Deep Sea Carbon and The Climate

Not everything that sinks into the ocean’s depths is lost forever. On the contrary, millions of tons of carbon are stored in the biggest ecosystem on the globe, the deep sea, as part of complex cycles of exchange spanning millennia. Despite years of research, much of its details remain unknown, especially how different kinds of stored carbon are being introduced, stored, sequestered, and released. Gerhard Herndl and his team have been working on what role microbes play in these carbon cycles. With their new research project NEREIDES, funded by the European Research Council, they are setting out to retrieve new insights into this crucial ecosystem from the depths of the sea.  

One current candidate for the missing carbon is organic carbon suspended deep in the ocean. Herndl adds, “We are not sure why this organic carbon is not being metabolized more efficiently.” Research indicates that it is produced by a wide variety of bacteria, archaea, and even fungi in a process called the microbial carbon pump. There are different theories that try to explain its exact origins.  

It could be that the diversity of carbon molecules is just so immense in the deep sea and that only certain microbes can use them, so that in the vastness of the ocean the organisms cannot find the molecules they can eat. Also, the microbes’ waste still contains organic carbon molecules but usually of lower complexity and with less energy content. The remaining organic carbon may just be the unusable remnants of their consumption. 

In their current research project NEREIDES—funded by Herndl’s second ERC Advanced Grant after his first in 2010—the scientists will go on two expeditions in the North Atlantic to take measurements of deep-sea carbon. There, they are concentrating on the larger particulate carbon particles and study their origins and interactions with the microbial carbon pump. This could also help scientists to better understand how the ocean’s carbon cycles interact with human efforts of drawing more carbon dioxide from the atmosphere into the oceans.

The scientist takes a sceptical stance towards many technological solutions of carbon dioxide removal. For example, he and his team showed that increasing algal growth using deep-sea nutrients and then harvesting them to store them in the deep ocean does not work as planned. The concentrated carbon in the algae gets metabolized quickly and is again releases into the atmosphere. “Planting mangroves and seaweed helps biodiversity, but does not store much carbon in the long run,” Herndl adds. “Changing the ocean’s acidity would require such enormous amounts of minerals to be dumped into it that this cannot be economically feasible—not to speak of unforeseen ecological effects.” 

While many researchers like Herndl are critical of these efforts, there are companies and countries pushing for these solutions, as such negative emissions have already been included in the Paris Agreement’s climate targets. Herndl sees himself as a voice for scientific rigor and fundamental research. “Scientists must take care to not become complicit in delaying the necessary transformation of the economy into an actually sustainable one with zero emissions by doing research that suggest technical solutions of capturing and storing carbon from the atmosphere to combat the climate crisis,” he cautions. “Marine carbon sinks should not be seen as technological solutions when the problem’s root of continued carbon emissions is not being sufficiently addressed by politics.” 

Both Herndl’s research and his policy work are supported by the Environmental and Climate Research Hub (ECH). He adds, “The hub provides us with a great platform for interdisciplinary exchange. I have been learning about methane production in marshes or the distribution of microplastics in the environment.”