Quantification of biomolecular condensate volume reveals network swelling and dissolution regimes during phase transition

Accurate determination of biomolecular condensate volume reveals that destabilization of condensates can lead to either swelling or shrinking of condensates, giving fundamental insights into regulation of the volume of cellular condensates. Determination of the volume of biomolecular condensates and coacervate protocells is essential to investigate their precise composition and impact on (bio)chemical reactions that are localized inside the condensates. It is not a straightforward task, as condensates have tiny volumes, are highly viscous and prone to wetting. Here we examine different strategies to determine condensate volume, and introduce two new methods, with which condensate volumes of 1 μL or less (volume fraction 0.4%) can be determined with a standard deviation of 0.03 μL. Using these methods we show that the swelling or shrinking of condensates depends on the degree of physical crosslinking. These observations are supported by Flory-Huggins theory and can have profound effects on condensates in cell biology.