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

Every June 8, World Oceans Day invites us to reflect on the health of this vast blue frontier that regulates life on Earth. This year, the commemoration 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 you have to 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 water chemistry, and to what extent they can influence the climate. In her book "Antropocene she explains how human activity is transforming the seas and why the Mediterranean has become a preview of the future awaiting the rest of the oceans.

When we talk about plastics in the ocean, almost everyone's mental image is of "garbage patches" or a turtle trapped in a net. However, her object of study is much more subtle and, perhaps for that reason, more disturbing. 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 reaching the oceans also contribute to global warming?

— It is an active line of research. A few years ago an article was published indicating that, with 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 greenhouse gas production using different types of plastics. The objective is to see which one releases more and which one releases less, but also to put these figures into context: we want to understand if this amount of gas is significant compared to other natural or anthropogenic sources that we already know. We need to know if plastic has an important contribution or if, compared to other sources, its impact is less. It is one more piece of the climatic puzzle that we are trying to fit together.

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

— It's not quite like that, and it's important to clarify this to avoid generating false expectations. Bacteria capable of degrading PET plastic have been found, but these bacteria were found in terrestrial landfills, not in the sea. Because of this, many studies are being conducted to see if there are other species that degrade other types of plastics or if they have genes that activate these enzymes. However, to date, no bacteria have been clearly seen in the sea that degrade plastic. 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 remediation at this time. We don't want the message to be "bacteria will rid us of plastic," because then people will think 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 take samples monthly in Blanes, at a point one kilometer from the coast that is much cleaner and pristine; the department of marine biology and oceanography of ICM-CSIC has been collecting data in this area for more than twenty years. On the other hand, when we take samples in Barcelona, we find that the water is much more contaminated due to the pressure of the city and 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 cannot cope with so much water. Then, the overflows open and everything reaches the sea: organic matter, but also fecal bacteria. When we analyze the water after a rain event, 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 we use massively in the summer. All 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 enclosed sea – only connected to the Atlantic through the Strait of Gibraltar – and subjected to such strong human pressure, many global problems are amplified or manifest much more rapidly here. What happens in the Mediterranean also happens in the oceans, only it is observed sooner and with greater intensity here.

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

— When the surface water heats up, it becomes lighter and stops mixing with the cold water below. This creates a kind of lid 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 lid of heat prevents new oxygen from reaching the bottom. As organisms continue to breathe the little oxygen that remains, it eventually runs out, and the consequences are fatal. Furthermore, this warming not only affects fish; it also disrupts ocean currents, changes water chemistry, and causes the extreme climate phenomena we are already suffering.

Is the ocean really the lung of the planet?

— 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 it is an essential transport route. The oxygen it produces is vital, especially for the 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 research in all disciplines. In the end, it is about moving forward, and for too long half of the population has been barred. There was half of human intelligence that could have contributed to the advancement of science and technology and that was nullified. That women are now contributing to it is enriching at all levels and an act of intelligence. Now there are 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 slow down the entry of new material, but what is already in the food chain will continue to circulate for a long time. Nature is slower than our actions. We see very rapid changes caused by warming, but restoring balance takes a long time. However, in the last chapter of my book, I talk precisely about examples from the past where serious environmental damage was reversed thanks to international cooperation. We banned DDT from pesticides, we eliminated lead in gasoline – which is already banned in almost all countries – and we slowed down 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 too. We can prevent the damage from becoming even more severe and, in some cases, reverse it if we take action now.