Drivers and Consequences of Size Declines in Unicells.

The communities of unicellular microbes (bacteria, protists and yeasts) that underpin ecosystems are changing. In warmer conditions, protists tend to shrink, but the consequences of these changes in size are unclear. We show preliminary evidence that warming-mediated declines in cell size observed in protists also apply to bacteria and yeasts. Predicting the consequences of these warming-mediated size declines requires that the relationships between cell size and key functional traits are well-characterised. We show that the critical relationship between unicellular size and energy use-that is, metabolic scaling-has been systematically mis-estimated in the past. Projections of the effects of warming on unicellular respiration change from superlinear to sublinear once the metabolic scaling relationship is updated, with worrying consequences for the biological carbon pump and other ecosystem services. Other size-function scaling relationships (e.g., photosynthesis) are likely to have been similarly mis-estimated. Next, we show that theory on the relationships between size, temperature and demography is more ambivalent than previously recognised, leaving uncertainty as to how warming will alter the dynamics of unicellular populations. Finally, we identify pathways for improving our capacity to predict future changes in unicellular size, and decrease the uncertainty surrounding the consequences of these changes.