In recent years, microbial biogeography has been extensively studied, revealing that several protists show restricted, rather than cosmopolitan, distributions. However, it remains unclear to what extent microbial communities are impacted by past tectonic and paleoclimate events. This is because no fossil record exists for many micro-organisms, complicating analyses on historical biogeography. When fossil records do exist, such as is the case for diatoms (a group of unicellular algae), they are largely restricted to marine environments which harbor rich fossil records dating back millions of years. Consequently, the link between past climate and diatom evolution has been extensively studied for marine communities. In contrast, we know very little about long-term dynamics of freshwater diatoms, owing to a scarce pre-Pleistocene fossil record. However, freshwater and marine diatoms differ fundamentally: freshwater populations are smaller and less interconnected, freshwater habitats are ephemeral over geological time, and freshwater diatoms diversify and turn over faster than marine diatoms (see here for a good read on this topic). Thus: insights from long-term evolution of marine diatoms are not necessarily relevant for freshwater communities, and this despite the fact that the majority of diatom diversity can be found in freshwater environments. In a recent publication in Science Advances, we tackled the knowledge-gap on long-term freshwater diatom evolution by analyzing fossil and recent lacustrine sediments from Antarctica spanning the Middle Miocene (15-14 million years), Late Pleistocene (130-11.7 thousand years) and Holocene. This allowed investigating long-term dynamics in the diversity and biogeography of Antarctica’s freshwater diatom flora. Our dataset spans several major climate perturbations: 1) the Middle Miocene Climate Transition, MMCT (14 Ma), which was one of the most significant climate shifts driving Continental Antarctica’s transition from a temperate/subpolar climate to a polar desert, and 2) the Pleistocene glacial-interglacial cycles. We dovetailed our Antarctic dataset with an assessment of the contemporary freshwater diatom floras from the Northern and Southern Hemispheres, focusing on polar, alpine, and temperate sites. This allowed examining Antarctica’s diatom flora in a global context. We found that Miocene Continental Antarctica was home to a diverse and unique freshwater diatom flora, which was much more species rich than Continental Antarctica today. We found 212 morphospecies belonging to 49 genera. Furthermore, our Miocene diatom flora was dominated by species and genera that are present-day absent from Continental Antarctica. In fact, most of the species are new to science, and we have evidence for four new diatom genera! Using our global dataset on genus-level, we found that the Miocene flora is compositionally distinct from modern Continental Antarctica, but instead shows clear affinity with modern Arctic and Sub-Antarctic samples, as well as temperate lakes of the Southern Hemisphere. Even more exciting: we found three diatom taxa in the Miocene Antarctic material which today are confined to Antarctica’s neighboring continents. This shows that Antarctica’s Miocene diatom flora was evolving in tandem with other former Gondwanan landmasses! Strikingly, whereas Miocene Antarctica was, like the modern Arctic, home to predominantly aquatic taxa, modern Continental Antarctica is dominated by aerophiles, i.e., diatoms that preferably inhabit soils. This is indicative of a large shift in community composition that happened since the Middle Miocene. Our Miocene flora is thus witness of major, but selective, extinction among an ancient Antarctic diatom flora, likely in response to the MMCT. We hypothesize that aerophilic diatoms were more likely to survive cooling, because they are adapted to extreme conditions (see this, this and this study). However, extinction did not stop at the MMCT! Several diatoms became locally extinct in Continental Antarctica during the last glacial period. Today, such species are confined to sub-Antarctica and Maritime Antarctica, but they inhabited Continental Antarctica during the previous interglacial. Altogether, our data showed that extinction has been a dominant factor shaping Antarctic diatom communities since the Middle Miocene, resulting in the depauperation of the modern Antarctic diatom flora. However, in situ speciation and new colonizations ultimately shaped Antarctica’s highly adapted flora, which shows high levels of endemism (see also this study). In fact, molecular phylogenies of Antarctic diatoms are directly indicative of post-MMCT colonizations and local diversification. This is nicely illustrated for the diatom species complex surrounding Pinnularia borealis (see this study). Last but not least, our results provide a more holistic view on the scale of biodiversity turnover in ancient Antarctica than the fragmentary perspective offered by macrofossils, and underscore the sensitivity of lacustrine microbiota to large-scale climate perturbations. Pinseel E., Van de Vijver B., Wolfe A.P., Harper M., Antoniades D., Ashworth A.C., Ector L., Lewis A.R., Perren B., Hodgson D.A., Sabbe K., Verleyen E., Vyverman W. Extinction of austral diatoms in response to large-scale climate dynamics in Antarctica. Science Advances 7: eabh3233. full text
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