Exploring the size-dependent dynamics of photosynthetic cells in rainwater: The influence of atmospheric variables and rain characteristics

The presence of microalgae in the atmosphere raises health and environmental concerns. Despite recent scientific advances, our knowledge of the origins and dynamics of photosynthetic cells in relation to atmospheric processes is limited due to a lack of empirical data. To address this gap, we conducted a one-year survey, collecting and analyzing rainwater samples. This study proposes to investigate the temporal dynamics of photosynthetic cells based on their size in combination with a unique dataset of variables of interest: type of rain and its characteristics, local meteorology, concentrations of inorganic chemical species, and long-range air mass transport. The analysis of the biochemical composition of rainwater, along with its correlation with the origin of air masses using ions as tracers, provides evidence of the long-range transport of photosynthetic cells. Additionally, our study reveals distinct removal mechanisms from the atmosphere for photosynthetic cells depending on their size. Our results suggest that convective events with high-intensity rainfall led to the efficient removal of mediumsized photosynthetic cells (4-15 mu m) from the atmosphere. However, removal mechanisms for small (<4 mu m) and large-sized cells (>15 mu m) are not influenced by microphysical rainfall characteristics and seem to be governed by different atmospheric processes: dry deposition is proposed to be a significant mechanism for the removal of large-sized photosynthetic cells, while small-sized cells detected in rain are correlated with the horizontal wind speed and duration of rainfall, particularly during stratiform events. This implies that the removal of photosynthetic cells from the atmosphere is strongly influenced by environmental variables, which are expected to vary in response to global change. Therefore, it is crucial to enhance the monitoring of photosynthetic cells in relation to atmospheric processes and investigate the potential impact of the dissemination of genetic material from distant sources on recipient ecosystems.