/ 24 June 2024

Scientists unravel drivers of global zinc cycle in the oceans

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Crew on board the SA Agulhas II preparing to deploy seawater collection bottles between surface and depths of up to 4500 metres in the Southern Ocean. (Ryan Cloete)

A team of local and international scientists have unravelled the drivers of the global zinc cycle in the oceans, with implications for a changing climate.

In their study published in the journal Science, the researchers confirmed the important role of the Southern Ocean in global biological processes and in the carbon cycle, and the “underappreciated” part played by inorganic zinc particles in these cycles.

The Southern Ocean plays the greatest role in the global productivity of phytoplankton, which are microscopic marine algae responsible for absorbing atmospheric carbon dioxide. Zinc, which is present in trace quantities in seawater, is an essential micronutrient In these processes.

According to the researchers, zinc is critical to many biochemical processes in marine organisms, particularly for polar phytoplankton blooms. When phytoplankton blooms perish, the mineral is released again.

The scientists were baffled as there was an observed disjunct between zinc and phosphorus — another nutrient essential for life in the oceans — even though the nutrients share a region with phytoplankton. Instead, a “strong but inexplicable” coupling between zinc and dissolved silica or silicon dioxide is often seen.

Now, for the first time, the biogeochemical processes driving the oceans’ zinc cycle can be explained with confidence, said Alakendra Roychoudhury, a specialist in environmental and marine biogeochemistry at Stellenbosch University, and a co-author of the article.

Central hub

Since 2013, his research group in the department of earth sciences has joined three expeditions of South Africa’s polar research vessel, the SA Agulhas II.

“For students, it’s a great learning experience but, at the same time, they also realise their potential in terms of working in really harsh conditions,” he said.

“The waves are 10m, 15m high sometimes and your ship is rolling and you still have to work … It’s a very harsh reality for a lot of our students to go on these cruises and realise how hard this work is.”

Crossing the vast Southern Ocean on the way to Antarctica in both summer and winter, the team collected seawater samples from the surface and deep ocean as well as sediments — naturally occurring material that is broken down.

Ryan Cloete, the co-first author of the paper and now a postdoctoral fellow at the Laboratory of Environmental Marine Sciences in France, took part in two of these expeditions. 

“Studying the Southern Ocean is so important as it acts as a central hub for global ocean circulation,” he said. “Processes occurring in the Southern Ocean are imprinted on water masses which are then transported to the Atlantic, Indian and Pacific oceans.” 

Working with researchers from Princeton University, the universities of Chicago and California Santa Cruz, as well as the Max Planck Institute for Chemistry, the samples were subjected to detailed particle-by-particle analysis, using X-ray spectroscopic techniques at a synchrotron facility. This allowed the study of the samples at atomic and molecular levels. 

Warming oceans

This understanding of the global zinc cycle has important implications in the context of warming oceans, according to Roychoudhury. 

A warmer climate increases erosion, leading to more dust in the atmosphere and, consequently, more dust being deposited in the oceans. More dust means more scavenging of zinc particles, leading to less zinc being available to sustain phytoplankton and other marine life. 

From a climate perspective, basically, because more carbon dioxide is accumulating in the atmosphere, “we are all looking at what are the processes that are controlling or that can reduce carbon dioxide from the atmosphere”, he said.

“And one of these processes is through photosynthesis in the ocean but the problem is that large parts of the ocean are not very productive and the reason for that is because you don’t find a lot of these trace metals in the ocean, like zinc or iron, etcetera. 

“What we are looking at in these remote regions where there’s no obvious supply source … is where the zinc comes from and what happens to it so it can become available to these phytoplankton. We are looking at molecular scale processes but the implications are on a global scale in terms of the carbon uptake and climate change.”

Roychoudhury added that most of the oxygen that we breathe is actually produced in the ocean, through phytoplankton. 

In-situ seawater pump deployed from the SA Agulhas II research vessel in the Southern Ocean. The pump filters seawater and allows the study of ocean particles between surface and deep ocean. Image credit: Ryan Cloete

Phytoplankton uptake

According to the scientists, in summer, it seems that higher productivity leads to a greater abundance of zinc in the organic fraction of the surface ocean, which can readily become available for uptake by phytoplankton. But they also found high concentrations of zinc associated with debris derived from rocks and earth and from atmospheric dust, present in these samples.

In the open ocean, the interplay between zinc’s association or dissociation from particles is pivotal for replenishing dissolved zinc to support marine life. Cloete said that because of poor growing conditions in winter, zinc particles are “literally ‘scavenged’ by inorganic solids such as silica, abundantly available in the form of diatoms, as well as iron and aluminium oxides”. 

Diatoms are microalgae — unicellular organisms with a skeleton made of silica — explaining the strong association between zinc and silica in the oceans.

When zinc is bound to an organic ligand (an ion or molecule), it is easy for marine life such as phytoplankton to take it up. Zinc in a mineral phase, however, is not easy to dissolve and will therefore not be easily available for uptake. In this form, particulate zinc can form large aggregates and sink to the deep ocean, where it becomes unavailable for uptake by phytoplankton.

Important micronutrients

According to Cloete, the novel approach to studying the oceanic zinc cycle opens the door to investigating other important micronutrients. “Like zinc, the distribution of copper, cadmium and cobalt could also experience climate-induced changes in the future.”

Roychoudhury added that, until now, a lot of scientific effort has gone into looking at iron as one of the major trace metals that is also limiting this productivity. 

“But now we are progressively finding that it’s not just iron. These other trace metals also play a role because a lot of these trace metals are involved in these enzymatic reactions. If zinc is not available for example, in that enzyme, the phytoplankton can substitute zinc with say, for example, cadmium or cobalt, which in terms of the size of the atom, are similar size. 

“For that enzyme to function, if zinc is not available, they can take that other metal and still start to function, so there is quite an intricate relationship in what ultimately limits a lot of the productivity. We are basically going towards that, to see how these other trace metals play a role.”

Roychoudhury said the findings reaffirmed the Southern Ocean’s global influence in regulating the climate and the marine food web. 

“The earth system is intricately coupled through physical, chemical and biological processes with self-correcting feedback loops to modulate variability and negate climate change. Our findings are a prime example of this coupling where biochemical processes happening at the molecular level can influence global processes like the warming of our planet.”