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.

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.

Oceanographic characteristics of the Adriatic Sea - Support to secondary HAOP spread through natural dispersal.

Vessels, specifically ballast water and hull fouling, are a major vector for the introduction of non-indigenous species (NIS) in European seas. The Mediterranean is one of the world's marine regions where their invasion is heaviest. The shallow Adriatic basin is a highly sensitive area that is already experiencing its consequences. The secondary spread of NIS over a wider area through natural dispersion is a complex process that depends on a wide range of oceanographic factors. This work analysed the dataset of the BALMAS project, in whose framework twelve ports in the Adriatic Sea were subjected to a Port Baseline Survey (PBS), to estimate the natural spread of NIS organisms from their port of arrival to the wider Adriatic basin. Its findings indicate that the prevailing water circulation patterns facilitate the natural dispersal of harmful aquatic organisms and pathogens (HAOP).

Seasonal variations in extracellular enzymatic activity in marine snow-associated microbial communities and their impact on the surrounding water.

Seasonal changes of microbial abundance and associated extracellular enzymatic activity in marine snow and in seawater were studied in the northern Adriatic during a three-year period. Marine snow was present during the entire period of investigation, although in higher concentrations during summer than during winter. Microorganisms densely colonized marine snow and aggregate-associated enzymatic activity was substantially higher (up to 105 times) than in seawater. Alkaline phosphatase activity (APA) and aminopeptidase activity in marine snow showed seasonal variations with higher values in late spring-summer than in autumn-winter, probably in response to changes in the quantity and quality of organic matter. The highest cell-specific bacterial activity was found for phosphatase, followed by peptidase, and the lowest was for glucosidases. Differential hydrolysis of marine snow-derived organic matter points to the well-known phosphorus limitation of the northern Adriatic and indicates preferential utilization of phosphorus- and nitrogen-rich organic compounds by microbes, while hydrolysis of polysaccharides seemed to be less important. In oligotrophic conditions during summer, organic matter released from marine snow might represent a significant source of substrate for free-living bacteria in seawater. For the first time microorganisms producing APA in marine snow were identified, revealing that dense populations of bacteria expressed APA, while cyanobacteria did not. Cyanobacteria proliferating in marine snow could benefit from phosphorus release by bacteria and nanoflagellates.

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.

Long-term fluctuations in Cystoseira populations along the west Istrian Coast (Croatia) related to eutrophication patterns in the northern Adriatic Sea.

An exploration of historical data suggested that eutrophication patterns might drive long-term fluctuations in Cystoseira populations along the west Istrian Coast (northern Adriatic Sea, Croatia). The regimes of northern Italian rivers, which flow approximately 100km west of the study area, mainly modulate the eutrophication levels of the northern Adriatic Sea. A regression of Cystoseira populations from the 1970s through the 1990s corresponded to increased levels of eutrophication in the study area. During the late 1990s, the density of sea urchins, which are efficacious macroalgal predators, decreased, likely due to an intense formation of pelagic mucilage aggregates that resulted in mass mortality episodes of macrozoobenthic species. During the 2000-2013 period, an oligotrophication of the northern Adriatic formed the basis for the recovery of Cystoseira taxa, whose abundances from 2009 to 2013 were similar to those characterising the most flourishing Mediterranean Cystoseira assemblages.

Spreading patterns of the invasive Caulerpa cylindracea Sonder along the west Istrian Coast (northern Adriatic Sea, Croatia).

The northern Adriatic Sea represents the northernmost and thus the coldest biogeographic sector of the Mediterranean Sea. In 2004, the invasive green alga Caulerpa cylindracea was recorded for the first time in the northern Adriatic at a site of the west Istrian Coast. Until 2010, additional C. cylindracea mats have only formed up to 7 km northward from the first colonisation site. Subsequently, the alga was also recorded at sites widespread along the entire coast. Both the first 2004 colonisation event and the 2011-2014 colonisation of distant sites occurred during periods of winter seawater temperatures higher than 9 °C. In general, algal spreading was markedly slow. Approximately 10 years after the first record, C. cylindracea has affected less than 1% of the entire west Istrian coastline. The colonisation predominantly occurred in ports and urbanised bays (seaside resorts) suggesting that anthropogenic activities might enhance algal diffusion.

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.

Meteorological and oceanographic conditions in the northern Adriatic Sea during the period June 1999-July 2002: influence on the mucilage phenomenon.

Mucilage events (formation of very large organic aggregates and gelatinous surface layers) have been documented several times during the past two centuries in the northern Adriatic Sea (NA), while their frequency has significantly increased since 1988. In this work, meteorological and oceanographic conditions in the NA during the period June 1999-July 2002 are described and their relation to the outbreak and fate of the mucilage phenomenon was investigated. Salinity and temperature data were collected during approximately monthly cruises along three transects in the NA. Relevant meteorological situations (air temperature, rainfall, wind) were selected from large-scale ECMWF analyses and from the Local Analysis and Prediction System (LAPS; Emilia Romagna Meteorological Service), while sea conditions (waves) were analysed by means of the Wave Adriatic Model (WAM). Data for air temperature, rainfall, and wind from several meteorological stations in the region were used. Average seasonal cycles of sea temperature and salinity simulated with statistical models, based on historical data collected in the NA since 1972, were used to determine thermal and haline anomalies. The monthly anomaly variability of maximum and minimum air temperatures, rainfall amount and number of rainy days did not appear to be relevant for the mucilage phenomenon outbreak. In contrast, both vertical and horizontal thermohaline gradients in the region were more developed during late spring and summer of 2000 and particularly of 2002, when the mucilage events were of greatest extent in space and time, compared to 2001 (short-lived event) and 1999 (no event). These more pronounced gradients were due to a combination of several unusual conditions: sharp heating of the sea surface in May-June, domination of eastwards transport of freshened waters formed in the Po Delta area, and intrusion of very high salinity intermediate waters originating in the eastern Mediterranean. Moreover, in winter of both 2000 and 2002 very dense and cold water formed and remained in the bottom layer until spring, contributing to increase the stratification degree of the water column. The duration of the mucilage events and their spatial distribution in the region depend strongly on meteorological changes. Recurrent anticyclonic conditions, characterized by low wind and calm sea, favour extended events in time (up 2 months in 2002). In contrast, highly perturbed weather, particularly due to strong "bora" wind, can be determined in sharp decay of the event (e.g. in July 2000).

Changes in nutrient concentrations and ratios during mucilage events in the period 1999-2002.

Nutrient and chlorophyll a concentrations and salinity were measured, approximately monthly, from June 1999 to July 2002 at 20 stations along three transects in the northern Adriatic Sea, north of the line Susak Island-Senigallia, with the aim of confirming or rejecting the hypothesis that changes in nutrient ratios may have an important role in the mucilage phenomenon. The data analyses were focused on the two major water types identified in the region: lower salinity (32-37) and oxygenated surface waters (type 1) in which the mucilage phenomenon primarily developed, and high salinity water originating from other parts of the Adriatic (type 4). Marked variability of dissolved inorganic nitrogen (DIN) in type 1 waters was roughly correlated with extreme fluctuations of the Po River flow rate during the investigated period. In contrast, the orthophosphate (PO4) concentration was primarily controlled by phytoplankton assimilation. The nutrient discharges and DIN/PO4 ratios (median 120) in the freshened surface layer were much higher and more variable in the period before the mucilage event in 2001 than in 2000 (median 75), and particularly in 2002 (median 30), although intensity and duration of the 2001 event were the lowest. However, in that period of 2000 and 2002 significant transversal transport of freshened waters occurred, despite the unusually low Po flow rates. In summer, in conditions of low freshwater discharge and the prevailing of semi-enclosed circulation in the region, more efficient DIN assimilation by phytoplankton occurred, probably due to a faster recycling of PO4. However, in 2002 this process appeared to have already started in March. Changes of the orthosilicate (SiO4)/DIN ratio were mainly dependent on DIN concentrations. In the more saline waters (type 4) the nutrient concentrations, particularly DIN, were much lower and no significant relationships were noticed among the studied parameters. Nutrient concentration and ratio changes do not trigger mucilage events, although very probably they have an essential role in combination with several other physical (pulsing freshwater discharge, marked stratification, minimal water exchange) and biological (e.g., increased plankton excretion, limited bacterial degradation) factors.

Circulation and horizontal fluxes in the northern Adriatic Sea in the period June 1999-July 2002. Part II: nutrients transport.

Nutrient concentrations (orthophosphate, orthosilicate and dissolved inorganic nitrogen-DIN) were measured on three transects in the northern Adriatic approximately monthly in the period June 1999-July 2002. The net nutrient transports across the three transects were estimated from these concentration values and calculated water fluxes [Grilli F, Paschini E, Russo A, Precali R, Supic N. Circulation and horizontal fluxes in the Northern Adriatic Sea in the period June 1999_July 2002. Part I: geostrophic circulation and current measurement. Sci Total Environ 2005-this issue], with the aim to verify their possible role in the mucilage phenomenon. The nutrient transports in the northern Adriatic were very variable, seasonally and from year to year, both in intensity and direction. Some differences were noticed also among the nutrient species. At the northernmost transect Po Delta-Rovinj minimal values and variable directions were recorded, probably due to the prevailing of eastward transversal transports, observed already in late winter, and generally dominant in spring and summer. Northward transport was often measured at the central transect Cesenatico-Cape Kamenjak, particularly in spring 2000 and 2002, and summer 2001, as well as for only DIN in summer 1999. In contrast, southward directions prevailed at the southernmost transect Senigallia-Susak Island. In October significant southward transports occurred at all transects and for all nutrients, particularly strong in 2000, when exceptionally high Po River discharges occurred, and the Western Adriatic Current (WAC) was well developed. However, the nutrient transports in the upper water column were low or northwards at both transect Cesenatico-Cape Kamenjak and Senigallia-Susak Island in June and July 2000, 2001, and 2002, when the mucilage phenomenon developed, but were opposite in 1999, a year without events. The results suggest that high variations of nutrient fluxes and their ratios, in conditions of reduced water dynamics, are essential for the development of the phenomenon, rather than the absolute amounts of the nutrient inputs.