The production and release of chemicals from human activities are on the rise. Understanding how the aquatic environment is affected by the presence of an unknown number of chemicals is lacking. We employed the chemical activity concept to assess the combined effects of hydrophobic organic contaminants on the phyto-plankton species Rodomonas salina. Chemical activity is additive, and refers to the relative saturation of a chemical in the studied matrix. The growth of R. salina was affected by chemical activity, following a chemical activity-response curve, resulting in an Ea50 value of 0.078, which falls within the baseline toxicity range observed in earlier studies. The chlorophyll a content exhibited both increases and decreases with rising chemical activity, with the increase possibly linked to an antioxidant mechanism. Yet, growth inhibition provided more sensitive and robust responses compared to photosynthesis-related endpoints; all measured endpoints correlated with increased chemical activity. Growth inhibition is an ecologically relevant endpoint and integrates ther-modynamic principles such as membrane disruption. Our study utilized passive dosing, enabling us to control exposure and determine activities in both the medium and the algae. The concept of chemical activity and our results can be extended to other neutral chemical groups as effects of chemical activity remain independent of the mixture composition.

Hydrophobic organic contaminants (HOCs) affect phytoplankton at cellular to population levels, ultimately impacting communities and ecosystems. Baseline toxicants, such as some HOCs, predominantly partition to biological membranes and storage lipids. Predicting their toxic effects on phytoplankton populations therefore requires consideration beyond cell uptake and diffusion. Functional traits like lipid content and profile can offer insights into the diverse responses of phytoplankton populations exposed to HOCs. Our study investigated the vulnerability of five phytoplankton species populations to varying chemical activities of a mixture of polycyclic aromatic hydrocarbons (PAHs). Population vulnerability was assessed based on intrinsic sensitivities (toxicokinetic and toxicodynamic), and demography. Despite similar chemical activities in biota within the exposed algae, effects varied significantly. According to the chemical activity causing 50% of the growth inhibition (Ea₅₀), we found that the diatom Phaeodactylum tricornutum (Ea₅₀ = 0.203) was the least affected by the chemical exposure and was also a species with low lipid content. In contrast, Prymnesium parvum (Ea₅₀ = 0.072) and Rhodomonas salina (Ea₅₀ = 0.08), both with high lipid content and high diversity of fatty acids in non-exposed samples, were more vulnerable to the chemical mixture. Moreover, the species P. parvum, P. tricornutum, and Nannochloris sp., displayed increased lipid production, evidenced as 5–10% increase in lipid fluorescence, after exposure to the chemical mixture. This lipid increase has the potential to alter the intrinsic sensitivity of the populations because storage lipids facilitate membrane repair, reconstitution and may, in the short-term, dilute contaminants within cells. Our study integrated principles of thermodynamics through the assessment of membrane saturation (i.e. chemical activity), and a lipid trait-based assessment to elucidate the differences in population vulnerability among phytoplankton species exposed to HOC mixtures.

The presence of hydrophobic organic contaminants (HOCs) in marine environments poses a threat to aquatic communities, yet the risks and effects of chemical mixtures at the community level remain understudied. This study investigated the impact of a chemical mixture of four polycyclic aromatic hydrocarbons (PAHs) on natural phytoplankton and bacterioplankton collected from the Baltic Sea using chemical activity as a quantitative measure of chemical pollution. Chemical activity provides an approach for assessing the impact of chemical mixtures on populations and communities by linking exposure, bioaccumulation, and biological effects. After three days of exposure, chemical activity levels close to 0.1 – similar to contamination levels found at highly polluted sites – resulted in significant shifts in both phytoplankton and bacterioplankton communities. Phytoplankton diversity decreased, with sensitive taxa such as the green algae Pseudoscourfieldia marina, small cryptophytes, picocyanobacteria (Synechococcus sp.), and photosynthetic picoeukaryotes, showing reductions of 40-94%. Notably, the diatom Chaetoceros sp., became dominant in the treated group. Diatoms are known for their tolerance to HOCs. Bacterial community composition also showed shifts, with significant reductions in diversity (both α - and ꞵ-diversity) and a marked increase in the dominance of Proteobacteria taxa (98 % in treated samples). These findings indicate that the chemical mixture disrupted community structure, favoring more tolerant taxa and leading to a reduction in biodiversity within both phytoplankton and bacterioplankton communities. Our study thus demonstrates that the effects of environmental relevant chemical activity of chemical mixtures have ecological impacts on aquatic ecosystems. These findings have potential implications for future research on the resilience and adaptability of phytoplankton and bacterioplankton communities under chemical stress. 

Contaminated sediments can release pollutants and thereby acting as a secondary source of contamination to the water column. These pollutants, such as hydrophobic organic compounds (HOCs) like polycyclic aromatic hydrocarbons (PAHs) and methylmercury (MeHg), are commonly sorbed to sediment particles due to their strong affinity for organic matter. Contaminant release, through sediment resuspension and desorption or diffusion, enhances contaminant bioavailability and risk to aquatic organisms. Passive samplers provide an approach for assessment of bioavailable contaminant concentrations, by assessing the contaminant freely dissolved fraction. This study aimed to determine whether passive samplers would be a good proxy for assessment of contaminant bioaccumulation and toxicity in aquatic organisms using a red macroalga Ceramium tenuicorne as a benchmark organism. An experiment was performed using an artificial sediment spiked with PAHs or MeHg, polyethylene and diffusive gradients in thin films passive samplers, and the red alga. Sediment resuspension was induced to create turbidity gradients (0.5–4.5 NTU) in the system. The objectives were to: (i) establish the relationship between contaminant uptake by passive samplers and bioaccumulation in the alga, including the influence of sediment resuspension on this relationship, and (ii) determine whether resuspension affected the relationship between toxicity (photosynthesis efficiency) and PAH exposure. After 7 days of exposure, contaminant concentrations in passive samplers and algae were measured, along with photosynthesis inhibition. Results showed significant relationships between passive sampler uptake and algal bioaccumulation for both PAHs and MeHg. A predictive model established for the PAHs demonstrated that bioaccumulation was driven primarily by contaminant release from the sediments represented by the uptake in the passive sampler and, to some extent, by turbidity. The estimated bioaccumulation data accounting for turbidity fitted the photosynthesis inhibition dose-response curve. These findings support the use of passive samplers as proxies for bioaccumulation and toxicity in low-turbidity environments and provide a foundation on which to base future field studies for contaminated sediment risk assessment.