Tropical rainforests are often referred to as an Earth’s lungs and are hotspots for biological diversity. However, few are aware that half of the oxygen we breathe comes from the ocean and is produced by microscopic algae. Even fewer realized the diversity of algae and other unicellular protists, for which estimates of the number of species exceed 1 million.
The small size of protists living in the water column (called planktonic protists) make their study challenging even for specialists, and the smallest protists that belong to pico- (cells smaller than 3 micrometers) and nanoplankton (3-20 micrometers) are often indistinguishable, or even undetectable using a conventional light microscope. For these reasons, the true extent of their diversity has remained hidden for decades. Only recently, at the beginning of the 21st century, has the application of molecular methods revealed the existence of novel protistan groups and thousands of unknown species, completely reshaping our understanding of the tree of life. Over the last 20 years, our knowledge of the diversity of protists in different aquatic habitats has been gradually increasing, but we are still at the beginning.
In our study, we were interested in the diversity of pico- and nanoplanktonic protists in an estuary of the Vistula River that enters the Baltic Sea at the Gulf of Gdansk (Poland). The Baltic Sea is an intercontinental European sea and is very unique; it is young (~ 10 000 years), small, and fresh: salinity is about 5 times lower than in the ocean. This low salinity made us wonder whether freshwater protists, introduced with the Vistula waters, can survive in the Baltic Sea.
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We applied novel sequencing methods to reveal sequences of almost 1,000,000 genes originating from protists actively living in the Vistula river, at its entrance to the Gulf of Gdansk, and in the Gulf waters. Sequences that differed less than 3% were grouped together into so-called Operational Taxonomic Units (OTUs) that are believed to approximately correspond to species. We found over 1,200 OTUs, many of which belonged to groups not known to occur in the Gulf of Gdansk or even in the Baltic Sea, such as, for example, Radiolarians. Moreover, the number of protists species estimated to be present in the Vistula River estuary based on our single study was higher than estimates for the whole Baltic Sea based on a multiannual effort by many research groups. Certainly, many more still awaits discovery!
Coming back to our questions about the survival of freshwater protists in the Baltic Sea, the short answer is no. While virtually all protists that lived in the Vistula River could be easily detected at the mouth of the river at a salinity of about 3.5, in the Gulf of Gdansk the protistan communities were already very different. More detailed analyses revealed that the freshwater protists were diluted together with the Vistula water, and therefore there was so few of them in the Gulf of Gdansk. In contrast, those that lived in the Gulf of Gdansk were more vulnerable to unfavorable conditions close to the river mouth, mainly to too low salinity. Such relationships were observed in all seasons, indicating that the observed pattern might be more general.
As it often happens in science, our study provided as many new questions as answers. It would be interesting to see, for instance, how protists change their metabolism, behavior, or activity due to salinity change, or how they deal with predators and exploit resources in a new environment. We hope to find the answers in the future.
These findings are described in the article entitled Diversity and community composition of pico- and nanoplanktonic protists in the Vistula River estuary (Gulf of Gdańsk, Baltic Sea), recently published in the journal Estuarine, Coastal and Shelf Science, and in the article Tideless estuaries in brackish seas as possible freshwater-marine transition zones for bacteria: the case study of the Vistula river estuary, recently published in the journal Environmental Microbiology Reports. This work was led by Kasia Piwosz from the National Marine Fisheries Research Institute and Czech Academy of Sciences, and conducted in cooperation with Marcin Gołębiewski from Nicolaus Copernicus University, Joanna Całkiewicz from the National Marine Fisheries Research Institute, and Simon Creer from Bangor University.