Lipids of different phytoplankton groups differ in sensitivity to degradation: Implications for carbon export.

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.

Osmotrophy of dissolved organic compounds by coccolithophore populations: Fixation into particulate organic and inorganic carbon.

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 14C-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.

Effects of high temperature and nitrogen availability on the growth and composition of the marine diatom Chaetoceros pseudocurvisetus.

The assimilation of inorganic nutrients by phytoplankton strongly depends on environmental conditions such as the availability of nitrogen and temperature, especially warming. The acclimation or adaptation of different species to such changes remains poorly understood. Here, we used a multimethod approach to study the viability and physiological and biochemical responses of the marine diatom Chaetoceros pseudocurvisetus to different temperatures (15, 25, and 30 °C) and different N:P ratios. Nitrogen limitation had a greater effect than high temperature on cell growth and reproduction, leading to a marked elongation of setae, decreased phosphorus assimilation, increased lipid accumulation, and decreased protein synthesis. The elongation of setae observed under these conditions may serve to increase the surface area available for the uptake of inorganic and/or organic nitrogen. In contrast, high temperatures (30 °C) had a stronger effect than nitrogen deficiency on cell death, nitrogen assimilation, chlorophyll a accumulation, the cessation of setae formation, and cell lipid remodelling. Significant changes in thylakoid lipids were observed in cells maintained at 30 °C, with increased levels of digalactosyldiacylglycerol and sulfoquinovosyldiacylglycerol. These changes may be explained by the role of galactolipids in thylakoid membrane stabilization during heat stress.

Osmotrophy of dissolved organic carbon by coccolithophores in darkness.

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.

Global warming and oligotrophication lead to increased lipid production in marine phytoplankton.

Earth temperature is rising and oligotrophication is becoming apparent even in coastal seas. In this changing environment, phytoplankton use carbon and nutrients to form important biomolecules, including lipids. However, the link between lipid production and changing environment is still unexplored. Therefore, we investigated the phytoplankton lipid production in the diatom Chaetoceros pseudocurvisetus cultures under controlled temperatures ranging from 10 to 30 °C and nutrient regimes mimicking oligotrophic and eutrophic conditions. Results were compared to plankton community's lipid production in the northern Adriatic at two stations considered as oligotrophic and mesotrophic during an annual monthly sampling. In order to gain detailed information on the investigated system, we supplemented lipid data with chlorophyll a concentrations, phytoplankton taxonomy, cell abundances and nutrient concentration along with hydrographic parameters. We found enhanced particulate lipid production at higher temperatures, and substantially higher lipid production in oligotrophic conditions. Enhanced lipid production has two opposing roles in carbon sequestration; it can act as a retainer or a sinker. Lipid remodeling, including change in ratio of phospholipids and glycolipids, is more affected by the nutrient status, than the temperature increase. Triacylglycerol accumulation was observed under the nitrogen starvation.

The Ecology of One Cosmopolitan, One Newly Introduced and One Occasionally Advected Species from the Genus Skeletonema in a Highly Structured Ecosystem, the Northern Adriatic.

The diatom genus Skeletonema is globally distributed and often an important constituent of the phytoplankton community. In the marine phytoplankton of the northern Adriatic Sea, we found three species of the genus Skeletonema: Skeletonema menzelii, Skeletonema marinoi and Skeletonema grevillei. Making use of the steep ecological gradients that characterise the northern Adriatic, along which we could observe those species, we report here on the ecological circumstances under which those species thrive and how their respective populations are globally connected. This is the first detailed ecological study for the species S. grevillei. This study is also the first report for S. grevillei for the Adriatic Sea and Mediterranean together with additional electron microscopic details on fresh in situ samples for this species. S. marinoi appears to clearly prefer strong freshwater influence and high nutrient concentrations delivered by low salinity waters. It can outcompete other diatom species and dominate microphytoplankton blooms. S. grevillei on the other hand appears to thrive in high nutrient concentrations triggered by water column mixing. It also appears to prefer higher salinity waters and coastal embayments. Genetic analysis of S. grevillei demonstrated a peculiar dissimilarity with isolates from coastal waters off Yemen, India, Oman and China. However, a closely related sequence was isolated from coastal waters off Japan. These results indicate that S. grevillei is an introduced species, possibly transported by ballast waters. S. menzelii is a sporadic visitor in the northern Adriatic, advected from rather oligotrophic middle Adriatic waters and never dominates the phytoplankton community in the northern Adriatic.

Adaptation of marine plankton to environmental stress by glycolipid accumulation.

A systematic investigation of non-phosphorus containing glycolipids (GL) was conducted in the northern Adriatic Sea during two years at two stations with different nutrient loads. GL concentration varied both spatially and temporally, with values of 1.1-21.5 µg/L and 0.4-44.7 µg/L in the particulate and the dissolved fraction, respectively. The highest concentrations were measured during summer in surface waters and at the more oligotrophic station, where GL yields (% of total lipids) were often higher than 20% and 50% in the particulate and dissolved fractions, respectively. To obtain more insight into factors governing GL accumulation autotrophic plankton community structure (pico-, nano- and microplankton fractions), chlorophyll a, heterotrophic bacteria and nutrient concentrations were measured together with hydrographic parameters and sunlight intensity. During the investigated period smaller autotrophic plankton cells (pico- and followed by nanoplankton) prevailed in abundance over larger cells (microplankton), which were found in large numbers in freshened surface samples. Several major findings resulted from the study. Firstly, during PO4 limitation, particularly at the oligotrophic station, enhanced glycolipid instead of phospholipid accumulation takes place, representing an effective phosphate-conserving mechanism. Secondly, results suggest that at seawater temperatures >19 °C autotrophic plankton considerably accumulate GL, probably to achieve thermal stability. Thirdly, high sunlight intensities seem to influence increased GL accumulation; GL possibly plays a role in cell mechanisms that prevent/mitigate photooxidation. And finally, substantial accumulation of GL detected in the dissolved fraction could be related to the fact that GL do not contain biologically relevant elements, like phosphorus, which makes them an unattractive substrate for enzyme activity. Therefore, substantial portion of CO2 could be removed from the atmosphere in P-limited regions during summer via its capture by plankton and conversion to GL.

Toxin-producing Ostreopsis cf. ovata are likely to bloom undetected along coastal areas.

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.

AFM imaging of extracellular polymer release by marine diatom Cylindrotheca closterium (Ehrenberg) Reiman & J.C. Lewin.

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.