“Bacteria are the scavengers of the ocean”

© N.Pansiot/Tara expéditions

Among the researchers collaborating on this 10th expedition is Jean-François Ghiglione of the CNRS, Observatoire oceanologique, Banyuls-sur-mer. Aboard Tara for a few days, he communicates his enthusiasm and shares his knowledge in a most understandable way. Once Jeff starts talking about his research, it’s hard for him to stop. Here he focuses on bacteria in the marine environment.  

  

First of all, can you tell us how bacteria live in the sea?

Bacteria have been here since life appeared on Earth. They have adapted and are able of performing all the functions in the ecosystem: They fix CO2 (as do cyanobacteria), they eat organic matter, and can grow in extreme regions without oxygen. They are “prokaryotes”, unicellular organisms about 1 micron in size, differentiated from the rest of the animal and plant kingdom. One liter of seawater contains on average 100,000 bacteria, and between 2,000 and 3,000 different species. In fact, bacteria are the most abundant organisms in the sea. They are everywhere, even if we don’t see them. They get eaten by viruses, flagellates and ciliates.

Bacteria are the scavengers of the ocean: they assimilate half of the organic carbon that comes from waste material in the food chain (from phytoplankton to fish). This gives them a key role in the global carbon balance because they’re the only organisms in the sea capable of transforming this kind of waste.

 

What is the interaction between bacteria and plastic?

We know that bacteria are the only organisms capable of degrading plastics in the marine environment. We also know that some pathogenic bacteria can attach themselves to plastic. These findings bring up 2 questions for scientists: Can bacteria be considered a source of hope for the degradation of plastics at sea? Can pathogenic bacteria represent a potential health hazard?

We will conduct 3 studies at the observatory of Banyuls-sur-Mer. First, we’ll observe the diversity of bacteria living on the microplastics collected aboard Tara. Then we’ll conduct research on pathogenic bacteria, and finally we’ll focus on the question of biodegradable plastics.

 

How does bacterial degradation of plastics work?

Plastic is a relatively recent human invention, dating back to the 60s and 70s. It has already been observed that bacteria manages to degrade plastic under certain conditions. The lifetime of a piece of plastic at sea is estimated to be about 100 years. When plastic first enters the water, physics comes into play. The plastic is broken into pieces by the currents and the action of UV rays. Then bacteria get involved, but their work of disintegration is very slow.

Bacteria confront 2 difficulties in “attacking” plastic. First they must succeed in attaching themselves to the microplastic which is very “slippery” (hydrophobic). To get around this problem, bacteria send molecules outside the cell to become “anchored.” Once the first bacteria are attached, others will be able to stick to each other and grow.   The colony then forms what is called a biofilm. In fact, so many bacteria grow on a particle of plastic that they form a speckled film visible to the naked eye.

The second difficulty encountered is huge! Ordinarily plastics are not “edible.” They must be oxidized (especially by UV rays) for bacteria to be able to feed on them. Since plastic particles are too large to be eaten as is, the bacteria begin to “attack” by giving off cell enzymes (exoenzymes). Different species of bacteria are involved in this very slow process of degradation, until they manage to break up the plastic into tiny molecules they can finally eat. This ultimate assimilation means the plastic is  eliminated by being transformed into the biomass, or CO2.

 

What are pathogenic bacteria?

They’re the ones that cause diseases such as Salmonella, for example. They have been studied  extensively since the time of Pasteur, because they cause diseases in humans. In general, one can find these pathogens in the sea near sewage treatment plants, but they are rapidly diluted. Note that the function of wastewater treatment plants is not to eliminate pathogenic bacteria, but to eliminate organic matter.

The question we ask ourselves is this: Can pathogenic bacteria “hide” in microplastics?

Bacteria love to live on substrata where they are sheltered from predators. We can imagine that pathogenic bacteria manage to live longer at sea by hiding on plastic dumped by sewage treatment plants. Some stations use filtration media –  small plastic circles put into the holding basins to promote growth of bacteria and improve the degradation of organic matter. Unfortunately, we have found many of these filtering devices during the Tara campaign Mediterranean, near the coast and even in the open sea. Without being alarmist, we wonder about the potential health risk of spreading pathogenic bacteria and we will, among other things, study this problem using samples collected aboard Tara.

 

Interview by Noëlie Pansiot

 

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- Oceans of plastic.