Projected area calculation for microalgae using three-dimensional models

Light acquisition and sinking properties of microalgae fundamentally affect how species perform in aquatic environments. Both properties are the function of their projected area (Ā), a crucial morphological trait of microalgae. Despite their importance, species-specific Ā values have not been computed for microalgae. The reason for this is that although using an analytical approach Ā can be calculated for every convex shape, a vast majority of planktic algae are concave and currently not known how to calculate the projected area of concave shapes. Applying shape-realistic 3D models of microalgae and a novel numerical approach combined with computer simulation, we calculated the projected area for more than 800 microalgae. Validating this approach using convex shapes and the analytical Ā = surface area/4 formula we found, that the proposed method achieves less than 5 % estimation bias. We also studied how Ā values of species can be predicted by easy-to-measure morphological metrics (such as Gald/width ratio, compactness, relative surface area extension, relative elongation, surface area constant, volume constant). We have found that the metrics do not show a sufficiently close relationship with the projected area to allow us to estimate species-specific Ā values. We demonstrated that the morphological differences among species can result in up to 6-fold differences in Ā values for the same volume. Spindle-form, filamentous species and loosely packed coenobia are the most efficient adaptations to maximize Ā. This study provides an innovative methodology and a huge dataset containing Ā values of 844 planktic freshwater microalgae. Although the dataset contains species only for the Pannonian and Dinaric ecoregions, because of the cosmopolitan nature of planktic algae it can be used for other ecoregions. Species-specific projected area values multiplied by cell counts give a new metric that characterizes the shading property of the given phytoplankton assemblage and enables us to better understand the changes in light availability and study the vertical processes in the water.