| Summary: | Accumulation of reserve compounds (i.e., lipids and chrysolaminarin) in diatoms depends on the environmental conditions, and is often triggered by stress conditions, such as nutrient limitation. Manipulation of CO<sub>2</sub> supply can also be used to improve both lipids and carbohydrates accumulation. Given the high diversity among diatoms, we studied the two marine model diatoms—<i>Thalassiosira pseudonana</i> and <i>Phaeodactylum tricornutum</i>, a freshwater diatom, <i>Asterionella formosa</i>, and <i>Navicula pelliculosa</i>—found in fresh- and sea-water environments. We measured the accumulation of reserve compounds and the activity of enzymes involved in carbon metabolism in these diatoms grown at high and atmospheric CO<sub>2</sub>. We observed that biomass and lipid accumulation in cells grown at high CO<sub>2</sub> differ among the diatoms. Lipid accumulation increased only in <i>P. tricornutum</i> and <i>N. pelliculosa</i> grown in seawater in response to elevated CO<sub>2</sub>. Moreover, accumulation of lipids was also accompanied by an increased activity of the enzymes tested. However, lipid accumulation and enzyme activity decreased in <i>N. pelliculosa</i> cultured in fresh water. Chrysolaminarin accumulation was also affected by CO<sub>2</sub> concentration; however, there was no clear relation with lipids accumulation. Our results are relevant to understand better the ecological role of the environment in the diatom adaptation to CO<sub>2</sub> and the mechanisms underpinning the production of storage compounds considering diatom diversity.
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