Competitive interaction between Tl(I) and K was successfully predicted by the biotic ligand model (BLM) for the microalga Chlorella sp. (Chlorophyta; University of Toronto Culture Collection strain 522) during 96‐h toxicity tests. Because of a greater affinity of Tl(I) (log K = 7.3–7.4) as compared to K (log K = 5.3–6.3) for biologically sensitive sites, an excess of 40‐to 160‐fold of K is required to suppress Tl(I) toxic effects on Chlorella sp., regardless of [Tl(I)] in solution. Similar excess of K is required to suppress Tl(I) toxicity to Synechococcus leopoliensis (Cyanobacteria; University of Texas Culture Collection strain 625) and Brachionus calyciflorus (Rotifera; strain AB‐R1F). The mechanism for the mitigating effect of K on Tl(I) toxicity was investigated by measuring ²⁰⁴Tl(I) cellular uptake flux and efflux in Chlorella sp. Potassium shows a competitive effect on Tl(I) uptake fluxes that could be modeled using the BLM‐derived stability constants and a Michaelis—Menten relationship. A strong Tl efflux dependent only on the cellular Tl concentration was measured. Although Tl efflux does not explain the effect of K on Tl(I) toxicity and uptake, it is responsible for a high turnover of the cellular Tl pool (intracellular half‐life = 12–13.5 min). No effect of Na⁺, Mg²⁺, or Ca²⁺ was observed on Tl⁺ uptake, whereas the absence of trace metals (Cu, Co, Mo, Mn, Fe, and Zn) significantly increased Tl uptake and decreased the mitigating effect of K⁺. The importance of K⁺ in determining the aquatic toxicity of Tl⁺ underscores the use of ambient K⁺ concentration in the establishment of Tl water‐quality guidelines and the need to consider K in predicting biogeochemical fates of Tl in the aquatic environment.

Uptake of inorganic Tl(I) and dimethylthallium, (CH₃)₂Tl⁺, by Chlorella spp. (Chlorophyta) and the diatom Stephan‐odiscus hantzschii (Heterokontophyta) were measured using radio‐tracer techniques in water from Lakes Erie and Superior (North America). Uptake of both Tl(I) and dimethylthallium was bioactive. Uptake of [²⁰⁴Tl]‐Tl(I) was greater in Lake Superior water than in Lake Erie water due to the greater K content in Lake Erie that inhibits Tl(I) uptake by phytoplankton but not that of [²⁰⁴Tl]‐dimethylthallium. Volume‐based bioconcentration factors for Tl(I) after 72 h of exposure were 5 × 10⁴ and 1.1 × 10⁴ for Chlorella sp. and S. hantzschii; for dimethylthallium they were 7.8 × 10² and 8.3 × 10³. Both Tl(I) and Tl(III) were concentrated similarly by Chlorella spp. These results suggest that chlorophytes, but not diatoms, accumulate Tl(I) to a greater extent than dimethylthallium. Greater bioaccumulation factors of inorganic Tl are possible in waters containing low amounts of K⁺; water quality guidelines seeking to protect biota from deleterious effects of Tl should consider the role of K.