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Crustose coralline algae (CCA) occur from the tropics to the poles in photic benthic environments. Here, we report on some of the world’s southernmost and coldest CCA sites in Terra Nova, Ross Sea, Antarctica at 74°41′ S. The recently described red alga Tethysphytum antarticum is investigated for its skeletal architecture, its mineralogical and geochemical composition, as well as for its taxonomic classification. A phylogenetic analysis based on molecular genetics and the sequencing of the photosystem II protein D1 (psbA) gave a perfect match with T. antarcticum. Histological sections and micro-CT-scans provide new diagnostic details for the conceptacles (the reproductive organs of the alga). X-ray diffractometry and electron-microprobe measurements yielded a clear high-Mg calcite (~8 mol%) composition of the skeletal parts. Detailed back-scattered electron imaging of polished petrographic thin sections revealed a two-layered thallus (vegetative plant tissue), comprising an organic-rich irregularly calcified basal layer with rectangular cells, overlain by the main thallus. Elemental maps show relatively increased sulphur in the basal layer, clearly tied to organic cell walls. MgCO3 and SrCO3 were targeted with semiquantitative elemental mappings and in an ontogenetic quantitative spot transect. Compared with temperature (−1.95 °C to +1.08 °C), the MgCO3 (mol%) reflects this world’s coldest CCA site temperature with the lowest MgCO3 content of 7.9 ± 1.6 mol%. The along transect variability, however, shows with ~6 mol% a larger MgCO3 variability than expected for the 3 °C intra-annual temperature amplitude in Terra Nova Bay. This implies that in low amplitude settings the biomineralisation control on Mg/Ca ratios can outcompete its temperature sensitivity. Mark-recapture studies, next to the environmental logger station La Zecca are suggested, to perform a detailed growth rate and biomineralisation quantification.

Brackish‐water carbonates are far less studied than their marine or limnic counterparts. However, their association with few, specialized species enables the documentation of fine‐scale changes in the depositional environment. The Cenozoic Mainz Basin (Germany) was only sporadically connected to the North Sea and the Paratethys, exposing several transitions from marine to fresh water influence. Focusing on one outcrop of the Rüssingen Formation of Mainz‐Weisenau (Aquitanian, Miocene), we present a detailed analysis of the faunal and sedimentological responses to changing salinities and water depth, including algal reef growth and facies development. The deposits include allochthonous limestones surrounding an autochthonous reef complex and several smaller reef patches. The allochthonous facies is dominated by the gastropod Hydrobia inflata, and the reef facies is mainly made up by the green alga Cladophorites sp. The algal thalli are overgrown by cryptocrystalline, organic precipitations, and laminated, chemical precipitations. Locally, quiver‐shaped structures of Trichoptera sp. protective cases occur. The depositional setting was a shallow, low energy, and brackish environment supersaturated by carbonate. We could not confirm a general trend of reducing salinities as reported for the Rüssingen Formation. Our results question previously reported episodic desiccation events, because apparent caliche horizons actually represent thin beds of increased Cladophorites growth. Set‐up, distribution of the reef facies, and reef debris indicate short‐time variations of temperature, salinity and water depth. We conclude that these variations are based on the geographic position at the edge of an algal reef barrier, separating the Mainz Basin from the Rhine Rift Valley., This study investigates the record of small‐scale changes in the depositional environment of Miocene brackish‐water deposits from the Mainz Basin (Germany) by facies analysis. Set‐up, distribution of the reef facies, and reef debris indicate short‐time variations of temperature, salinity and water depth. The apparent caliche horizons actually represent thin beds of increase Cladophorites growth. The example demonstrates that these brackish deposits are a sensitive recorder of palaeoenvironmental change., Open access funding enabled and organized by Projekt DEAL. WOA Institution: FRIEDRICH‐ALEXANDER‐UNIVERSITAET ERLANGEN‐NURNBERG Blended DEAL: ProjektDEAL

Coralline algae (Corallinales, Rhodophyta) that form rhodoliths are important ecosystem engineers and carbonate producers in many polar coastal habitats. This study deals with rhodolith communities from Floskjeret (78°18′N), Krossfjorden (79°08′N), and Mosselbukta (79°53′N), off Spitsbergen Island, Svalbard Archipelago, Norway. Strong seasonal variations in temperature, salinity, light regime, sea-ice coverage, and turbidity characterize these localities. The coralline algal flora consists of Lithothamnion glaciale and Phymatolithon tenue. Well-developed rhodoliths were recorded between 27 and 47 m water depth, while coralline algal encrustations on lithoclastic cobbles were detected down to 77 m water depth. At all sites, ambient waters were saturated with respect to both aragonite and calcite, and the rhodolith beds were located predominately at dysphotic water depths. The rhodolith-associated macrobenthic fauna included grazing organisms such as chitons and echinoids. With decreasing water depth, the rhodolith pavements were regularly overgrown by non-calcareous Polysiphonia-like red algae. The corallines are thriving and are highly specialized in their adaptations to the physical environment as well as in their interaction with the associated benthic fauna, which is similar to other polar rhodolith communities. The marine environment of Spitsbergen is already affected by a climate-driven ecological regime shift and will lead to an increased borealization in the near future, with presently unpredictable consequences for coralline red algal communities.