Marta Sebastián: ‘Bacteria are key to most of the chemical transformations on our planet’

Marta Sebastián landed at the Institut de Ciències del Mar (ICM-CSIC) 15 years ago, after graduating in Marine Sciences in Cadiz, obtaining her PhD in biological oceanography in Malaga and specialising in marine microorganisms ecology during her stay in the United States as a postdoctoral researcher. She is fascinated by these small beings invisible to the human eye that play a key role in global biogeochemical cycles. Nowadays, she studies them using techniques such as DNA sequencing, which allows us to delve into their biogeography, diversity and metabolic potential, which defines everything they could do. She answers these questions on board the oceanographic vessel Sarmiento de Gamboa during the MICOLOR 2 campaign, which took place in the Atlantic Ocean this May.

1. What was your role in this latest Atlantic campaign?

The expedition aimed to study microorganisms associated with marine particles. These particles are organic, i.e. carbon particles. They are produced at the ocean surface by primary producers and travel to the bottom like snowflakes, which is known as marine snow. During the journey, these particles are transformed by micro-organisms, and this has an impact on the amount of carbon that travels to the seabed. In this last campaign, I was in charge of filtering large volumes of water with filters of different sizes to study the community of microorganisms in particles of different sizes. Also, together with a PhD student, we have done experiments to see how these particles transform over time.

2. How is a day on the ship?

Well, the days on the boats are long, but I love them because there is always a moment for you to go out and look at the sea, or when a dolphin is sighting off the port side and we all run out to see them. On this campaign we would arrive at about 7am at a station (a point in the ocean) and work on it until 7pm, when we would set course for the next station. During the day we used different devices to collect particles: a marine snow catcher, which is a 100-litre volume bottle with which we would collect particles from the ocean. We also collected water with a rosette of 24 12-litre bottles that were closed at different depths, from 1500 metres to the surface. Finally, we used plankton nets to collect zooplankton. 

3. What results do you expect from the samples collected?

We hope to have a good understanding of the communities associated with marine particles and how they vary with depth as the particles travel to the bottom of the ocean. 

4. Why are these communities so important?

Prokaryote communities (bacteria and archaea) are key to most chemical transformations on our planet. Phytoplankton capture a large amount of atmospheric CO2 during the process of photosynthesis, and this carbon is trapped in the form of particles. If these particles degrade very quickly, the CO2 returns to the atmosphere, but if, on the other hand, the particles go to the bottom of the ocean, the carbon is removed from the surface and sequestered for hundreds of years, dampening global warming. It is therefore important to know how the identity of prokaryotes affects this degradation, and what factors influence whether it is faster or slower.

5. Is it true that they are highly resilient?

Yes, prokaryotes have been found in ice and sediments that have managed to revive after thousands of years. Some bacteria can form spores, but the vast majority of prokaryotes go into a state of dormancy (or non-growth), although it is not yet clear how they do this. In fact, I am currently working on a project to study this dormancy process in marine bacteria.

6. Have you noticed any changes in communities due to global change?

Some colleagues have noticed that cells are getting smaller and smaller, but it is still too early to know if there have been changes in diversity, for example, because we have had techniques to study diversity for just over 20 years.

7. What techniques are we talking about?

The study of marine prokaryotes has evolved in parallel with the development of techniques. In the 1970s, it was done with epiflorescence microscopes, which made it possible to realise that prokaryotes were very abundant in the ocean. In the late 1980s, flow cytometers began to be used, allowing the abundance of prokaryotes to be counted quickly and some properties of the communities to be seen, such as whether the cells are larger or smaller. From the 1990s onwards, molecular techniques began to be used, allowing the diversity and identity of the most abundant prokaryotes to be seen. Finally, since the mid-2000s, the development of massive sequencing techniques has revolutionised the way microbial communities are studied.

8. Why? 

Thanks to the latest DNA sequencing techniques, we can reveal the identity of a large part of the prokaryotes that make up a community, study their biogeography, their metabolic potential or how active they are. This is important because, although morphologically bacteria all look the same to us, they can be much more different from each other (evolutionarily speaking) than a human and a mosquito. By combining all these techniques, we can learn a lot about how communities function and how they may evolve in the future.

9. To what extent is multidisciplinarity important when studying marine microbial communities? 

I have a very multidisciplinary background, as a marine science degree gives you knowledge of physics, chemistry, geology and ocean biology. This is very valuable, and although I specialised in biology, for example, the distribution of plankton is determined by ocean currents and other physical structures in the ocean, so it is important to take this into account.

10. What direction should future research take?

I think they should continue to combine techniques that allow us to have a predictive understanding of how a change in the diversity of the marine microbiome can affect ecosystem functioning.