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Extracellular polysaccharide production by marine diatoms is a significant route by which photosynthetically produced organic carbon enters the trophic web and may influence the physical environment in the sea. This study highlights the capacity of atomic force microscopy (AFM) for investigating diatom extracellular polysaccharides with a subnanometer resolution. Here we address a ubiquitous marine diatom Cylindrotheca closterium, isolated from the northern Adriatic Sea, and its extracellular polymeric substance (EPS) at a single cell level. We applied a simple procedure for AFM imaging of diatom cells on mica under ambient conditions (in air) to achieve visualization of their EPS with molecular resolution. The EPS represents a web of polysaccharide fibrils with two types of cross-linking: fibrils association forming junction zones and fibril-globule interconnections with globules connecting two or more fibrils. The fibril heights were 0.4-2.6 nm while globules height was in the range of 3-12 nm. Polymer networks of native gel samples from the Northern Adriatic and the network formed by polysaccharides extracted from the C. closterium culture share the same features regarding the fibril heights, pore openings and the mode of fibril association, proving that the macroscopic gel phase in the Northern Adriatic can be formed directly by the self-assembly of diatom released polysaccharide fibrils.

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Mass appearances of the toxic dinoflagellate genus Ostreopsis are known to cause dangerous respiratory symptoms in humans exposed to aerosols. The outbreaks can appear in shallow marine waters of temperate regions around the globe. We followed a massive bloom event on a public beach on the northern Adriatic coast near Rovinj, Croatia. We identified the responsible species and the produced toxins as well as the dynamics of the event with respect to environmental conditions. Ostreopsis cf. ovata appeared in masses from September through October 2010 on a public beach near Rovinj, Croatia but stayed undetected by public health organizations. Respiratory symptoms were observed whenever humans were exposed to substrate samples containing large numbers of Ostreopsis cells. During the mass abundance of O. cf. ovata also exposure to the aerosols on the beach evoked respiratory symptoms in humans. Our measurements showed high cell abundances and high toxin contents with a stable relative contribution of putative Palytoxin and Ovatoxins a-e. Artificial beach structures proved to dramatically reduce settling of the observed Ostreopsis biofilm. Blooms like those reported herein have a high potential to happen undetected with a high potential of affecting the health of coastal human populations. Increased monitoring efforts are therefore required to understand the ecology and toxicology of those bloom events and reduce their negative impact on coastal populations.

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The northern Adriatic (NA) is a favorable basin for studying the adaptive strategies of plankton to a variety of conditions along the steep gradients of environmental parameters over the year. Earlier studies identified phosphorus (P)-limitation as one of the key stresses within the NA that shape the biological response in terms of biodiversity and metabolic adjustments. A wide range of reports supports the notion that P-limitation is a globally important phenomenon in aquatic ecosystems. In this study P stress of marine microphytoplankton was determined at species level along a trophic gradient in the NA. In P-limitation all species with considerable contributions to the diatom community expressed alkaline phosphatase activity (APA), compared to only a few marginal dinoflagellate species. Nevertheless, APA expressing species did not always dominate the phytoplankton community, suggesting that APA is also an important strategy for species to survive and maintain active metabolism outside of their mass abundances. A symbiotic relationship could be supposed for diatoms that did not express APA themselves and probably benefited from APA expressed by attached bacteria. APA was not expressed by any microphytoplankton species during the autumn when P was not limiting, while most of the species did express APA during the P-limitation. This suggests that APA expression is regulated by orthophosphate availability. The methods employed in this study allowed the microscopic detection of APA for each microphytoplankton cell with simultaneous morphologic/taxonomic analysis. This approach uncovered a set of strategies to compete in P-limited conditions within the marine microphytoplankton community. This study confirms the role of P-limitation as a shaping factor in marine ecosystems.

Summary The evolutionary and ecological story of coccolithophores poses questions about their heterotrophy, surviving darkness after the end‐Cretaceous asteroid impact as well as survival in the deep ocean twilight zone. Uptake of dissolved organic carbon might be an alternative nutritional strategy for supply of energy and carbon molecules. Using long‐term batch culture experiments, we examined coccolithophore growth and maintenance on organic compounds in darkness. Radiolabelled experiments were performed to study the uptake kinetics. Pulse–chase experiments were used to examine the uptake into unassimilated, exchangeable pools vs assimilated, nonexchangeable pools. We found that coccolithophores were able to survive and maintain their metabolism for up to 30 d in darkness, accomplishing about one cell division. The concentration dependence for uptake was similar to the concentration dependence for growth in Cruciplacolithus neohelis, suggesting that it was taking up carbon compounds and immediately incorporating them into biomass. We recorded net incorporation of radioactivity into the particulate inorganic fraction. We conclude that osmotrophy provides nutritional flexibility and supports long‐term survival in light intensities well below threshold for photosynthesis. The incorporation of dissolved organic matter into particulate inorganic carbon, raises fundamental questions about the role of the alkalinity pump and the alkalinity balance in the sea.

Osmotrophy is one of the main modes of mixotrophic acquisition of carbon by phytoplankton, but historically it has been under‐investigated and its physiological and ecological relevance remains poorly understood. Here, we investigate osmotrophy in coccolithophores. Coccolithophores are one of the major contributors to the ocean biomass inhabiting both euphotic and subeuphotic depths in the marine environment. Coccolithophores demonstrate the potential to utilize a wide array of organic compounds in darkness. In experiments with BioLog Ecoplates, we screened a wide array of organic compounds as potential carbon sources, and observed that the major types of organic compounds taken up by coccolithophores were primarily carbohydrates along with a few amino acids and polymers. Furthermore, in subsequent radiotracer experiments, the uptake rates of 14C‐labeled dissolved organic carbon compounds in the dark were low relative to the maximal rates of photosynthetic carbon fixation in the light. The time course of uptake for some compounds suggests constitutive capacity for their transport, while for others the transport appears to be activated. Nonetheless, the collective slow uptake rate of a large array of organic compounds found in seawater, might be the only way that osmotrophy could fuel significant coccolithophore growth in the deep euphotic and subeuphotic zones in the sea.