Acidophilic microorganisms thrive in environments where the external pH is orders of magnitude lower than their intracellular pH. Verrucomicrobial methanotrophs of the family Methylacidiphilaceae, including Methylacidiphilum and Methylacidimicrobium, inhabit extremely acidic geothermal environments and can grow at a pH < 1.0 and temperatures up to 65 °C. We analyzed and compared their membrane fatty acid compositions at pH 3.0 across strains with different temperature optima. Thermophilic Methylacidiphilum strains almost exclusively contain saturated fatty acids, while the mesophilic Methylacidimicrobium strains we studied incorporate 16–47% unsaturated fatty acids. Notably, the thermophile Methylacidiphilum fumariolicum SolV increases unsaturated fatty acid content in response to a 10 °C temperature decrease but not to a decrease in pH from 3.0 to 1.7. Genomic analysis revealed a conserved fatty acid biosynthesis pathway. Despite constitutive expression of predicted pH homeostasis genes, SolV did not upregulate them upon changing the pH from 3.0 to 1.7. However, genes involved in methane oxidation were strongly upregulated, suggesting a potential metabolic adaptation to extreme acidity.

Acidophilic microorganisms thrive in environments where the external pH is orders of magnitude lower than their intracellular pH. Verrucomicrobial methanotrophs of the family Methylacidiphilaceae, including Methylacidiphilum and Methylacidimicrobium, inhabit extremely acidic geothermal environments and can grow at a pH < 1.0 and temperatures up to 65 °C. We analyzed and compared their membrane fatty acid compositions at pH 3.0 across strains with different temperature optima. Thermophilic Methylacidiphilum strains almost exclusively contain saturated fatty acids, while mesophilic Methylacidimicrobium strains incorporate 16–47% unsaturated fatty acids. Notably, the thermophile Methylacidiphilum fumariolicum SolV increases unsaturated fatty acid content in response to a 10 °C temperature decrease. Genomic analysis revealed a conserved fatty acid biosynthesis pathway. Despite constitutive expression of predicted pH homeostasis genes, SolV did not upregulate them upon changing the pH from 3.0 to 1.7. However, genes involved in methane oxidation were strongly upregulated, suggesting a potential metabolic adaptation to extreme acidity.

Diatoms are one of the most important organisms contributing to aquatic primary productivity and their sterols are frequently used as markers for their presence and abundance. In this study, the sterol composition of >100 diatom cultures was analyzed and its distribution was compared to the diatom phylogeny to identify typical diatom biomarkers. Forty-four sterols were detected, 11 of them being commonly present as major sterols (contributing >10% to the total sterols). 24-Methylcholesta-5,24(28)-dien-3β-ol is the most common sterol in diatoms, being present in 67% of all cultures analyzed, followed by the △⁵ sterols, cholest-5-en-3β-ol (cholesterol), 24-methylcholest-5-en-3β-ol, and 24-ethylcholest-5-en-3β-ol. 24-Methylcholesta-5,22E-dien-3β-ol, previously suggested to be specific for diatoms, was only the fifth most common sterol; this sterol was absent in some of the major diatom groups, and high relative concentrations seem to be restricted to pennate diatoms. No sterols are restricted to specific phylogenetic groups of diatoms. Cluster analyses, however, do reveal distinct sterol distributions: Thalassiosirales typically contain high relative abundances of 24-methylcholesta-5,24(28)-dien-3βol, high relative abundances of cholesta-5,22E-dien-3β-ol are typical for Cymatosirales, high relative abundances of 24-ethylcholesta-5,22E-dien-3β-ol are characteristic for related Amphora, Amphiprora, and Entomoneis species, and a combination of high relative abundances of 24-methylcholest-5-en-3β-ol, 24-methylcholesta-5,24(28)-dien-3β-ol, and 24-ethylcholest-5-en-3β-ol is typical for Attheya species. High contributions of 24-methylcholesta-5,22E- dien-3β-ol (>50% of all sterols) seem to be restricted to pennate diatoms. None of the major sterols found in diatoms can be used as an unambiguous diatom biomarker because all of them have been reported as common sterols in other algal groups.

The presence of the C₃₀ sterol gorgosterol (22,23-methylene-23,24-dimethylcholest-5-en-3ß-ol) and its analogues in some marine and freshwater environments is generally associated with invertebrate animals or dinoflagellates since there have been no reports of them in other microalgal classes. Here we show that two unialgal cultures of different species of the marine diatom Delphineis contain gorgosterol in addition to sterols more commonly found in diatoms. Our findings suggest that some reports of gorgosterol in seawater and marine sediments may well have an origin, at least in part, from diatoms.