Ocean acidification reduces thallus strength in a non-calcifying foundation seaweed

Climate change is causing unprecedented changes in terrestrial and aquatic ecosystems through the emission of greenhouse gases, including carbon dioxide (CO₂). Approximately 30% of CO₂ is taken up by the ocean ('ocean acidification', OA)¹, which has profound effects on foundation seaweed species. Negative physical effects on calcifying algae are clear², but studies on habitat-forming fleshy seaweeds have mainly focused on growth and less on thallus strength^3,4. We exposed the habitat-forming brown seaweed Fucus vesiculosus to OA corresponding to projected climate change effects for the year 2100, and observed reduced apical thallus strength and greater loss of exposed individuals in the field. The tissue contained less calcium and magnesium, both of which are important for creating structural alginate matrices. Scanning electron microscopy (SEM) revealed tissue voids in the OA samples that were not present in seaweeds grown under ambient pCO₂. We conclude that under OA, weakened F. vesiculosus will be at a significantly higher risk of physical damage and detachment.