Salinity is a significant barrier to the distribution of diatoms, and though it is generally understood that diatoms are ancestrally marine, the number of times diatoms independently colonized fresh waters and the adaptations that facilitated these colonizations remain outstanding questions in diatom evolution. Resolving the exact number of freshwater colonizations will require large-scale phylogenetic reconstruction with dense sampling of marine and freshwater taxa. A more tractable approach to understanding the marine--freshwater barrier is to study a group of diatoms with high diversity in each habitat. The "centric" diatom order Thalassiosirales affords an excellent opportunity to study the origin and evolution of diatoms in fresh waters. Thalassiosirales is a well-supported monophyletic group common in marine, brackish, and freshwater habitats. Thalassiosirales species historically are classified into the marine Thalassiosiraceae or freshwater Stephanodiscaceae, reflecting the more generally held hypothesis that diatoms are naturally split along marine--freshwater lines. The fossil record suggests that Stephanodiscaceae traces to a single colonization of freshwater in the mid-Miocene, and in addition, Stephanodiscaceae species share a suite of complex cell wall characters, which has been interpreted as corroborating evidence for their monophyly. I reconstructed the phylogeny of Thalassiosirales and used the phylogeny to test these and other hypotheses and to address a number of other problems related to the marine--freshwater boundary in diatoms. Phylogenetic analyses showed strong evidence for multiple colonizations of freshwater and reject all previous colonization hypotheses. Results further show that part of Stephanodiscaceae is an early diverging lineage within Thalassiosirales, indicating that these two distantly related and separately derived Stephanodiscaceae lineages independently evolved a similar set of complex morphological features upon or shortly after the colonization of fresh waters. Finally, marine and freshwater diatoms, including Thalassiosirales, show several important differences in silicon physiology. In addition to containing an order of magnitude more silica in their cell walls, freshwater diatoms have a drastically lower enzymatic affinity for silicic acid, the dissolved form of silica used by diatoms. I sequenced the silicon transporter genes from marine and freshwater Thalassiosirales and show that physiological differences are not due to differences in the coding sequence. / text

The Gulf of Bothnia consists of two sub-basins in the northern Baltic Sea: the Bothnian Sea (salinity 4-5‰) and Bothnian Bay (salinity 2-3‰). Changing nutrient concentrations and signs of eutrophication has recently been observed in the Gulf of Bothnia. Many rivers enter this sea area, and potentially river inflows constitute a source of nutrient pollution via waste-water emissions. The aim of this study was to elucidate effects of waste-water emissions in four rivers in northern Sweden, Luleå, Skellefteå, Umeå and Söderhamn. My approach was to compare nutrient concentrations at upstream and downstream sampling stations related to the position of waste-water treatment plants. Temporal data from 2006 to 2021 were used and statistically analyzed using non-parametric tests to establish spatial and temporal patterns for nutrient discharged to the coast. The results showed that there are statistically differences in dissolved inorganic phosphorus (DIP) in the form of phosphate (PO4), ammonia (NH4) and total nitrogen (TotN) between the upstream and downstream of Luleå and Umeå wastewater treatment plants. No statistically significant differences were observed in the upstream and downstream data for Söderhamn and Skellefteå. This suggest that better management and mitigation of nutrient loading from wastewater treatment plants that serve higher populations is paramount to achieve the zero-eutrophication goal in the Gulf of Bothnia.