Jelena Godrijan

The impact of atmospheric deposition on marine ecosystems requires consideration of the interfacial processes within the sea surface microlayer, the uppermost layer at the air–sea interface. This study examines the impact of anthropogenic and biomass burning aerosol particles on the plankton community structure and organic matter accumulation in the system derived from in situ‐collected microlayer compared to the underlying water through a 4‐day microcosm experiment conducted in August 2020 in the Central Adriatic area. Realistic aerosol additions (1 mg L ⁻¹ ) had different impacts on surface layers, revealing complex interactions between nutrient availability, toxicity, and plankton competition. A sudden nutrient supply, particularly of nitrogen, favored larger phytoplankton, while other aerosol constituents, such as trace metals and carbonaceous compounds, likely had a toxic effect, particularly on smaller species. The neuston in the microlayer showed greater sensitivity to pollutants, possibly due to higher UV exposure enhancing toxicity compared to the underlying water counterparts. Shifts in neuston diversity promoted the accumulation of organic carbon and surfactants in the microlayer, with potential implications for suppressing the air–sea CO ₂ exchange.

Coccolithophores are abundant microalgae that play a key role in the ocean carbon cycle through photosynthesis and their production of intricate calcium carbonate coccoliths. Coccolithophores are particularly abundant in the Great Calcite Belt within the Southern Ocean, although these communities remain largely understudied due to their remote location. Mesoscale oceanographic features can influence carbonate export in these regions, although their impact on coccolithophore communities and life cycles remains unclear. We have applied a combination of scanning electron microscopy (SEM) and molecular DNA metabarcoding approaches to characterize coccolithophore populations across a mesoscale meander system in the Southern Indian Ocean. DNA metabarcoding, using haptophyte‐specific 18S rRNA primers, revealed a rich diversity of coccolithophores in this region. This diversity was absent in previous studies using broad‐specificity 18S eukaryote primers, indicating that coccolithophores are likely to be massively underrepresented in many large‐scale sequencing studies. Scanning electron microscopy analysis revealed a similar community, dominated by Umbellosphaera tenuis , Gephyrocapsa huxleyi , and Syracosphaera halldallii , with broadly similar trends in community structure, richness, and diversity. Community composition and life cycle phases were strongly influenced by a mesoscale oceanographic frontal system associated with significant gradients in temperature, salinity, and nutrient availability. Our study identifies a high diversity of coccolithophores in the Great Calcite Belt and suggests that hydrographic gradients across mesoscale features can cause fine‐scale heterogeneity in coccolithophore communities that diverge from the broad‐scale patterns caused by latitudinal gradients.

Coccolithophores are major marine phytoplankton that contribute to ocean carbon cycling through both organic carbon fixation and calcium carbonate biomineralization, yet the functional basis of their interactions with phycosphere bacteria remains poorly resolved. Here, we present the nuclear genome of the haploid phase of the coccolithophore Calcidiscus leptoporus, a coccolithophore that calcifies in both life-cycle stages. We combined host genome analysis, genome-resolved characterization of associated bacteria, and a four-month vitamin-manipulation experiment to test how B-vitamin biosynthetic complementarity relates to host performance and phycosphere community assembly. Metabolic reconstructions indicate partitioned B-vitamin biosynthtic potential: the host encodes pathways for B2, B5, B6, and B9, including a rare fused B5 biosynthesis gene, but lacks complete pathways for B1, B3, B7, and B12. These missing functions were distributed among recurrent bacterial taxa, with no single bacterial MAG encoding the full set of host-missing vitamins. Across the experiment, bacterial community composition was structured primarily by the experimental phase, while vitamin treatments secondarily influenced which taxa became enriched at later stages. Consistent with genome-inferred auxotrophy, B-vitamin availability constrained long-term growth under the tested conditions, with B1 alone providing partial rescue and vitamin-replete treatments showing equal or stronger responses. Together, these data establish C. leptoporus as a genomic model for coccolithophore biology and holobiont interactions, while providing a testable framework linking vitamin economies to phycosphere assembly and host performance.

This study evaluates the impact of sampling protocols on the measurement of particulate inorganic carbon (PIC) in ocean waters, an essential component for understanding the global carbon cycle and climate regulation. The study compares four protocols for estimating PIC in discrete water column samples, focusing on the effects of filter pore size (0.4 vs. 0.8 μm) and rinsing agents (pH‐adjusted MilliQ water with NH₄OH vs. potassium tetraborate buffer). Five coccolithophore strains were selected to represent variations in PIC content resulting from species‐specific differences in coccolith mass, coccolith number per cell, and life cycle phase. Discrete samples were analyzed using inductively coupled plasma mass spectrometry. Statistical analyses show no significant differences in PIC concentrations between protocols, filter types, or rinsing agents, confirming the robustness and precision of the measurement method. In addition, the non‐calcifying strain provided insights into the measurement uncertainty and enabled us to quantify the precision of the sampling method. These results suggest that researchers can use any tested protocol without compromising data quality. This will improve the reliability and comparability of PIC measurements and contribute to a more precise understanding of ocean carbon dynamics and climate regulation.

Marine life is threatened by global warming and its indirect consequences, which include, among others, increased stratification leading to phosphorus (P) and nitrogen depletion in the upper water column. Phosphorus plays a key role in all biochemical systems; storage of metabolic energy, formation of genetic material, and subcellular compartmentalization. Our multi‐year study of lipid biogeochemistry in the northern Adriatic (Mediterranean), which is becoming warmer and nutrient‐poorer, particularly regarding P, has shown that under conditions of P scarcity, phospholipids are relatively more abundant and smaller plankton cells dominate. Consistent with the field data, experiments with seven phytoplankton monocultures, comprising microplankton, nanoplankton, and picoplankton, confirmed a relatively higher phospholipid content in the smaller phytoplankton species and, in particular, an increase in those grown under stress conditions in general, including, unexpectedly, P‐limitation. We suggest two reasons for the observed “P paradox” of P‐limited phytoplankton: (1) cell geometry: volume of the plasma membrane relative to the volume of the entire cell is greater in smaller cells and, therefore, the proportion of plasma membrane phospholipids to intracellular lipids is greater in smaller cells, (2) higher proportion of densely packed saturated fatty acids found in stressful conditions, including P oligotrophy, additionally increase the proportion of membrane phospholipids relative to intracellular lipids. Our findings contribute to the understanding of P cycling in the sea. In addition, our data suggest that higher phospholipid export to deep waters is possible by smaller plankton.

The future of life on Earth depends on how the ocean might change, as it plays an important role in mitigating the effects of global warming. The main role is played by phytoplankton. Not only are phytoplankton the base of the oceans' food web, but they also play an important role in the biological carbon pump (BCP), the process of forming organic matter (OM) and transporting it to the deep sea, representing a sink of atmospheric CO₂. Lipids are considered important vectors for carbon sequestration. A change in the phytoplankton community composition as a result of ocean warming is expected to affect the BCP. Many predictions indicate a dominance of small at the expense of large phytoplankton. To gain insight into interplay between the phytoplankton community structure, lipid production and degradation, and adverse environmental conditions, we analyzed phytoplankton composition, particulate organic carbon (POC) and its lipid fraction in the northern Adriatic over a period from winter to summer at seven stations with a gradient of trophic conditions. We found that at high salinity and low nutrient content, where nanophytoplankton prevailed over diatoms, the newly fixed carbon is substantially directed toward the synthesis of lipids. Lipids produced by nanophytoplankton, coccolithophores, and phytoflagellates, are more resistant to degradation than those produced by diatoms. The difference in lipid degradability is discussed as a difference in the size of the cell phycosphere. We hypothesize that the lipids of nanophytoplankton are less degradable due to the small phycosphere with a poorer bacterial community and consequently a lower lipid degradation rate compared with diatoms. The lipid chemical composition of the different phytoplankton groups could have a different susceptibility to degradation. Results suggest a successful lipid carbon sink of nanophytoplankton and, thus, a negative feedback on global warming.

Coccolithophores are typically thought of as photoautotrophs, yet a few genera inhabit sub-euphotic environments with insufficient light for photosynthesis, suggesting that other carbon acquisition strategies are likely. Field experiments were performed in the northwest Atlantic (a region with potentially abundant coccolithophores). Phytoplankton populations were incubated with ¹⁴ C-labeled dissolved organic carbon (DOC) compounds, acetate, mannitol, and glycerol. Coccolithophores were sorted from these populations 24 hours later using flow cytometry, and DOC uptake was measured. DOC uptake rates were as high as 10 ⁻¹⁵ moles cell ⁻¹ day ⁻¹ , slow relative to photosynthesis rates (10 ⁻¹² moles cell ⁻¹ day ⁻¹ ). Growth rates on the organic compounds were low, suggesting that osmotrophy plays more of a survival strategy in low-light situations. Assimilated DOC was found in both particulate organic carbon and calcite coccoliths (particulate inorganic carbon), suggesting that osmotrophic uptake of DOC into coccolithophore calcite is a small but notable part of the biological carbon pump and alkalinity pump paradigms.

The Mediterranean basin continuously receives anthropogenic aerosols from industrial and domestic activities from the European region, as well as high rates of aeolian material in the form of mineral dust from northern Africa. Moreover, combustion dominates over natural dust, whereas vegetation fires frequently burn throughout the Mediterranean coastal zone, especially during hot and dry summers. Once in the atmosphere, aerosols become an important external source of nutrients but also of toxins to the marine ecosystem through atmospheric deposition (AD), affecting the quality and quantity of organic matter (OM) produced by phytoplankton in the photic zone, and altering the CO2 uptake. AD onto sea surface cannot be completely understood without considering the interfacial processes within the sea surface microlayer (SML). As the uppermost millimeter of the sea surface, the SML represents the natural interface of the major environmental importance. It could serve as the first indicator of increasing human impact and climate change due to fast response of its biological and physico-chemical properties. However, surprisingly little research assessed the impact of AD on surface plankton communities, distinguishing between the SML and the water column bellow.This work is designed to assess the magnitude and temporal variability of atmospheric concentrations and deposition fluxes of nutrients and trace metals, and to gain insight into the AD impacts on the nature of enrichments of organic compounds within surface layers in a typical Mediterranean coastal environment. The field campaign was conducted during the period of retrieval of sea surface oligotrophic conditions (February-July 2019) at the Adriatic coastal area. On-line black carbon (BC) concentrations were measured while the aerosol particles (PM10), wet and total deposition samples as well as the SML and underlying water (ULW; 0.5 m depth) samples were collected simultaneously. The first comprehensive insight into concentration levels of macro nutrients (N, P) and trace metals (e.g. Cu, Pb, Cd, Ni, Zn, Co) in atmospheric samples, their transport history, source apportionment and deposition fluxes to the coastal Adriatic area will be presented. The temporal dynamics of SML biology as well as concentrations of inorganic and organic constituents enabled the assessment of their sources and the nature of the enrichments taking place within the SML. Due to their significance throughout the Mediterranean coastal area, open-fire episodes and Saharan dust inputs were especially considered. In order to better understand the impacts of ambient AD from diverse sources on the physiology and biomass of the natural plankton population and consequently on the chemistry of the surface layers (SML and ULW), we further conducted the first in situ bioassay incubation experiment of its kind at the Adriatic Sea. We experimentally examined the impact of locally collected anthropogenic aerosols, that had different levels of biologically important nutrients, trace metals and organic pollutants, in contrast to the material mimicking biomass burning events.Acknowledgment: This work has been supported by the Croatian Science Foundation under the IP-2018-01-3105 project: Biochemical responses of oligotrophic Adriatic surface ecosystems to atmospheric deposition inputs.