How microplastics can create 'dead zones' in aquatic ecosystems

The biogeochemical flow of phosphorous and nitrogen is one of the Stockholm Resilience Centre's key planetary boundaries that we're currently breaching. 

As these essential nutrients cease to cycle and become permanently deposited in the world's waterways, they promote algae over all other organisms in these ecosystems. 

These algae rapidly multiply, mature and begin decomposing, consuming oxygen at a significant rate (choking all fish and many keystone plant species) whilst emitting methane. 

If microplastics enter waterways through wastewater, they tend to increase nutrient density through adsorption, whilst fostering microbial communities called biofilms that incubate bloom-forming algae.  

For example, one study demonstrated "that MPs had the potential to exacerbate M. aeruginosa [a bloom-forming algae] blooms."¹ 

Microplastics increase nutrient density 

When exposed to air and sun, microplastics oxidise to form functional groups that attract ionic forms of nitrogen (cationic ammonium) and phosphorous (anionic phosphates).  

In some cases, chemically weathered microplastics attract anionic metals which, in turn, attract phosphates. 

Despite their size, microplastics have high surface-area-to-volume ratios as a result of their irregular shapes, greatly expanding available space for adsorbed nutrients.  

In essence, microplastics function as a meta-pollutant, whose increasing concentration simultaneously increases concentrations of other pollutants.  

A study that compared different levels of microplastics in aquatic ecosystems found that "Nutrients positively related to microplastics abundance."² Another study investigated these adsorption mechanics in more detail, concluding that: 

"the hydrogen bond formed between the electron pair on nitrogen and the hydroxyl hydrogen of molecular phenol, the hydrogen bond connected to the nitrogen of amine, and the hydrogen bond formed between the hydrogen of the hydroxyl group and the free amine group played an essential role in the adsorption performance of microplastics."³ 

What are biofilms? 

Biofilms are complex, layered microbial communities that adhere to surfaces and create a protective extracellular polymeric substances (EPS) matrix. This matrix enhances adhesion, protects microbes from environmental stressors, and traps organic matter through adhesion, porousness, electrostatic forces, hydrogen bonding, and shared hydrophobia.  

These microcultures can significantly alter nutrient concentrations in their ecosystems; some bacteria within biofilms fix nitrogen, converting atmospheric nitrogen into bioavailable forms like ammonium. whilst others decompose organic matter, releasing nitrogen and phosphorus. 

Microplastics provide ideal surfaces for biofilm formation due to their durability, hydrophobicity, and irregular texture, which create microhabitats that enhance microbial colonization; simultaneously, as we've explored, they tend to increase nutrient density. 

A review of the research concluded that "Several genera of filamentous cyanobacteria [bloom-forming algae], such as Phormidium, Rivularia, and Leptolyngbya have been found on ocean microplastics," citing a study from Oman which "reported that 4% of the plastisphere community was occupied by Microcystis (cyanobacteria) indicating a possibility of transportation of harmful algal bloomers to marine waters through the propagules attached to the plastic surface."⁴ Another study called for greater understanding of "drifting plastic debris as a potential vector microalgae dispersal."⁵ 

Microplastic-based biofilms foster algal blooms 

The problem, however, is that biofilms can support a suite of microbes that form and sustain algal blooms, adding extra fuel to the fire in ecosystems already primed to blow. 

Whether it's the cyanobacteria which bloom, the microbes that decompose matured blooms, or those that fix nitrogen in anaerobic environments (further promoting nitrogen-hungry cyanobacteria), these microbial communities constituted by microplastics could be the deciding factor. 

For instance, a recent study found that "PVC [polyvinyl chloride] and PP [polypropylene] MPs [microplastics] can promote microbial nitrification and nitrite oxidation, while PP can significantly promote alkaline phosphatase (ALP) activity and the abundance of the phosphorus-regulation (phoR) gene."⁶ 

Because the enzyme responsible for nitrogen fixation (nitrogenase) is oxygen-sensitive, bacteria have mechanisms to protect its functioning as oxygen levels fall; simultaneously, in anoxic conditions, iron oxides in sediment let go of their phosphorus as they become less oxidised, reducing from iron(III) to iron(II). 

One study found that "At all temperatures, algal density increased with an increasing MPs concentration until MPs concentrations reached 0.4 mg/mL".⁷ 

In short, biofilms make algal blooms more likely by supporting various microbial activities, including the promotion of cyanobacteria, nitrogen production, and oxygen depletion through decomposition, which form and sustain them. 

¹ Responses of bloom-forming Microcystis aeruginosa to polystyrene microplastics exposure: Growth and photosynthesis. Li et al. Water Cycle. Vol. 3, 2022.  

² Influence of microplastics on nutrients and metal concentrations in river sediments. He et al. Environmental Pollution. Vol. 253. 2020.

³ Adsorption of different pollutants by using microplastic with different influencing factors and mechanisms in wastewater: a review. Zhao et al. Nanomaterials (Basel). 2022.  

⁴ Plastisphere community assemblage of aquatic environment: plastic-microbe interaction, role in degradation and characterization technologies. Dey et al. Environmental Microbiome. 2022.

⁵ Drift plastic debris as a potential vector dispersing Harmful Algal Bloom (HAB) species. Masó et al. Scientia Marina. 2003.

⁶ Effects of microplastics on nitrogen and phosphorus cycles and microbial communities in sediments. Yin et al. Environmental Pollution. Vol. 1. 2023.  

⁷ Extracellular polymeric substances in green alga facilitate microplastic deposition. Gopalakrishnan, Kishkore and Donna Kashian. Chemosphere. Vol. 286. 2022.