Whether freshwater plant communities use carbon dioxide or bicarbonate for photosynthesis is largely related to the bicarbonate concentration in their local environment, according to a new study, the first global evaluation of bicarbonate use among aquatic plants. Because changes in land use and human activities are causing large-scale increases in bicarbonate, the biodiversity of freshwater ecosystems is likely to be greatly impacted by these events, the authors say. Unlike terrestrial plants, many freshwater aquatic plants have evolved the ability to use bicarbonate in addition to carbon dioxide for photosynthesis, to compensate for the low concentrations and potential depletion of dissolved CO2 in water. However, while bicarbonate use is shared amongst a wide variety of species across most taxonomic groups, it isn't ubiquitous across all freshwater plants. Lars Iversen and colleagues investigated the link between bicarbonate levels in a given freshwater ecosystem - which is largely dependent on the geochemistry of the catchment area - and the distribution of freshwater plants that use bicarbonate. The authors generated a detailed global map of the distribution of bicarbonate plants and CO2 plants and combined it with a geological map illustrating geological bicarbonate. The results show that the proportion of CO2 plants to bicarbonate plants across the globe is significantly related to the bicarbonate concentration in the local geology. The way the composition of freshwater plants is structured by catchment geology is a major departure from plant communities on land, which are largely influenced by variations in temperature and rainfall.