<p>Sedimentary bulk organic matter and various molecular organic components exhibit strong CO<sub>2</sub>-dependent carbon isotope fractionation relative to dissolved inorganic carbon sources. This fractionation (ε<sub>p</sub>) has been employed as proxy for paleo-<em>p</em>CO<sub>2</sub>. Yet, culture experiments indicate this CO<sub>2</sub>-dependent ε<sub>p</sub> is highly specific at genus and even species level, potentially hampering the use of bulk organic matter and non-species specific organic compounds. In recent years, significant progress has been made towards a CO<sub>2</sub> proxy using controlled growth experiments with dinoflagellate species, also showing highly species-specific ε<sub>p</sub> values. These values were, however, based on motile specimens and it remains unknown whether these relations also hold for the organic-walled resting cysts (dinocysts) produced by these dinoflagellate species in their natural environment. We here analyze dinocysts isolated from core-tops from the Atlantic Ocean and Mediterranean Sea, representing several species (<em>Spiniferites elongatus</em>, <em>S</em>. (<em>cf</em>.) <em>ramosus</em>, <em>S</em>. <em>mirabilis</em>, <em>Operculodinium centrocarpum</em> sensu Wall & Dale (1966) (hereafter referred to as <em>O</em>. <em>centrocarpum</em>) and <em>Impagidinium aculeatum</em>) using Laser ablation – nano Combustion – Gas Chromatography – Isotope Ratio Mass Spectrometry (LA/nC/GC-IRMS). We find that the dinocysts produced in the natural environment are all significantly more <sup>13</sup>C-depleted compared to the cultured motile dinoflagellate cells, implying higher overall ε<sub>p</sub> values and, moreover, exhibit large isotope variability. Where several species could be analysed from a single location, we often record significant differences in isotopic variance and offsets in mean δ<sup>13</sup>C values between species, highlighting the importance of single-species carbon isotope analyses. The most geographically expanded dataset, based on <em>O</em>. <em>centrocarpum</em>, shows that ε<sub>p</sub> correlates significantly with various environmental parameters. Importantly, <em>O</em>. <em>centrocarpum</em> shows a CO<sub>2</sub>-dependent ε<sub>p</sub> above ~240 μatm <em>p</em>CO<sub>2</sub>. Similar to other marine autotrophs, relative insensitivity at low <em>p</em>CO<sub>2</sub> is in line with a carbon concentrating mechanism being active at low <em>p</em>CO<sub>2</sub>, although we here cannot fully exclude that we partly underestimated ε<sub>p</sub> sensitivity at low <em>p</em>CO<sub>2</sub> values due to the relatively sparse sampling in that range. Finally, we use the relation between ε<sub>p</sub> and <em>p</em>CO<sub>2</sub> in <em>O</em>. <em>centrocarpum</em> to propose a first<em> p</em>CO<sub>2</sub> proxy based on a single dinocyst species.</p>