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Abstract

Global patterns of diversity in marine algae have been reported at the genus level but rarely at the species level. In this study, we investigate the global distribution of species diversity for marine green algae and evaluate the effect of water temperature upon the species composition. We calculated and mapped the species richness and phylogenetic diversity of green algae in 5° x 5° latitude-longitude cells. We then compared and grouped the different spatial patterns of phylogenetic species composition of green algae around the world. Finally, we fitted a locally weighted scatterplot smoothing (LOWESS) curve to the sea surface temperature (SST) data to determine how the species diversity varied with SST. We found hotspots of species richness and phylogenetic diversity in green algae in the seas around western Europe and the coasts of India, Brazil, South Africa, and northeastern Australia, and about half of these hotspots were located in the Atlantic Ocean. Both the species richness and the phylogenetic diversity of green algae were found to be higher in warm waters. In the tropics, the greater the species richness, the more the phylogenetic species composition was dispersed over multiple clusters. Conversely, in the temperate zone, the greater the species richness, the higher the phylogenetic clumpedness of the species in a community. The species in a community were evenly distributed on the phylogenetic tree in waters warmer than 23°C. On the other hand, the phylogenetic species composition was clumped in waters cooler than 23°C and this phylogenetic clumpedness increased as SST decreased. Our results indicate that the evolutionary processes of species composition in local communities of green algae have adapted to local environmental conditions, and suggest a phylogenetically clumped species composition of marine green algae in the temperate zones.

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