Study reveals the microbial battlegrounds within estuaries - and the part played by microplastics

Estuaries are known hotspots for biodiversity and are turbulent mixing zones where freshwater and seawater microbes confront one another. 

A research team at Umeå University, Sweden, set out to explore how this mixing, termed as community coalescence, affects the ecology of microbial communities and whether microplastics, serving as microbial ‘ferries’ from rivers into the sea, impact the mixing outcomes.

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“Imagine that some microbes are like excellent football players who thrive on teamwork and cooperation. They form strong associations with one another and create numerous cooperations,” said postdoctoral researcher Máté Vass.

“When one microbe does well, its peers benefit, too, just like in football. But what happens when a more competitive player suddenly invades and disrupts the community? Can this new competitor replace a cooperative microbe, or will its arrival cause trouble for the entire team?”

River-sea mixing

The team addressed these questions with the help of a series of small-scale laboratory experiments, where river water was mixed into seawater and vice versa to mimic numerous estuarine environments. The authors emphasized that the answers to the above-mentioned questions depend on which side of a river–sea mixing one stands at. 

If one looks at seawater microbes, they not only tolerate the salty water better than microbes dispersed from freshwater rivers but also form a greater number of competitors than those of in rivers. Consequently, when one of them is being disturbed by a newcomer microbe from a river, its disappearance will not necessarily cause changes in the rest of the sea community. 

On the other hand, if river microbes are more cooperative than sea microbes, even a single kick-out causes cascading effects on its cooperative peers upon reaching the sea, causing a loss of river microbes in estuaries.

The authors said if one cooperative species declines as a consequence of mixing, it will tend to pull others down with it and destabilize the entire community. This was the case in river microbes whose diversity decreased and their establishment in the mixed environment was hard to predict. However, the community mixing, an ecological phenomenon known as ‘community coalescence’, overall elevated species diversity, the number of species found in the mixed community, as well as increased the number of competitive associations among microbes, making the mimicked estuarine community more stable eventually.

Challenging findings

The team’s findings challenge previous knowledge in ecology which sometimes suggests more cooperative communities upon mixing, and further, that increasing species diversity can have a destabilizing effect on microbial communities. But the latter possibly depends on the ratio of competitors and facilitators.

The key members of their study were diatoms, ciliates, golden-brown algae and aquatic fungi; abundant groups of microeukaryotes of rivers in the northern part of Sweden from where the samples for the researchers’ study were sourced. The decline of these microeukaryotes upon reaching the sea raised the team’s next question, namely, whether river microbes have had more chance to survive the mixing by reaching the sea reinforced on tiny plastic particles.

Role of microplastics

In a follow-up study, the team examined the role of microplastics during community mixing.

As the concentration of microplastics in the Baltic Sea can be as much as 3,300 particles per cubic meter, the team was wondering how such pollutants, where a large proportion is washed out via rivers, can influence the microbial world of estuaries. 

Similarly to their previous experiment, they mimicked estuaries by mixing river and seawater communities in laboratory settings, but this time they supplemented their experiment with the addition of 5 mm plastic (LDPE) particles. They aimed to test all scenarios involving the presence or absence of plastic pollution in the river and sea samples used for their coalescence experiment.

In concert with their earlier findings, the coastal sea communities seemed to be resistant (see Glossary) towards both hitchhiked bacteria and microeukaryotes to a great degree. The authors emphasized that even though many microbes from rivers grow well on this plastic type, involving opportunistic pathogens such as Pseudomonas and Brevundimonas species, these microbes could not take the fight with sea microbes well, leading to their unsuccessful establishments in estuarine conditions.

Carrier particles

It is well-established that plastic particles can be overgrown by numerous microbial species and be carried across aquatic environments by floating with river currents. Previously, many researchers emphasized that microplastics pose a threat to human health as numerous pathogens can colonize such particles and reach new environments. 

The authors’ estimations suggested that only a small fraction of microbes coming from the river can be spread successfully into the sea. Hence, the authors assumed that pathogens of river origin have a low chance of being spread across aquatic ecosystems. Nevertheless, it remained unanswered whether virulence and/or antibiotic resistance genes of plastic-dispersed bacteria can be shared in the form of gene exchanges before their populations are outcompeted by sea microbes.

The team’s studies show how complex and lively microbial communities are in aquatic environments where mixing events are common. Aquatic microbes can constantly adapt and respond to environmental changes, whether such changes are induced by the natural water currents or pollutants like microplastics. Understanding how these microbes interact and change when travelling from river into the sea, can better predict the effects of environmental changes and human activities on our aquatic ecosystems and think of ways to protect them in the future.

The studies were led by Prof Agneta Andersson and his postdoctoral researcher Máté Vass from Umeå University, Sweden.

Glossary

·         Community coalescence: the phenomenon by which previously isolated microbial communities mix and reassemble into a new community. This, for example, occurs in river-sea confluences, tree outplanting, kissing, or when leaf-colonizing microbes mix with soil microbes during leaf-fall.

·         Community stability: Here, stability is inferred from community cohesion, the degree to which members of a community are connected as a result of their interactions with each other.

·         Community resistance: A key property which allows communities to remain unchanged in the face of a disturbance.

·         Ecosystem function: A process (e.g., nutrient cycling, biomass production) or community property (e.g., resistance) which contributes to the overall ecosystem services produced by the diversity of organisms in the landscapes in which we live.