Increasing Carbon Dioxide Gives Seaweed a Bad Acid Trip

New research published in Current Biology shows that ocean acidification weakens the structural strength of the common seaweed, Fucus vesiculosus, also known as Bladderwrack. Experiments led by Alexandra Kinnby and colleagues found that seaweeds grown under elevated carbon dioxide conditions, simulating future ocean acidification, developed weaker tissue and were at higher risk of breaking off. Field trials showed the weakened plants were more easily dislodged and lost from coastal habitats compared to those grown under normal carbon dioxide levels. The results indicate that as oceans continue absorbing human carbon emissions, the tissue integrity of seaweeds like Fucus vesiculosus may decline, threatening these important habitat-forming species.

Acidification Alters Tissue Composition and Structure

The study found that elevated CO₂ actually increased the growth rate and photosynthesis of Fucus vesiculosus. However, despite the larger size, the seaweeds developed significantly weaker tissue when grown under high CO₂ conditions. Measurements showed the force required to break the algae’s fronds was reduced by over 25% compared to those grown under normal conditions. The results reveal a tradeoff – larger but more fragile seaweeds. Even the thick basal tissue showed some reduction in strength when grown under simulated ocean acidification. The findings contradict the notion that enhanced growth from CO2 necessarily yields more robust marine plants.

Interestingly, the study found that seaweeds originating from sheltered, calm waters showed a greater decline in tissue strength under high CO₂ compared to those from wave-exposed shores. The force required to puncture or break samples from sheltered habitats dropped more steeply when grown under acidified conditions. The researchers suggest seaweeds from rougher areas may be pre-adapted to withstand physical stresses, making them less sensitive to CO₂-induced weakening. However, for fragile seaweeds adapted to tranquil waters, ocean acidification could substantially reduce their structural integrity, even with increased growth.

The researchers attributed the loss of strength under high CO₂ to changes in tissue composition and structure. Analysis showed seaweed samples grown under acidified conditions had significantly lower tissue content of the minerals calcium and magnesium compared to those from normal CO₂. These minerals are crucial in fortifying the fibrous polysaccharide matrices that give seaweed fronds their mechanical strength. Microscope images also revealed a more porous and less dense internal structure in samples from the high CO₂ treatments. The weakening of tissue strength appears directly linked to the loss of reinforcing minerals and the development of a more fragile, loosely packed internal matrix under simulated ocean acidification.

Experiments Give Scientists a Chance to Test the Future

The researchers conducted controlled laboratory experiments growing Fucus vesiculosus under present-day CO2 conditions of 400 parts per million (ppm) as well as an elevated level of 1100 ppm, projected for the year 2100. This enabled them to simulate realistic current and future ocean acidification scenarios.

The seaweeds grew for 90 days under these different CO₂ levels. The team then compared several key response variables between the two treatments, including growth rate, tissue breaking strength, composition, and survival rates when transplanted back into coastal habitats.

Importantly, the study included Fucus vesiculosus originating from both sheltered, calm-water habitats and wave-exposed, high-energy shores. This difference allowed them to determine if prior exposure to hydrodynamic forces affects the seaweed’s sensitivity to reduced pH conditions.

Seaweed Responses Assessed Over 90-Day Experiment

The researchers grew Fucus vesiculosus samples under different CO₂ concentrations for 90 days to allow effects to manifest. They measured several key response variables after the exposure period.

The growth rate was analyzed by measuring the increase in total thallus area. A thallus is the vegetative body or fronds of a seaweed. Photosynthetic efficiency was tracked by assessing chlorophyll fluorescence. The team also quantified drag forces on the seaweeds to determine if the projected larger surface area under high CO₂ created increased hydrodynamic loading.

A key response was the force required to break the algae’s fronds and puncture its tissue. This enabled a direct comparison of structural strength between the CO₂ treatments. The tissue composition was also analyzed to detect any mineral content changes that could underlie alterations to the seaweed’s cell wall structure and strength.

Finally, the researchers tested the real-world implications by transplanting samples back into coastal habitats after the CO₂ exposure. They measured survival rates to see if acidification-weakened fronds were more prone to breaking off under natural wave forces.

Findings Underscore Risks of Ocean Acidification

The findings have important implications for ocean acidification driven by human carbon emissions. The world’s oceans absorb around 30% of the CO2 released from human activities, causing seawater pH to drop. While negative impacts have already been observed in calcifying species like corals and shellfish, consequences for foundational habitat-formers like seaweeds have been less studied until now.

This work shows that the common intertidal seaweed, Fucus vesiculosus, exhibits significantly weaker tissue when grown under realistic future CO₂ levels. Field results further demonstrate these acidification-weakened individuals are more prone to breaking off and being lost from seaweed-dominated coastal ecosystems. Such habitat degradation would have cascading effects on the many species relying on seaweeds for food and shelter.

Together, the findings add to growing evidence that ocean acidification may pose a substantial threat to foundational seaweeds worldwide. More research is still needed to confirm if the mechanisms discovered in Fucus vesiculosus also weaken other macroalgae species as carbon emissions continue altering ocean chemistry. However, this study highlights an urgent need to curb CO2 emissions to help maintain the structure and functioning of vulnerable coastal ecosystems.

READ THE ARTICLE
Kinnby, A., Cervin, G., Larsson, A., Edlund, U., Toth, G. and Pavia, H. (2023) “Ocean acidification reduces thallus strength in a non-calcifying foundation seaweed,” Current Biology, 33(18), p. 941-942. Available at: https://doi.org/10.1016/j.cub.2023.07.056.

Cover Fucus vesiculosus. Image: Canva.