By Vilhelmiina Haavisto, Freelance Writer

Published September 30th, 2021

Microscopic life in our oceans plays a vital role in taking up carbon dioxide from the atmosphere—but can we rely on them to curb our emissions?

From the Amazon rainforest to Australia, wildfires made more intense and frequent by climate change have wreaked havoc around the world in recent years. However, a study recently published in Nature reports that a group of microscopic life forms in the ocean, known as phytoplankton, appear to benefit from these destructive fires.

Wildfires release large amounts of carbon into the atmosphere. For example, the carbon dioxide emissions from Australia’s 2019 to 2020 wildfires alone are estimated to have exceeded the entire country’s reported carbon dioxide emissions for 2018.

Like plants, phytoplankton carry out photosynthesis, using light and carbon dioxide from the atmosphere to grow in a process known as primary productivity. Primary productivity is one of the first steps to making energy and nutrients available for the rest of the non-photosynthesizing levels of the food web.

Small in size, but extremely large in numbers, phytoplankton account for half of all the primary production occurring on Earth, forming the base of the marine food web and providing many other important services to the surrounding ecosystems. However, primary production is often limited by the nutrient-poor conditions in the open ocean. Mixing between the surface and deeper layers is a key source of essential nutrients for phytoplankton, including nitrogen, phosphorus, and iron.

The researchers showed that phytoplankton benefitted from iron that was transported in the ash from the Australian wildfires and deposited in highly iron-limited surface waters of the Southern Ocean. The supplementary iron spurred an increase in primary productivity to such an extent, that an estimated 49% to 95% of all the carbon released by the wildfires was captured by the ensuing phytoplankton bloom.

‘[Phytoplankton] account for half of all the primary production occurring on Earth.’

The area of the bloom was measured using satellites and was estimated to exceed the surface area of Australia itself, in what the researchers described as an ‘unprecedented anomaly’ in primary productivity in the region.

Dr Meike Vogt, a scientist working on marine ecosystem modelling at ETH Zurich and who was not directly involved in the research, highlights that this study is the first to quantify the response of marine ecosystems to large-scale nutrient fertilization due to wildfires. She calls this study ‘intriguing’ and its findings ‘relevant’, noting that the addition of any nutrient in waters where it is limiting will likely stimulate the resident phytoplankton, although this particular bloom was clearly ‘anomalous and big’.

Indeed, large-scale iron fertilization of the oceans has been suggested as a strategy for stimulating marine primary productivity to capture anthropogenic carbon dioxide, and has been tested in regions around the world, including the Southern Ocean, with mixed results.

One key limitation of iron fertilization seems to be that the captured carbon does not always remain in the ocean in the long-term, and may instead be released back into the atmosphere once the phytoplankton bloom wanes. Thus, iron fertilization would need to be maintained continuously to substantially influence carbon cycling in the long run.

The burial of carbon in ocean floor sediment is vital for long-term storage of atmospheric carbon dioxide and determines the ultimate effects of phytoplankton activity on climate change. However, the extent to which carbon captured by large phytoplankton blooms is exported to the deep ocean, and the relative importance of the myriad of factors contributing to successful export, are not yet well understood.

Dr Vogt emphasises that uncovering which organisms are responsible for the export of carbon is a key area of further research; these findings would greatly expand our understanding of marine microbial carbon cycling and may enable us to better forecast ecosystem and food web responses.

‘Monitoring carbon export to the deep ocean is much more resource-intensive than measuring and tracking phytoplankton blooms via satellite.’

‘We are beginning to understand who is where and why’, she says, ‘but now we want to know what they are doing and how this affects ocean carbon uptake and storage.’ However, monitoring carbon export to the deep ocean is much more resource-intensive than measuring and tracking phytoplankton blooms via satellite, as it ideally requires intensive on-site sampling before, during, and after the bloom event.

The effects of iron addition also seem to be highly variable, with the magnitude of the phytoplankton response differing considerably. Indeed, some have argued that the Southern Ocean may be unusually responsive to such interventions because of its highly iron-limited nature, and that blooms of such scales in response to iron addition might not occur elsewhere.

Its effects may also extend far beyond the initially fertilized zone, and it is unclear whether side effects of the iron and ensuing blooms on distant marine ecosystems could ultimately cancel out any initial carbon capture-related benefits.

Other changes in ocean chemistry may also influence phytoplankton communities in ways that are not always beneficial. For example, ocean acidification off the coast of the Canary Islands, which occurs because of increased carbon dioxide concentrations, was found to favour a species of microscopic algae that produces toxins. These kinds of blooms are often difficult to predict and control, and can disrupt marine food webs, reduce export of carbon, and threaten nearby fisheries and associated livelihoods.

Moreover, recent research by Dr Vogt and her colleagues indicates that plankton communities could shift their global distribution in response to warming, potentially threatening the ecosystem services that these communities provide in different parts of the world’s oceans.

Clearly, there is much left to discover about the dynamics of carbon cycling in the oceans, and how this is and will continue to be affected by changing environmental conditions. Long-term data on phytoplankton primary productivity, diversity, and function will be extremely important, as it will bolster efforts to model and predict the responses of these communities to the diverse, interacting effects delivered by climate change.

‘Wildfires may soon begin contributing to global carbon emissions.’

‘Many marine microbes have been around for hundreds of millions of years and will prevail [in the face of climate change]’, Dr Vogt asserts, ‘but whether us mammals and other animals do is another question.’

Until 2019, the IPCC considered wildfires to be carbon neutral, as they are a key part of the global carbon cycle; after the ash settles, forests regrow and reprise their role as large-scale carbon sinks. However, with their mounting intensity and frequency, wildfires may soon begin contributing to global carbon emissions, even with help from nutrient-boosted phytoplankton blooms.

More pressing than wildfires, however, are anthropogenic greenhouse gas emissions and their impacts on all of Earth’s ecosystems. Despite agreements made and targets set, a recent Climate Action Tracker report finds that none of the world’s largest polluters are on track to achieve their climate pledges. It is crucial that we do not let speculation about carbon capture using phytoplankton totally distract us from the urgent need to drastically reduce global carbon emissions.

We should absolutely not plan to rely on phytoplankton to mop up even part of the carbon we put out, especially considering how much we have yet to learn about these communities and how climate change, pollution, and other anthropogenic disturbances to our oceans could influence their activity.

Featured Image: Secchi Disk Study

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