Marine environments worldwide are increasingly threatened by warming, deoxygenation, and ocean acidification. Foraminifera may respond to these stressors by altering their test geochemistry and morphology. We integrated morphological features with assemblage and geochemical records of benthic foraminifera from the Danish Straits at the entrance to the Baltic Sea, covering the Last Interglacial period (LIG, MIS5e), to assess potential indications of environmental changes. Using synchrotron radiation X-ray microcomputed tomography (SRµCT), we analyzed Elphidium clavatum in terms of size, surface area, volume, thickness, and pores. Pore patterns and test thickness are evaluated simultaneously to assess both metabolic and mechanical constraints under environmental changes. The initial chamber size (proloculus) may indicate the reproduction mode of foraminifera in response to environmental stress, including salinity and oxygen variations. During the early-mid LIG, the mechanical constraints of E. clavatum remain relatively strong, which are characterized by high pore density, thickness, and low porosity, coinciding with higher bottom water salinity and oxygen content. In the mid-late LIG, the higher porosity, larger proloculus size, lower test thickness and pore density of E. clavatum enhanced the metabolic function and survival rates. These traits reflect adaptation to an increasingly stressful environment with lower salinity and oxygen levels at the Danish Straits, as indicated by declining faunal diversity and increasing E. clavatum abundance. Our study demonstrates that benthic foraminiferal morphological features, including proloculus size, test thickness, and pore patterns, serve as indicators for assessing stress levels and reconstructing bottom water conditions in brackish and potentially hypoxic environments during the LIG.

Phytoplankton have short generation times, flexible reproduction strategies, large population sizes, and high standing genetic diversity, traits that should facilitate rapid evolution under directional selection. We quantified local adaptation of copper tolerance in a population of the diatom Skeletonema marinoi from a mining exposed inlet in the Baltic Sea and in a non-exposed population 100 km away. We hypothesized that mining pollution has driven evolution of elevated copper tolerance in the impacted population of S. marinoi. Assays of 58 strains originating from sediment resting stages revealed no difference in the average tolerance to copper between the two populations. However, variation within populations was greater at the mining site, with three strains displaying hyper-tolerant phenotypes. In an artificial evolution experiment, we used a novel intraspecific metabarcoding locus to track selection and quantify fitness of all 58 strains during co-cultivation in one control and one toxic copper treatment. As expected, the hyper-tolerant strains enabled rapid evolution of copper tolerance in the mining exposed population through selection on available strain diversity. Within 42 days, in each experimental replicate a single strain dominated (30-99% abundance) but different strains dominated the different treatments. The reference population developed tolerance beyond expectations primarily due to slowly developing plastic response in one strain, suggesting that different modes of copper tolerance are present in the two populations. Our findings provide novel empirical evidence that standing genetic diversity of phytoplankton resting stage allows populations to evolve rapidly (20-50 generations) and flexibly on timescales relevant for seasonal bloom progressions.