Microplastics and bacteria: a double-edged blade

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Original article: Microplastica e batteri: una lama a doppio taglio, by Luigi Gallucci

The knowledge about the direct effects of microplastics and their organic additives on marine bacteria is very few. To understand how our dear microorganisms are affected by the rampant use of plastic, researchers of the University of the Balearic Islands (Palma de Mallorca), in collaboration with the University of Antofagasta (Chile), assessed its quantitative and qualitative effects on the marine microbial population.

Plastic, microplastic, what do we need to worry about?

The concept of “waste“, as far as plastic is concerned, is harshly condemned nowadays. Every day we use, even without noticing, plastic products: small water bottles, the shopping bag, the paper of our favorite snack, or the packaging of products of various kinds. Whether you want them to be useless and replaceable or not, this whole range of products inevitably ends up somewhere. In most cases, if properly disposed of, it does not present a problem for the environment, but when it is thrown incorrectly?
All this, in one way or another, arrives in our beautiful seas. Here plastic can suffer different destinies. Essentially, we can have 4 types of final products resulting from the incorrect disposal of this material (Fig. 1):

  • Macroplastic (>250 mm)
  • Mesoplastic (1-25 mm)
  • Microplastics (1-1,000 μm)
  • Nanoplastics (<1 μm)
microplastics and bacteria illegally disposed
Figure 1 – Illegally disposed plastic [Bibliography: Carlos Miguel Forero]

The most problematic of these are microplastics since they are part of that series of debris called “marine litters” that have devastating effects on the marine environment. We also know that plastic waste not only breaks down into smaller components but also:

  • Absorb on its surface molecules of hydrophobic pollutants compounds (PCB, DDT, and its metabolites).
  • Release substances such as phthalates, bisphenol A, alkylphenols, and other polluting chemical intermediates in degradation processes.

Impacts on biota, including the microbiota, are largely unknown. The greatest danger comes from the passage of this debris into the food chain – the highest percentage is represented precisely by microplastics. On the other hand, according to a recent study, these microplastics can contaminate even the human placenta, carried by blood flow and possible harmful outcomes for the fetus’s health.

Current knowledge on the relationships between microplastic particles and bacteria

Several studies have deepened the relationships between plastic waste and marine microbiota. We know that some marine bacteria can degrade plastic waste, most notably the bacterium Ideonella sakaiensis 201-F6 (Yoshida, 2016) (Fig.2). In contrast, others, including some pathogens, can survive on plastic residues, spreading more quickly.

ideonella sakaiensis isolated from Yoshida bacteria
Figure 2 – Ideonella sakaiensis isolated from Yoshida et al. It’s a bacterium that can grow on microplastic fragments. [Bibliography: sciencemag.org]

The study evaluated several factors of the relationship between bacteria and microplastics. Among the most critical factors are growth assessment, protein overproduction, direct physical interactions between bacteria and MPs (microplastics), phosphorus acquisition mechanisms and/or atmospheric nitrogen fixation’s rate (N2). In addition, it is the first research carried out on diazotrophic microorganisms that can fix atmospheric nitrogen in a more biologically helpful form, such as ammonia.

The researchers started by reviewing other studies. Thanks to them, we know that microplastics can be an anthropogenic stress factor affecting microbial diversity and the nitrogen cycle. They also detected changes in microbial communities associated with floating plastics through metagenomic analyses, thus assuming that biological responses are also species-specific.

The study: can microplastic concentration influence bacteria?

Initially, five marine nitrogen-fixing species were selected – two cyanobacteria strains and three heterotrophic bacteria strains. Before starting the trial, cells were acclimatized and cultured in their respective ideal culture media to reach their exponential stage. In this case, the culture media consisted of:

  • Liquid component (MB, marine broth)
  • SSW (synthetic seawater)

 The cells were acclimated at 25° C at 120 r.p.m. in a rotary shaker with 12 h of darkness and 12 h of light (low-intensity fluorescent light). Then placed in contact with the microplastics and additives under the same initial culture conditions for 72 h.

The research can then be divided into two phases based on the concentrations of MPs and additives. In the first, the five selected species were exposed to significant concentrations but comparable to environmental ones. In the second, called by researchers “Worst Case” (worst case scenario), only two model bacterial strains were used, respectively a cyanobacterium and a heterotrophic bacterium among those previously selected.

Search results: phase 1

Most of the diazotrophic bacteria in phase 1 of the study were not significantly affected by the relationship with various microplastics, except for the significant increase in the growth phase of selected cyanobacteria. This change in the growth phase of cyanobacteria allowed the inference that these relationships are influential in a species-specific manner. The effects of organic additives deserve a different analysis. In this case, the bacteria suffered various influences depending on the additive taken into consideration, assuming a species-specific trend. Some of the selected strains underwent an increase in their growth phase, while others experienced a decrease. This phenomenon is due to the typical metabolism of individual bacterial strains (Fig. 3).

Analysis of species related to MPs fragments
Figure 3 – Influence of microplastics and additives on growth values in phase 1. The represented values are significant and error bars represent the standard errors between the various replications of the experiment. (n=3) [Bibliography: Fernández-Juárez V. et al.,2021]

Additives can be absorbed or released from microplastics, with different consequences in both cases. If absorbed, they bring advantages to sensitive strains and disadvantages to those that use these substances as a source of organic carbon. On the contrary, if released, they can be harmful to sensitive strains, bringing benefits to those strains that use them as a metabolic substrate. If present in high combinations with identical microplastic particles, the cumulative effect of different additives and microparticles may be zero.

Search results: phase 2 “Worst-Case”

In this phase, called the “worst-case scenario”, the researchers simulated a possible situation in which concentrations of microplastics and additives were well above the environmental critical levels of contamination of these substances. High concentration of MPs (microplastics) significantly stimulates the bacterial growth phase of both marine bacteria, especially with PVC residues. This change is mainly due to the increase in dissolved organic carbon (DOC) in the medium concerning the rise in plastic concentrations.

In this second phase, the production of proteins is also affected, especially those suitable or related to plastic degradation. These include alcohol dehydrogenase (ADH, Fig.4) and a carbon carrier (C4-ABCS), overproduced in the heterotrophic strain of Cobetia spp. but not found in the cyanobacteria Halothece spp.

Microplastics and bacteria enzymes
Figure 4 – Structural analysis of alcohol dehydrogenase (ADH) from Cobetia spp. [Bibliography: Fernández-Juárez V. et al.,2021]

On the other hand, the effect of additives’ high concentrations as in the first phase of the study seems to create contrasting and variable situations depending on the type of additive and the metabolism of the species under analysis. Thus, the concentration of additives has a minor influence on the effects on microbial populations since they are also present at lower concentrations.

Additional relational factors between microplastics and bacteria

Other factors may affect the microbiota, such as the size of micro-fragments and chemical-physical properties. The size of the fragments seems to play a fundamental role, influencing the surface area on which bacterial cells can proliferate. They will have a greater chance of adhering to the fragments if they are more extensive. Hydrodynamic and electrostatic factors also play a role in the adhesion processes, which can vary from case to case and affect the growth and absorption capacity of the nutrients of the cells.

Microplastics and bacteria analysis relationship between membranes and fragments of microplastic
Figure 5 – Direct physical interaction between MPs and selected bacterial species. In particular, in the F-I images the relationship between membranes and fragments of microplastics is highlighted. [Bibliography: Fernández-Juárez V. et al.,2021]

It seems that the adhesion favors the absorption (especially in oligotrophic waters) while favoring the formation of microbial biofilms, which in turn increase the surface suitable for the capture of nourishment. Smaller fragments appear to be detrimental to smaller heterotrophic marine bacteria and not to cyanobacteria. However, to investigate this aspect, the researchers specify that it will be helpful to carry out further studies. From the first superficial analyses, it seems that it plays a fundamental role in absorbing small fragments of plastic through direct physical contact. These tiny fragments create some invaginations (Fig.5) inside the cellular membranes, which could harm the health of the bacterial populations of smaller dimensions.

Conclusion

From this critical study, we can deduce that even microbial populations are affected by plastics in the oceans. The relationship between microplastics and marine bacteria is very complex, and further studies will be needed to get a complete picture of the situation. With the proper knowledge, it is also possible to develop strategies to preserve these wonderful environments with the help of the marine microbiota.

However, we deduce that even bacterial populations can be adversely affected by plastic pollution. We must keep in mind that the marine microbiota is at the basis of the nutrient cycles that gave rise to life on our planet, continuing to feed these cycles to this day. If this balance is damaged, what could happen to the delicate environmental balance? The researchers will try to predict possible scenarios. Still, it is up to all of us to work to prevent terrible scenarios that would lead to the disappearance of entire fundamental ecosystems.

Luigi Gallucci

Bibliography

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