Cristina Romera-Castillo: “In the Mediterranean we see the effects of the climate crisis sooner”

Every June 8th, World Oceans Day invites us to reflect on the health of this vast blue frontier that regulates life on Earth. This year, the anniversary arrives with an air of cautious victory following the historic adoption by the UN of the High Seas Treaty, a legal framework designed to protect biodiversity in waters that do not belong to any country and which represent two-thirds of the marine surface. However, to understand the global health of the ocean, sometimes one must look at what is small.

This is what oceanographer and CSIC scientist Cristina Romera Castillo does. From her laboratory, located at the Institute of Marine Sciences (ICM-CSIC), she investigates what happens when plastics decompose in the ocean, how they alter the chemistry of the water, and to what extent they can influence the climate. In her book Antropocéano she explains how human activity is transforming the seas and why the Mediterranean has become a preview of the future that awaits the rest of the oceans.

When we talk about plastics in the ocean, almost everyone's mental image is of "trash islands" or a turtle trapped in a net. However, her object of study is much more subtle and, perhaps for that reason, more unsettling. When a piece of plastic reaches the sea, what exactly happens to it?

— We study the degradation of plastic in the sea from a chemical point of view. When this material reaches the sea, it begins to degrade, and the factor that most affects this process is the sun's ultraviolet light. As it deteriorates, it releases chemical compounds into the marine environment. What many people forget is that plastic, for the most part, is carbon. Therefore, as it degrades, it releases this carbon in the form of compounds into the water. Our job is to quantify how much carbon it releases and, above all, to determine if this amount is high enough to impact the global carbon cycle or the biogeochemistry of the water. We are also very interested in its interaction with microscopic life: we have seen that there are bacteria that degrade these compounds and, in doing so, these microorganisms reproduce and generate CO₂. We are currently conducting experiments with bacteria and phytoplankton to understand how these released chemical compounds affect them.

Does the plastic that reaches the oceans also contribute to global warming?

— It is an active line of research. A few years ago, an article was published indicating that, through degradation and especially due to the incidence of ultraviolet light, plastic released methane; other subsequent studies pointed to the release of CO₂. What we are doing now is studying this production of greenhouse gases using different types of plastics. The goal is to see which one releases more and which one less, but also to put these figures into context: we want to understand if this amount of gases is significant with respect to other natural or anthropogenic sources that we already know. We need to know if plastic has an important contribution or if, in comparison with the rest of the sources, its impact is minor. It is one more piece of the climate puzzle that we are trying to fit together.

News about the discovery of bacteria that learn to feed on plastic are frequent. Are we close to having a biological solution to the problem? 

— It is not quite like that, and it is important to clarify it so as not to create false expectations. Bacteria have been found that have an enzyme capable of degrading PET plastic, but these bacteria were found in terrestrial landfills, not in the sea. As a result, many studies are being carried out to see if there are other species that degrade other types of plastics or if they have genes that activate these enzymes. But, today, no bacteria that degrades plastic so clearly has been seen in the sea. What does happen is that what floats in the sea is colonized by microorganisms –bacteria, phytoplankton, fungi– that create what we call a biofilm. This biofilm contributes to degrading the plastic, producing cracks or consuming some of its compounds, but it has not been seen to make it disappear completely. It is such an extremely slow degradation that it cannot be considered a solution or a remediation right now. We do not want the message to be "bacteria will rid us of plastic," because then people will think that they can continue throwing trash into the sea with total impunity. The most important thing is to avoid generating waste whenever possible.

What do the water samples from Barcelona and Blanes tell you?

— We carry out monthly sampling in Blanes, at a point one kilometer from the coast that is much cleaner and more pristine; the department of marine biology and oceanography at the ICM-CSIC has been collecting data in this area for over twenty years. In contrast, when we take samples in Barcelona, we find that the water is much more contaminated due to the pressure from the city and the discharges. In the city's beach area, there is a high concentration of plastics and a lot of dissolved organic matter. When it rains heavily, the sewage system does not have the capacity to handle so much water. Then, the overflows open and everything ends up in the sea: organic matter, but also fecal bacteria. When we analyze the water after a rain episode, the contamination is evident. As a scientist, I am concerned about this constant anthropogenic pressure, to which are also added the chemical compounds from the sunscreens that we use massively in the summer. All of this affects the biogeochemistry of the water.

Why is the Mediterranean such a valuable laboratory for understanding the future of the oceans?

— The Mediterranean behaves like a small-scale laboratory. Being a small and closed sea –only connected to the Atlantic through the Strait of Gibraltar–, and being subjected to such strong human pressure, here many global problems are seen to be increased or manifest much more quickly. What happens in the Mediterranean is already happening in the oceans, only that here it is observed sooner and with greater intensity.

How does global warming affect the survival of commercial species and corals?

— When the surface water warms up, it becomes lighter and stops mixing with the cold water below. This creates a kind oflid invisible that separates the two layers. The problem is that the animals living at the bottom need oxygen to breathe, just like us. Normally, oxygen enters from the air at the surface and goes down, but this heat lid prevents new oxygen from reaching the bottom. Since organisms continue to breathe the little oxygen that remains, it ends up running out, and the consequences are fatal. Furthermore, this warming not only affects fish; it also disrupts ocean currents, changes the water's chemistry, and causes the extreme weather phenomena we are already suffering.

Is the ocean really the planet's lung?

— The ocean does a lot for us: it regulates the climate; it sequesters CO₂, which helps to mitigate the effects of global warming; it provides food, and is an essential transport route. The oxygen it produces is vital, especially for the very organisms that live in it. We must be aware that the ocean affects all geographical regions of the planet, not just the coast. What happens at sea influences the climate of inland areas. There is a total interconnection that we often forget.

We are seeing a change of cycle in oceanography, with more and more women leading projects and expeditions.

— Undoubtedly, the participation of more and more women in research enriches investigation in all disciplines. In the end, it is about moving forward, and for too long half of the population has been vetoed. There was half of human intelligence that could have contributed to the advancement of science and technology and which was nullified. The fact that there are now women contributing to it is enriching at all levels and an act of intelligence. We are now more capable people providing solutions to problems that are global and urgent.

Are we still in time to repair the damage we have caused to the oceans?

— The damage, especially from microplastics, is impossible to clean up completely. If we stopped today, we would halt the entry of new material, but what is already in the trophic chain will continue to circulate there for a long time. Nature is slower than our actions. We see very rapid changes caused by warming, but recovering the balance requires a lot of time. However, in the last chapter of my book, I speak precisely about examples from the past in which serious environmental damage was reversed thanks to international cooperation. We banned DDT in pesticides, we eliminated lead in gasoline—which is now prohibited in almost all countries—and we curbed the CFCs that caused the hole in the ozone layer. Nature damaged by those actions has begun to recover. If we did it in the past, it would be possible to do it now as well. We can prevent the damage from becoming even more serious and, in some cases, reverse it if we take action now.