Microplastics May Be Silently Rewiring the Ocean’s Smallest Powerhouses

Marine microalgae are among the most consequential organisms on Earth. These microscopic photosynthetic drifters anchor the base of nearly every ocean food web, drive global biogeochemical cycles, and sequester vast quantities of carbon dioxide. Now, a systematic synthesis published in the journal Ecotoxicology has pulled together nearly a decade of experimental evidence to answer a deceptively simple question: what do microplastics actually do to these tiny engines of the ocean? The answer, drawn from 43 experimental studies selected out of 59 screened publications, is both more nuanced and more troubling than headlines often suggest. The field is expanding rapidly, but the researchers behind the review, led by Nágela Gardênia Rodrigues Santos of the Federal University of Maranhão in Brazil, conclude that our picture of microplastic toxicity in marine microalgae remains fragmented, methodologically inconsistent, and far from environmentally realistic.

The review’s methodology was deliberately rigorous. The team searched Web of Science and Scopus from 1970 all the way to August 2025, using predefined combinations of terms covering microplastics, marine microalgae, toxicity, and marine pollution. After removing duplicates and screening titles, abstracts, and keywords, 59 publications addressing microplastic–microalgae interactions were retained for bibliometric characterization. Of these, 43 original studies reporting direct experimental exposure of marine microalgae to plastic particles qualified for the comparative ecotoxicological synthesis. Strikingly, no eligible publications predate 2016, indicating that this research field is barely a decade old. Publication output grew from a single paper in 2016 to a peak of fourteen in 2024, a trajectory that mirrors the broader explosion of microplastic science but also reveals how young and incomplete the evidence base for marine primary producers remains compared with the meta-analyses already available for fish and aquatic invertebrates.

Geographically, the field is strikingly concentrated. China accounted for 54.24 percent of the 59 selected publications, followed by Spain and South Korea at 6.78 percent each, with Portugal, France, Italy, Belgium, and Brazil contributing smaller shares. Sixteen countries appeared in total, yet only seven publications involved international co-authorship, a mere 11.86 percent. The authors suggest this insularity may contribute to the methodological heterogeneity that pervades the literature, since research groups working in isolation tend to adopt different experimental designs, particle characteristics, and exposure conditions. The dominance of Chinese output aligns with global bibliometric trends in aquatic ecotoxicology and has been linked to expanded research funding, notably through the National Natural Science Foundation of China since 2015.

When it comes to what is being tested, one polymer dominates the experimental record. Polystyrene appeared in 25 of the 43 studies, followed by polyethylene in 10, polyvinyl chloride in 5, and polymethyl methacrylate in 3. Polyamide, polylactic acid, polybutylene succinate, polyethylene terephthalate, tire wear particles, and gear-derived microplastics each featured in a single study. The review’s authors point out an important disconnect here: polystyrene’s laboratory ubiquity likely reflects the commercial availability of standardized fluorescent spheres and microbeads rather than its environmental prevalence, since polyethylene and polypropylene are often more abundant in surface waters. This mismatch between what scientists test and what organisms actually encounter in the ocean has significant implications for extrapolating laboratory findings to real-world risk assessment.

The organisms under scrutiny show a similar taxonomic skew. Diatoms, the glass-walled phytoplankton that underpin much of coastal and oceanic productivity, were investigated in 22 studies, or 51.2 percent of the experimental dataset. Chlorophyta, the green algae, featured in 10 studies, and dinoflagellates in 8. Other groups, including haptophytes, raphidophytes, cryptophytes, cyanobacteria, and red algae, were barely represented. The authors highlight this as a critical gap, noting that coccolithophores, for example, play a central role in the marine calcium carbonate cycle yet remain almost entirely unstudied in the microplastic context. Because microplastic toxicity is increasingly recognized as species-specific, expanding taxonomic diversity is essential for predicting how entire phytoplankton communities, not just a handful of laboratory workhorses, might respond to chronic plastic exposure.

So what do the experiments actually show? The most frequently evaluated endpoints were grouped into six functional categories, and growth and population parameters led by a wide margin, assessed in 40 of the 43 studies. Growth inhibition, growth rate, cell density, and cell viability were the workhorse measurements. Cellular stress and damage indicators, particularly oxidative stress driven by overproduction of reactive oxygen species, appeared in 29 studies, with documented consequences including lipid peroxidation, membrane damage, organelle injury, and reduced esterase activity. Photosynthetic physiology, encompassing photosynthetic rate, efficiency, photochemistry, and chlorophyll fluorescence, was examined in 20 studies, while structural and interaction parameters such as aggregation appeared in 25, photosynthetic biochemistry in 14, and broader biochemical or metabolic responses in 13.

The mechanisms of toxicity emerging from this literature are multifaceted. Smaller particles appear to exert greater toxicity through enhanced surface interactions with algal cells, whereas larger particles can inhibit growth indirectly through shading, physically blocking the light that photosynthetic cells depend on. Direct physical interference has been demonstrated in the diatom Skeletonema costatum, where cell–particle contact disrupts normal function. Microplastics can also act as vectors for other contaminants: work by Prata and colleagues showed that the presence of microplastics can alter the toxicity of pharmaceuticals such as procainamide and doxycycline in the microalga Tetraselmis chuii. Meanwhile, species-specific hetero-aggregation between plastic particles and phytoplankton, first characterized by Long and colleagues, can reshape particle dynamics in the water column, modifying light availability and nutrient exchange in ways that ripple through entire microbial communities.

Perhaps the most conceptually important section of the review concerns what the authors call the dynamic nature of microplastics. Particles in the ocean are not static objects. Aging and weathering alter surface chemistry, charge, roughness, and functional groups, changing how particles aggregate, adsorb contaminants, leach additives, and interact with cells. Dissolved organic matter and algal extracellular polymeric substances can coat particles in an eco-corona; one study showed that this coating actually reduced the toxicity of polystyrene nanoplastics to marine Chlorella by decreasing particle–cell interactions and oxidative stress. Biofilms growing on plastic surfaces can modify particle density, transport, and bioavailability, while weathering may enhance the release of polymer additives that contribute chemical toxicity of their own. Yet these environmentally transformed particles remain severely underrepresented in experimental work, meaning most laboratory findings rest on pristine, commercially manufactured plastics that bear little resemblance to weathered marine debris.

The review is refreshingly candid about the limits of the evidence. Most experiments rely on short-term laboratory assays with simplified exposure scenarios, often using concentrations exceeding those typically found in nature. Indeed, studies employing environmentally relevant concentrations and particle-size distributions have reported limited or no effects on microalgal growth under the conditions tested. The authors therefore argue that current evidence supports the identification of potential toxicity mechanisms far more strongly than it supports quantitative ecological risk prediction. They also flag transient effects, in which initial physiological vulnerability gives way to adaptive recovery, as a phenomenon that complicates interpretation and underscores the need to quantify actual particle bioavailability in exposure media.

Critical endpoints remain almost entirely unexplored: aggregate formation, particle sinking dynamics, DNA damage, relative electron transfer rates, and long-term exposure under realistic conditions all fall into this category. These are not academic omissions. Because microalgae sit at the base of marine food webs, alterations in their growth, lipid composition, and photosynthetic performance could propagate upward, influencing trophic transfer of both energy and plastic particles themselves. The review proposes a practical path forward: pairing robust apical endpoints such as growth inhibition, already standardized in OECD algal testing guidelines, with complementary photosynthetic physiology measures, while deploying biochemical biomarkers for mechanistic interpretation. The authors stop short of claiming laboratory responses translate directly into ecological harm. Instead, their synthesis delivers something arguably more valuable: a clear-eyed map of what we know, what we have merely assumed, and what the next decade of microplastic ecotoxicology must urgently address if science is to keep pace with one of the planet’s most pervasive pollutants.

Subject of Research: Ecotoxicological effects of microplastic exposure on marine microalgae

Article Title: Microplastic toxicity in marine microalgae: an ecotoxicological synthesis of experimental biological responses

Article References: Santos, N. G. R., Pinho, K. F. B., Reis, I. D. C. S., Luvizotto-Santos, R., & Jorge, M. B. (2026). Microplastic toxicity in marine microalgae: an ecotoxicological synthesis of experimental biological responses. Ecotoxicology, 35(8), Article 178. https://doi.org/10.1007/s10646-026-03181-x

Image Credits: AI Generated

DOI: 10.1007/s10646-026-03181-x

Keywords: microplastics, marine microalgae, ecotoxicology, oxidative stress, photosynthesis, phytoplankton, polystyrene, polyethylene, primary productivity, eco-corona, marine pollution, systematic review