffness throughout, indicative of reductions in crosslinking between DOPA residues of mussel foot proteins (Mfps). Given the demonstrated sensitivity of the plaque during the adhesive’s cure window, Chapter 3 explores the implications observed interactions with the seawater environment, given the dynamic conditions that mussels experience as inhabitants of the nearshore. Five-day excursions in pH and dissolved oxygen measured at a local shellfish farm were replicated in fluctuating laboratory experiments, wherein pH excursions (<5.0) delayed plaque strengthening when applied early in the curing process, while hypoxia decreased adhesion strength after the adhesive had fully matured (<2 mg L-1). In both cases, adhesion strength was rescued after re-immersion in open-ocean seawater conditions. Altogether, these results emphasize that mussel attachment strength is vulnerable to environmental modification, but only if deleterious pH or oxygen conditions are maintained during critical periods of the biomaterial’s lifespan.
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