The role of the diatom Cylindrotheca closterium in a mucilage event in the northern Adriatic Sea: coupling with high salinity water intrusions

The temporal and spatial distribution of mucilaginous aggregates,during the mucilage event of 2002 in the northern Adriatic,was studied in relation to waters of different salinity consideringthe aggregate’s age and proportion of Cylindrotheca closteriumin its microphytoplankton community. The results showed thatthis epipelic diatom is a dominating species in the microphytoplanktoncommunity of freshly formed aggregates, in spite of its negligibleabundance in the water column. The appearance and distributionof these aggregates in the water column was associated withwater of higher salinity. The same pattern was observed in thesummer of 2001, when a shorter and limited mucilage event occurred.These results suggest the importance of oligotrophic high salinitywater intrusions from the central Adriatic for a selective accumulationof C. closterium cells. Coupling between the increased numberand minimum growth state of C. closterium-accumulated cellsmight result in a hyperproduction of its own mucilage in theseaccumulation zones formed under the influence of high salinitywater intrusions. In contrast, in the upper lower salinity waterlayers, under the influence of a freshwater input, the phytoplanktoncommunity of the freshly formed aggregates was similar to thatof the surrounding water.     

Organic aggregates formed by benthopleustophyte brown alga Acinetospora crinita (Acinetosporaceae,Ectocarpales)

This work presents the elemental, polysaccharide, and fatty acid compositions of benthic aggregates formed by the filamentous brown alga Acinetospora crinita, which are widely spread on the rocky bottoms of the Mediterranean Sea. The aggregates can be characterized as mineralized centers in which regeneration of nutrients and recycling of dissolved organic matter actively occur and favor the development of an abundant phytoplankton community. Analyses of the stable isotopes of C and N display their marine origin and could provide evidence of the processes that occur inside/outside of the aggregates. The monosaccharide compositions of Adriatic and Tyrrhenian mucilages produced by brown alga A. crinita were quite similar. In particular, the Adriatic sample compositions resembled the average composition of the Tyrrhenian high molecular weight exopolymers, and the observed differences could be ascribed to different degradation stages. The fatty acid patterns found for the aggregates were similar to those observed in the isolated A. crinita algae with variable contributions from embedded diatom species. The bacterial contribution to the fatty acid pool was quite low, most likely due to the known poor conditions for their heterotrophic growth.     

Microbial communities in northern Adriatic mucilaginous aggregates: insight into the early phase of aggregate formation

Insight into the initial phase of aggregate formation was provided by comparison between bacterial communities from freshly formed aggregates dominated by the epipelic diatom Cylindrotheca closterium and associated water masses. This study was performed from 2000 to 2006 in the northern Adriatic. The chemotaxonomic structures and physiological conditions were inferred from the fatty acid profiles of the cultured bacterial communities of all implicated components, fresh aggregates, their adjacent waters, oligotrophic high-salinity waters and halocline waters. The results showed similarity between bacterial communities of fresh aggregates and oligotrophic high-salinity water, suggesting their common origin and involvement in the formation of aggregates. In contrast, the origin of the water adjacent to aggregates was different from that of the other components but was similar to the halocline layer and was likely derived from northern Adriatic waters. The presence and activity of heterotrophic bacteria belonging to Alteromonadaceae, which are regularly observed on fresh aggregates, suggest that the early phase of aggregate formation corresponds to an abrupt change of environmental conditions due to the mixing of central and northern Adriatic waters. The initial colonisation of fresh mucilage ascribes to Alteromonas an important ecological function in aggregate community development related to the succession of phytoplankton and heterotrophic bacteria.     

Changes in Na+/K+-ATPase activity, unsaturated fatty acids and metallothioneins in gills of the shore crab Carcinus aestuarii after dilute seawater acclimation

We have assessed the activity of Na⁺/K⁺-ATPase, cAMP, free fatty acids (FFA) and metallothionein (MT) in the posterior gills of the brackish water shore crab Carcinus aestuarii during acclimation to 10 ppt dilute seawater (DSW). Following 3–18 days acclimation in DSW specific activity of Na⁺/K⁺-ATPase in native gill homogenates and partially purified membrane vesicles was progressively increased, from 1.7- to 3.9-fold. After short-term acclimation of crabs in DSW with added sucrose to make media isosmotic with the haemolymph the specific Na⁺/K⁺-ATPase activity in homogenates was not increased, relative to SW enzyme activity. Moreover, hyposmotic conditions led to depletion of cAMP in gills.In partially purified membrane vesicles isolated from posterior gills, fatty acids with compositions 16:0, 18:0, 18:1, 20:4 and 20:5 dominated in both SW- and DSW-acclimated Carcinus. During a year in which the metabolic activity of crabs was increased, the arachidonic/linoleic acids ratio (ARA/LA) for DSW-acclimated crabs was markedly increased relative to that in SW. Increased Na⁺ + K⁺-ATPase activity under hyposmotic stress may be modulated at least partially by the changed proportion of fatty acids in the purified membranes of posterior gills. Long-term acclimation of shore crabs to DSW resulted in a 2.6-fold increase in cytosolic metallothionein (MT) content in posterior gills over those in SW crabs. Assuming an antioxidant role of MT associated with intracellular zinc partitioning, the observed MT induction in posterior gills may be considered an adaptive response of C. aestuarii to hyposmotic stress.     

Fatty acid and phytoplankton compositions of different types of mucilaginous aggregates in thenorthern Adriatic

The biomarkers (fatty acid proportions and ratios characteristicof phytoplankton and bacteria) and phytoplankton species insmall (0.5–2.0 cm) and large (0.5–5 m) marine aggregateswere determined in samples collected in the northern AdriaticSea, during a mucilage event in 1997, as well as in 1993, 1994and 1998, when events were not observed. Types of aggregateswere identified according to various biomarker relationships,particularly those related to bacterial and phytoplankton activitiesand changes in the diatom species composition. Aged mucilaginousaggregates (in summer 1997) showed fatty acid proportions (16P/18P,3.9–7.7) characteristic of the highest phytoplankton activities,and also showed the highest bacterial fatty acid proportions(13.3–17.1%) and ratios (C15:br/C15:0, 4.4–6.0).They showed an different diatom community (dominated by Cylindrothecaclosterium) from that in surrounding waters. These characteristicssuggest a continuous renewal of the aggregate organic matter,supporting the hypothesis that aggregate is a ‘selfsustaining’community. In contrast, both freshly formed marine snow, dominantduring periods without mucilage events, and freshly formed mucilaginousaggregates presented biomarker proportions and ratios similarto those of suspended matter (bacterial fatty acids 3–6.7%,C15:br/C15:0 1.2–4.6, 16P/180.4–4.0), and diatomcomposition similar to that of the ambient water. This indicatesthat marine snow sinks more rapidly than large aggregates, beforesignificant changes can occur in its plankton composition. Otheraging aggregate types showed intermediate characteristics, suggestingthat their residence times in the water column were still sufficientto develop organic production–decomposition cycles thatmodified to various extents their biochemical composition.     

Characterisation and differentiation of oligotrophic waters by culturable particle-attached and free-living bacterial communities

This study reports the development of a tool to characterise and differentiate northern Adriatic waters, particularly oligotrophic, high-salinity waters, based on the cellular fatty acids of culturable heterotrophic bacterioplankton. The growth abilities and population dominance were observed for particle-attached and free-living bacteria cultured in three types of media: Marine Broth, diluted Marine Broth (1:10) and R2 broth. Three groups of water layers were distinguished by hierarchical clustering analysis: eutrophic, oligotrophic and oligotrophic nutrient-selected. Significant differences between the resulting groups were tested by two-way ANOVA (with replication). Eutrophic layers were characterised by readily culturable particle-attached and free-living fractions of the bacterial community in all three media, all dominated by fast-growing γ-Proteobacteria. In contrast, oligotrophic water layers with low productivity had a much weaker culturability and a different population dominance for the free-living community, as compared to their attached or growth-arrested counterparts, for all media. The free-living bacteria from strictly oligotrophic environments demonstrated minimum culturability in Marine Broth, while those from selective oligotrophic environments were culturable and were dominated either by Cytophaga–Flavobacter complex, α-Proteobacteria or γ-Proteobacteria. The conclusive evidence regarding the selective and refractory nature of organic compounds in these waters demonstrates the dominant culturability of the Cytophaga–Flavobacter complex and α-Proteobacteria in free-living communities in all growth-media. The response of fatty acid dominance ratios depends significantly on the trophic state and fraction (p < 0.05), although the effect of the trophic state is completely different in attached and free fractions. Both fractions were tested separately, demonstrating a significant influence of the trophic state (p < 0.05), while the effect of the media on the fatty acid response was not significant (p > 0.05). An interaction between media and trophic status was present in the attached fraction (p < 0.05), yet this was not observed in the free fraction (p > 0.05), indicating that any systematic difference between trophic states was the same for each media tested. Accordingly, the free-living fraction of bacterioplankton is a more informative attribute and can be used solely as an indicator of the water layer trophic condition.