Structural characterization of the extracellular stalk material of the diatom Didymosphenia geminata

The study represents new bioanalytical characterization of mainly organic components of the poorly investigated extracellular polymeric substances (EPS) of the enigmatic diatom Didymosphenia geminata, an invasive, worldwide expanding species endangering diverse ecosystems. This microalga attaches its siliceous cells to rocky substrates using fibrous stalks, which are made of an EPS-based matrix stabilized by crystalline calcite. The EPS were analyzed using selected methods, including microscopic, spectroscopic, and spectrometric techniques. We identified diverse types of biomolecules. The presence of lipids, condensed aromatics, and heteroaromatic compounds in the EPS has been confirmed using high-resolution mass spectrometry (HR-MS). Additionally, both sulfur-containing functionalities and carboxylic acids were determined too using infrared (IR) spectroscopy and nuclear magnetic resonance (NMR) spectroscopy. For the first time, lignin compounds have been detected as one of the components of the EPS of the D. geminata diatom, using HR-MS and fluorescence microscopy (FM) in combination with specific staining techniques. By increasing the understanding of the chemistry and structural features of the stalks, we aim to develop potential applications and methods for removing these stalks from affected regions in the future, or, alternatively, to use them as a large-scale source of sustainable biocomposite material.