Thallium (Tl) is the most toxic metal of all and is present at concentrations of concern over a wide geographical area. Tl can be released to the environment during coal mining, processing and use in coal-fired power plants and other coal based industrial processes, and through refining processes of ores for other metals such as copper, gold, lead, uranium, and zinc. For the past decades, Tl has been developed as a tracer to investigate paleo-oceanographic processes as well as geochemical cycles in the modern ocean. However, up to date, the roles of biology in the Tl cycle have seldom been considered. Here we present dose-response data from various families of phytoplankton to a wide range of Tl concentrations (1 ng/L to 1 mg/L). Growth rates, the maximum yield of photochemistry of photosystem II (F_v/F_m), the functional absorption cross-section of photosystem II (σ_PSII), and the change of whole metallome have been analysed. Of all phytoplankton studied here, cyanobacteria (Synechococcus) have the largest tolerance to Tl toxicity, whereby growth was even stimulated at high Tl concentration (1mg/L). Haptophytes, especially isochrysidales, accumulated significantly more Tl intracellularly than chlorophytes. Tl⁺ can be mistakenly imported for the chemically similar K⁺. Potassium transporters of various phytoplankton have also been identified from their genome using a novel orothogroup inference algorithm (OrthoFinder), and singular value decomposition (svd) analysis of potassium transporters data provides a fundamental framework to understand the contrasting bioaccumulation of Tl among different lineages. The potassium transporters from E. huxleyi are significantly different from all other phytoplankton in this study. Indeed, E. huxleyi accumulated most Tl available in the media at enviromental relevant concentrations (up to 100 ng/L). Based on the data in this study, under the average Tl concentration in seawater (10 ng/L), E. huxleyi can accumulate 1.6 µg Tl per g organic C produced by this organism. E. huxleyi is one of the most widely distributed phytoplankton, and considering massive yearly blooms of this species which cover an average of 1.4 × 10⁶ km² surface water, Tl cycles in those area should be largely affected.