ALBERT

Description: The diatom species Fragilariopsis kerguelensis is endemic to the Southern Ocean where it plays a key role in the ocean silica cycle due to its heavily silicified cell walls. Frustules from dead cells can sink to the ocean floor and therefore contribute about 90% to the diatom frustules making up the Antarctic opal belt, a band of heavily silicified sediments below the Antarctic Circumpolar Current. Recent studies showed that two morphotypes of F. kerguelensis can be found in core samples originating from the Southern Ocean. The morphotypes can be distinguished by the morphometric descriptor rectangularity and the abundance of the morphotypes can be linked to the origin of the samples from glacial or interglacial periods. It is, however, so far unknown whether these morphotypes also occur in current diatom assemblages of the Southern Ocean; if yes, how their occurrence is influenced by environmental conditions; and whether they represent different species, or rather phenotypic plasticity within a single species. For answering these questions diatom valves in water and sediment samples from the Southern Ocean, preserved on microscopic slides were analysed morphologically using a semi automated morphometry workflow. Histograms of the rectangularity were then plotted which all showed a bimodal distribution proving the existence of the two morphotypes in recent samples. In the next step the biogeographic distribution pattern of the two morphotypes was assessed by plotting the rectangularity distribution at each sample station. Whereas one morphotype was dominant in the northernmost samples, its dominance decreased towards the south, and the other morphotype became dominant in the southernmost locations investigated. This pattern could also be linked to the Sea Surface Temperature with a regression. After the previous findings the question arose if the morphotypes could be cryptic species or if they occur due to phenotypic plasticity within a single species. To answer this a genetic assessment with diatom strains showing different rectangularity values, isolated from the Southern Ocean and kept in live cultures at the Alfred-Wegener-Institut was done. Their ribosomal internal transcribed spacer (ITS) regions were amplified using PCR and then analysed with Sanger sequencing. The results made it possible to identify three potential species based on their genetic differences. One semi-cryptic species was solely made up of morphotype B whereas morphotype A was divided in two genetic clusters representing two cryptic species. Mating experiments were carried out to uncover possible reproductive barriers between the cryptic species. The results showed, that strains being assigned the same cryptic species commonly sexually reproduce, whereas sexual reproduction between strains from different cryptic species was only observed in a few exceptions. In summary, the two investigated morphotypes of Fragilariopsis kerguelensis can be found in the Southern Ocean today, with one dominating at higher, the other at lower Sea Surface Temperatures. Comparison of ribosomal ITS sequences and mating experiments indicate that the two morphotypes seem to belong to three different species, one semi-cryptic species belonging to one and two cryptic species to the other morphotype.

Description: Fragilariopsis kerguelensis (O’ Meara) Hust. is a ubiquitous diatom of the Southern Ocean. Its thick frustules are the numerically dominant component of the siliceous sediment layer covering large parts of the seafloor beneath. Morphometric variability of frustules of this diatom has been of interest for paleoenvironmental reconstructions. Recently, two morphotypes differentiated by the morphometric descriptor rectangularity were described from a Southern Ocean sediment core, the relative abundance of which correlated with reconstructed paleotemperatures. In the present study, we use semi-automated microscopic and image analysis methods to answer whether these morphotypes also appear in recent assemblages, and if yes, do their distributions reflect geographic location or environmental factors. Three transects from the water column, sampled along the Greenwich meridian with hand nets, and one sediment surface transect from the South Pacific, were analyzed. In each of these transects, both morphotypes were detected, and annual mean sea surface temperatures (SST) were found to be a good predictor of their relative abundances. The transition between dominance of one or the other morphotype appeared roughly between the Antarctic Polar Front and the Southern Boundary of the Antarctic Circumpolar Current. Although more extensive circumpolar sampling will be needed to confirm the generality of our conclusions, the observed morphometric cline is a novel aspect of the biology of this species and can in the future potentially be used for further developing paleoproxies especially for highly F. kerguelensis-dominated sediment in the Southern Ocean.

Description: The Southern Ocean is characterized by longitudinal water circulations crossed by strong latitudinal gradients. How this oceanographic background shapes planktonic populations is largely unknown, despite the significance of this region for global biogeochemical cycles. Here, we show, based on genomic, morphometric, ecophysiological and mating compatibility data, an example of ecotypic differentiation and speciation within an endemic pelagic inhabitant, the diatom Fragilariopsis kerguelensis. We discovered three genotypic variants, one present throughout the latitudinal transect sampled, the others restricted to the north and south, respectively. The latter two showed reciprocal monophyly across all three genomes and significant ecophysiological differences consistent with local adaptation, but produced viable offspring in laboratory crosses. The third group was also reproductively isolated from the latter two. We hypothesize that this pattern originated by an adaptive expansion accompanied by ecotypic divergence, followed by sympatric speciation.

Description: Climatic changes in the Southern Ocean have strong implications for the global marine carbon cycle, for example through changes in phytoplankton community composition. These shifts, in turn, can affect the strength and efficiency of the biological carbon pump, i.e. the process by which carbon is exported from the surface ocean to the deep sea via the aggregation and sinking of phytoplankton and other organic matter. At depth, carbon can be sequestered over long periods of time, effectively “buffering” increasing atmospheric CO2 concentrations. For the Southern Ocean, specifically the Weddell Sea, we only have limited data on carbon export due to the difficulties of accessing these remote and often ice-covered regions. Based on various phytoplankton bottle incubation experiments which simulated future climatic changes a possible shift in phytoplankton community composition from large diatoms to small flagellates such as Phaeocystis sp. is indicated, with unknown consequences for nutrient cycling and carbon export. To address these unknowns, we conducted in situ measurements and roller tank experiments with contrasting diatom-to-Phaeocystis ratios during a Polarstern cruise to the Southern Weddell Sea in spring 2021 to characterize aggregate formation degradation and sinking of marine snow. The same set of parameters were also assessed in controlled laboratory experiments with well-defined diatom-to-Phaeocystis ratios. Based on our results from field and laboratory, we hypothesised that a climate-mediated shift towards Phaeocystis in the future would reduce the efficiency of the biological carbon pump due to decreased silica-ballasting and increased concentrations of positively buoyant exopolymeric substances associated with Phaeocystis colonies. To our surprise, preliminary results reveal that higher Phaeocystis cell numbers relative to diatoms do not lead to a statistically significant reduction in aggregate mass density and size-specific sinking velocity. At the same time, there was a trend towards larger particles when Phaeocystis was abundant. Our results from field and laboratory observation together reveal that Phaeocystis-dominated communities do not impede carbon export in the Weddell Sea, but may actually enhance it.

Description: During the RV Polarstern expedition to the southern Weddell Sea, in Feb 2021, a breeding colony of notothenioid icefish (Neopagetopsis ionah, Nybelin 1947) of globally unprecedented extent has been discovered. The colony, at time of survey, covered at least ~240 square kilometres of the eastern flank of the Filchner Trough and was comprised of fish nests at a density of 0.26 nests per square metre, representing an estimated minimum total of ~60 million active nests, and an associated fish biomass of 〉 60,000 tonnes. The majority of nests were each occupied by 1 adult fish guarding 1735 eggs (±433 SD). Bottom water temperatures adjacent to the nests were up to 2 °C warmer than the surrounding bottom waters, indicating a spatial correlation between the modified Warm Deep Water (mWDW) upflow onto the Weddell Shelf and active nesting.

Description: A breeding colony of notothenioid icefish (Neopagetopsis ionah, Nybelin 1947) of globally unprecedented extent has been discovered in the southern Weddell Sea, Antarctica. The colony was estimated to cover at least �240 km2 of the eastern flank of the Filchner Trough, comprised of fish nests at a density of 0.26 nests per square meter, representing an estimated total of �60 million active nests and associated fish biomass of 〉60,000 tonnes. The majority of nests were each occupied by 1 adult fish guarding 1,735 eggs (±433 SD). Bot- tom water temperatures measured across the nesting colony were up to 2�C warmer than the surrounding bottom waters, indicating a spatial correlation between the modified Warm Deep Water (mWDW) upflow onto the Weddell Shelf and the active nesting area. Historical and concurrently collected seal movement data indicate that this concentrated fish biomass may be utilized by predators such as Weddell seals (Lep- tonychotes weddellii, Lesson 1826). Numerous degraded fish carcasses within and near the nesting colony suggest that, in death as well as life, these fish provide input for local food webs and influence local biogeo- chemical processing. To our knowledge, the area surveyed harbors the most spatially expansive continuous fish breeding colony discovered to date globally at any depth, as well as an exceptionally high Antarctic sea- floor biomass. This discovery provides support for the establishment of a regional marine protected area in the Southern Ocean under the Convention on the Conservation of Antarctic Marine Living Resources (CCAMLR) umbrella.