Iron concentration key in giant kelp production according to IMAS study

Study lead author Ellie Paine, of the Institute for Marine and Antarctic Studies, said it was the first time it had been scientifically proven that the growth of giant kelp Macrocystis pyrifera could not be sustained at oceanic dissolved iron concentrations.

"Iron has a significant influence on the function of the biological carbon cycle," Dr Paine said.

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"It sets the rate of photosynthesis and nitrogen uptake in both phytoplankton and seaweed."

The research used seawater sampled from the Southern Ocean at a depth of 20 metres, in a trace-metal-free incubation experiment.

A positive pressure 'bubble' was custom built to contain the experiment and study the importance of dissolved iron for photosynthesis, nitrogen uptake, growth rates and dissolved organic carbon release in giant kelp.

Dr Paine said they chose to use giant kelp because it was a fast-growing seaweed and had a high capacity to take up nutrients from seawater.

"It's also why many planned ocean afforestation endeavours focus on this species," Dr Paine said.

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"We took a closer look at dissolved inorganic iron, a trace metal that is essential for seaweed growth - and found that, under limiting iron concentrations, the seaweeds showed significant physiological stress and high mortality after two weeks.

"This indicated that the concentration of iron in the oceanic seawater was far too low to support kelp growth."

She said the study highlighted that iron availability in the open ocean was a crucial aspect of seaweed growth and should inform the ongoing ocean afforestation debate.

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