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The research cruise M197 with the RV METEOR sailed January 6th 2024 to February 6th 2024 from Limassol (Republic of Cyprus) to Catania (Italy), with a focus on investigating the seawater and sediment biogeochemistry in the Eastern Mediterranean Sea. The two specific foci of the research cruise were to (i) investigate the physical, chemical and biological factors regulating the productivity and sinking carbon flux in this region, and (ii) investigate natural and human induced changes in the region over the last few thousand years. On the research cruise, 30 stations were occupied. Seawaters were collected from the surface to the seafloor to measure chemical properties of the seawater, including major and trace nutrients, the carbonate system, microplastics, and the microbial communities inhabiting the water column. Experiments were conducted to assess rates of primary production and nitrogen fixation, and the nutrients regulating phytoplankton growth. Sinking carbon fluxes were determined using the thorium-234 approach. The types and abundance of zooplankton were determined via various nets and two camera systems. The deposition flux and chemistry of aerosols was also assessed. Sediment cores were collected to determine nutrient fluxes into the overlying water column as well as investigate past environmental conditions. Collectively our research will provide an important advance in the network of factors that regulate the chemistry and biology of this system and inform ocean biogeochemical models of this region to make more realistic predictions of climate change induced impacts.

Picoeukaryotes are an important, diverse and spatially variable component of marine microbial communities. However, little is known of their distribution in response to environmental heterogeneity. In this study, to understand the Nordic Seas picoeukaryotic community, eleven surface samples from different water bodies were collected in June 2015. Archaeplastida, mainly Prasinophyceae, was present in all samples and was the largest component in cold waters, while Rhizaria and Alveoata were most abundant in the samples influenced by warm waters. Multivariate analyses showed that samples could be discriminated into groupings, each with its specific dominant species and community structure could precisely reflect the environmental heterogeneity caused by different water masses. This study details the relationships between the picoeukaryotes and complex currents in the Nordic Seas, and provides insight for application of using picoeukaryotes as indicator in future bioassessment for arctic or boreal waters.

Marine microbial eukaryotes are ubiquitous, comprised of phylogenetically diverse groups and play key roles in microbial food webs and global biogeochemical cycling. However, their vertical distribution in the deep sea has received little attention. In this study, we investigated the composition and diversity of the eukaryotes of both 0.2-3 mu m and > 3 mu m size fractions from the surface to the hadal zone (8727 m) of the Mariana Trench using Illumina MiSeq sequencing for the 18S rDNA. The microbial eukaryotic community structure differed substantially across size fractions and depths. Operational taxonomic unit (OTU) richness in the >3 mu m fraction was higher than that in the 0.2-3 mu m fraction at the same depth. For the 0.2-3 mu m fraction, sequences of Retaria (Rhizaria) were most abundant in the surface water (53.5%). Chrysophyceae (Stramenopiles) sequences dominated mostly in the samples from water depths below 1795 m. For the > 3 mu M fraction, sequences of Dinophyceae (Alveolata) were most abundant in surface waters (49.3%) and remained a significant proportion of total sequences at greater depths (9.8%, on average). Retaria sequences were abundant in samples of depths >= 1000 m. Amoebozoa and Apusozoa sequences were enriched in the hadal sample, comprising 38 and 20.4% of total sequences, respectively. Fungi (Opisthokonta) sequences were most abundant at 1759 m in both size fractions. Strong positive associations were found between Syndiniales (mainly MALV-I and MALV-II) and Retaria while negative associations were shown between MALV-II and Fungi in a co-occurrence analysis. This study compared the community structure of microbial eukaryotes in different zones in the deep sea and identified a distinct hadal community in the larger size fraction, suggesting the uniqueness of the eukaryotes in the biosphere in the Mariana Trench.

Marine microbial eukaryotes are ubiquitous, comprised of phylogenetically diverse groups and play key roles in microbial food webs and global biogeochemical cycling. However, their vertical distribution in the deep sea has received little attention. In this study, we investigated the composition and diversity of the eukaryotes of both 0.2-3 mu m and > 3 mu m size fractions from the surface to the hadal zone (8727 m) of the Mariana Trench using Illumina MiSeq sequencing for the 18S rDNA. The microbial eukaryotic community structure differed substantially across size fractions and depths. Operational taxonomic unit (OTU) richness in the >3 mu m fraction was higher than that in the 0.2-3 mu m fraction at the same depth. For the 0.2-3 mu m fraction, sequences of Retaria (Rhizaria) were most abundant in the surface water (53.5%). Chrysophyceae (Stramenopiles) sequences dominated mostly in the samples from water depths below 1795 m. For the > 3 mu M fraction, sequences of Dinophyceae (Alveolata) were most abundant in surface waters (49.3%) and remained a significant proportion of total sequences at greater depths (9.8%, on average). Retaria sequences were abundant in samples of depths >= 1000 m. Amoebozoa and Apusozoa sequences were enriched in the hadal sample, comprising 38 and 20.4% of total sequences, respectively. Fungi (Opisthokonta) sequences were most abundant at 1759 m in both size fractions. Strong positive associations were found between Syndiniales (mainly MALV-I and MALV-II) and Retaria while negative associations were shown between MALV-II and Fungi in a co-occurrence analysis. This study compared the community structure of microbial eukaryotes in different zones in the deep sea and identified a distinct hadal community in the larger size fraction, suggesting the uniqueness of the eukaryotes in the biosphere in the Mariana Trench.