In northern Greece, European ground squirrels or sousliks (Spermophilus citellus) construct complex burrow systems by scratch-digging behavior. The present study investigated the presence of ...anatomical characters related to digging in the forelimb of S. citellus. The forelimb of 3 preserved specimens was dissected and several qualitative and quantitative variables on selected muscles were collected. In addition, selected osteological variables and indices were calculated in a sample of 207 sciurid postcrania representing 14 burrowing and nonburrowing genera. Both analyses showed that the forelimb of S. citellus was characterized by enlarged and powerful shoulder retractors, well-developed arm retractors with distal insertions upon a relatively robust humerus, enlarged elbow extensors associated with a long olecranon, and dominant pronators and carpal and digital flexors. Similar morphology is also encountered in other semifossorial mammals, indicating significant adaptations to scratch-digging behavior. However, the characters examined designate a more compromised morphology, associated with the generalized postcranium of sciurids. On the other hand, S. citellus exhibits a more specialized forelimb morphology, compared to that of other marmotines, for a semifossorial way of life, in association with the subgeneric derived morphology, lack of social interactions, and exploitation of a habitat with harder soils.
Recent estimates of nutrient budgets for the Eastern Mediterranean Sea (EMS) indicate that atmospheric aerosols play a significant role as suppliers of macro- and micro- nutrients to its Low Nutrient ...Low Chlorophyll water. Here we present the first mesocosm experimental study that examines the overall response of the oligotrophic EMS surface mixed layer (Cretan Sea, May 2012) to two different types of natural aerosol additions, “pure” Saharan dust (SD, 1.6 mg l-1) and mixed aerosols (A - polluted and desert origin, 1 mg l-1). We describe the rationale, the experimental set-up, the chemical characteristics of the ambient water and aerosols and the relative maximal biological impacts that resulted from the added aerosols. The two treatments, run in triplicates (3 m3 each), were compared to control-unamended runs. Leaching of approximately 2.1-2.8 and 2.2-3.7 nmol PO4 and 20-26 and 53-55 nmol NOx was measured per each milligram of SD and A, respectively, representing an addition of approximately 30% of the ambient phosphate concentrations. The nitrate/phosphate ratios added in the A treatment were twice than those added in the SD treatment. Both types of dry aerosols triggered a positive change (25-600% normalized per 1 mg l-1 addition) in most of the rate and state variables that were measured: bacterial abundance (BA), bacterial production (BP), Synechococcus (Syn) abundance, chlorophyll-a (chl-a), primary production (PP) and dinitrogen fixation (N2-fix), with relative changes among them following the sequence BP>PP≈N2-fix>chl-a≈BA≈Syn. Our results show that the ‘polluted’ aerosols triggered a relatively larger biological change compared to the SD amendments (per a similar amount of mass addition), especially regarding BP and PP. We speculate that despite the co-limitation of P and N in the EMS, the additional N released by the A treatment may have triggered the relatively larger response in most of the rate and state variables as compared to SD. An implication of our study is that a warmer atmosphere in the future may increase dust emissions and influence the intensity and length of the already well stratified water column in the EMS and hence the impact of the aerosols as a significant external source of new nutrients.
The effect of episodicity of Saharan dust deposition on the pelagic microbial food web was studied in the oligotrophic Eastern Mediterranean by means of a mesocosm experiment in May 2014. Two ...different treatments in triplicates (addition of natural Saharan dust in a single-strong pulse or in three smaller consecutive doses of the same total quantity), and three unamended controls were employed; chemical and biological parameters were measured during a 10-day experiment. Temporal changes in primary (PP) and bacterial (BP) production, chlorophyll a (Chla) concentration and heterotrophic bacteria, Synechococcus and mesozooplankton abundance were studied. The results suggested that the auto- and hetero-trophic components of the food web (at least the prokaryotes) were enhanced by the dust addition (and by the nitrogen and phosphorus added through dust). Furthermore, a 1-day delay was observed for PP, BP and Chla increases when dust was added in three daily doses; however, the maximal values attained were similar in the two treatments. Although the effect was evident in the first osmotrophic level (phytoplankton and bacteria), it was lost further up the food web, masked under the impact of grazing exerted by predators such as heterotrophic flagellates, ciliates and dinoflagellates. This was partly proved by two dilution experiments. This study demonstrates the important role of atmospheric deposition and protist grazing when evaluating the effect on oligotrophic systems characterised by increased numbers of trophic levels.
Large amounts of dust and atmospheric aerosols, originating from surrounding desert areas (e.g., Sahara and Middle East) are deposited annually on the surface of the Eastern Mediterranean Sea. These ...depositions can provide high amounts of micro (such as Fe, Zn, Co) and macro nutrients (such as P and N) to supplement nutrient-poor surface waters- that typically limit primary productivity and also dinitrogen (N2) fixation in many marine environments. Here, we studied the impact of the atmospheric deposition of dust and aerosols on N2 fixation in the Cretan Sea (Eastern Mediterranean Sea). Mixed polluted aerosols (hereafter A) and Saharan dust (hereafter SD) were added to nine mesocosms (3-m3 each) containing surface mixed layer seawater (~10 m), and N2 fixation was evaluated for 6 days during May 2012 (springtime). The addition of SD triggered a rapid (30 h) and robust (2-4 fold) increase in N2 fixation rates that remained high for 6 days and contributed 3-8% of the primary productivity. The A addition also resulted in higher N2 fixation rates compared to the unamended control mesocosms, although the responses were less profound (1.5-2 fold) and accounted for only 2-4% of the primary productivity. The microbial community responded differently to the two additions. Heterotrophic bacterial N2 fixers dominated the diazotroph community in A and the control mesocosms, while the non-filamentous cyanobacterial group Trichodesmium prevailed in the SD treatment (68% of all the operational taxonomic units, verified by qPCR analyses). Our results indicate that the aerosol source, its route prior to deposition, and its specific chemical composition, can alter the diazotrophic diversity and activity in the Eastern Mediterranean Sea and may thus impact both the N and C dynamics in this impoverished environment.
The effects of atmospheric deposition on plankton community structure were examined during a mesocosm experiment using water from the Cretan Sea (Eastern Mediterranean), an area with a high frequency ...of atmospheric aerosol deposition events. The experiment was carried out under spring-summer conditions (May 2012). The main objective was to study the changes induced from a single deposition event, on the autotrophic and heterotrophic surface microbial populations, from viruses to zooplankton. To this end, the effects of Saharan dust addition were compared to the effects of mixed aerosol deposition on the plankton community over 9 days. The effects of the dust addition seemed to propagate throughout the food-web, with changes observed in nearly all of the measured parameters up to copepods. The dust input stimulated increased productivity, both bacterial and primary. Picoplankton, both autotrophic and heterotrophic capitalized on the changes in nutrient availability and microzooplankton abundance also increased due to increased availability of prey. Five days after the simulated deposition, copepods also responded, with an increase in egg production. The results suggest that nutrients were transported up the food web through autotrophs that were favored by the Nitrogen supplied through both treatments. Although the effects of individual events are generally short lived, increased deposition frequency and magnitude of events is expected in the area, due to predicted reduction in rainfall and increase in temperature, which can lead to more persistent changes in plankton community structure. Here we demonstrate how a single dust deposition event leads to enhancement of phytoplankton and microzooplankton and can eventually, through copepods, transport more nutrients up the food web in the Eastern Mediterranean Sea.
Acid processes in the atmosphere, particularly those caused by anthropogenic acid gases, increase the amount of bioavailable P in dust and hence are predicted to increase microbial biomass and ...primary productivity when supplied to oceanic surface waters. This is likely to be particularly important in the Eastern Mediterranean Sea (EMS), which is P limited during the winter bloom and N&P co-limited for phytoplankton in summer. However, it is not clear how the acid processes acting on Saharan dust will affect the microbial biomass and primary productivity in the EMS. Here, we carried out bioassay manipulations on EMS surface water on which Saharan dust was added as dust (Z), acid treated dust (ZA), dust plus excess N (ZN), and acid treated dust with excess N (ZNA) during springtime (May 2012) and measured bacterioplankton biomass, metabolic, and other relevant chemical and biological parameters. We show that acid treatment of Saharan dust increased the amount of bioavailable P supplied by a factor of ~40 compared to non-acidified dust (18.4 vs. 0.45 nmoles P mg super(-1) dust, respectively). The increase in chlorophyll, primary, and bacterial productivity for treatments Z and ZA were controlled by the amount of N added with the dust while those for treatments ZN and ZNA (in which excessive N was added) were controlled by the amount of P added. These results confirm that the surface waters were N&P co-limited for phytoplankton during springtime. However, total chlorophyll and primary productivity in the acid treated dust additions (ZA and ZNA) were less than predicted from that calculated from the amount of the potentially limiting nutrient added. This biological inhibition was interpreted as being due to labile trace metals being added with the acidified dust. A probable cause for this biological inhibition was the addition of dissolved Al, which forms potentially toxic Al nanoparticles when added to seawater. Thus, the effect of anthropogenic acid processes in the atmosphere, while increasing the flux of bioavailable P from dust to the surface ocean, may also add toxic trace metals such as Al, which moderate the fertilizing effect of the added nutrients.
ABSTRACT
The planktonic food web in the oligotrophic Mediterranean Sea is dominated by small-sized (<20 μm) microbes, with nanoflagellates being the major bacterial grazers and the main participants ...in nutrient cycling. Phosphate is a key nutrient in the P-limited Cretan Sea (NE Mediterranean) and P-availability can affect its trophic dynamics. Here, we examined the grazing potential of heterotrophic (HF) and pigmented (PF) nanoflagellates as a response mechanism to phosphate amendment. Flagellate grazing effect on bacteria was quantified in P-amended nutrient-depleted water from the Cretan Sea over the course of 4 days using microcosm experiments. P-addition positively affected HF abundance, while PF abundance remained unchanged. At the community level, P-addition had a negative effect on PF bacterial removal rates. In the control, PF-grazing rate was significantly higher than that of HF throughout the experiment. Pigment analysis showed no changes in phytoplankton community composition as a result of P-addition, indicating that PF grazing rate declined as a physiological response of the cells. The present study emphasizes the dominant grazing role of PF under P-depleted conditions and reveals that during the late stratified season PF respond to P-addition by lowering their grazing rates, enhancing the relative importance of bacterial removal by HF.
Phosphate addition alters the microbial food web functioning in P-depleted waters of the Cretan Sea.
Extracellular release of dissolved organic matter (DOM) by phytoplankton is a significant process that drives the microbial loop, providing energy and nutrients to bacteria. A model based on Dynamic ...Energy Budget (DEB) theory is proposed for describing DOM release by phytoplankton under nitrogen-replete and nitrogen-limiting conditions. The model allows for variable biomass stoichiometry by partitioning biomass into structural mass and a number of reserves masses (C-reserves, N-reserves and generalized reserves), with each pool having a constant chemical composition. In the context of DEB theory, two alternative pathways of DOM release emerge from the theory, without additional assumptions; one relates to growth and lysis of the cells and one to rejection of unprocessed substrates due to stoichiometric constraints. We assume that these pathways correspond to the two conceptual mechanisms of DOM release, i.e., passive diffusion and active exudation respectively. Model parameters were estimated by fitting the model to literature data for the diatom Thalassiosira pseudonana. Model results suggest higher mass-specific growth, dissolved organic carbon (DOC) and nitrogen (DON) release rates during the nitrogen-replete phase of growth, while the percentage extracellular release (DOC release rate over primary production rate) is higher during the nitrogen-limited phase of growth. Moreover, nutrient limitation induces an increase in biomass C:N ratio which is followed by an increase in C:N ratio of the produced DOM. These results are in accordance with a wide range of experimental evidence. The relative contribution of the two release mechanisms affects the quality of DOM produced in terms of elemental and molecular composition and size fractionation, which, in turn, may have implications for the bioavailability of the produced DOM to bacteria. Moreover, the model predicts a close to linear relationship between (log-transformed) primary production and DOC release under nitrogen-replete conditions. Deviations from linearity are related to the prevalence of the active exudation mechanism and the reduction of the primary production rate under nitrogen-limiting conditions.
•A DEB model for DOM release by phytoplankton under N replete and limiting conditions.•Model predictions capture common observed patterns of phytoplankton physiology.•Passive diffusion and active exudation mechanisms emerge from theory.•Nitrogen availability affects the relationship of primary production and DOC release.•DOM composition depends on the operating mechanisms of DOM release.
The abundance and grazing effect of small (<5 μm) pigmented (PNF) and heterotrophic (HNF) nanoflagellates on prokaryotic picoplankton stock (i.e. heterotrophic bacteria (HB) and Synechococcus) were ...assessed during April 2016 at four stations along a longitudinal transect in the ultra-oligotrophic Eastern Mediterranean Sea and at two selected depths with varying nutrient and light conditions; the 5 m depth representing surface water and the 75 m depth representing the deeper euphotic layer, targeting the Deep Chlorophyll Maximum (DCM) zone at the sampling stations. Cells of size <3 μm dominated the small nanoflagellate community and accounted for 84 ± 8.2% and 73 ± 12.2% (mean ± sd) of total HNF and PNF, respectively. HNF abundance and their grazing effect on prokaryotic picoplankton did not differ between the two depths, while PNF exhibited higher abundance and grazing effect on prokaryotic picoplankton at 75 m compared to the surface (5 m). HNF dominated prokaryotic picoplankton consumption at 5 m accounting for 67–92% and 71–90% of the HB and Synechococcus consumption, respectively. On the other hand, PNF were the dominant grazers of prokaryotic picoplankton at 75 m accounting for 72–80% of the HB consumption and 60–74% of the Synechococcus consumption. HNF and PNF together consumed 4.4 ± 3.2% d−1 (mean ± sd) of the HB standing stock and 19.7 ± 16% d−1 (mean ± sd) of the Synechococcus standing stock. Moreover, a negative relationship between phosphate concentration and ingestion rates of PNF on prokaryotic picoplankton was observed. Pigment analysis showed that the major PNF groups were Prymnesiophytes followed by Pelagophytes/Chrysophytes. Prymnesiophytes was the group mostly associated with mixotrophy. Overall, our results pinpoint the role of small nanoflagellates as prokaryotic picoplankton consumers and the importance of mixotrophy as a trophic mode for PNF in the ultra-oligotrophic E. Mediterranean Sea.
Chlorophyll
a
(Chl
a
) is the predominant pigment in every single photosynthesizing organism including phytoplankton and one of the most commonly measured water quality parameters. Various methods ...are available for Chl
a
analysis, but the majority of them are of limited throughput and require considerable effort and time from the operator. The present study describes a high-throughput, microplate-based fluorometric assay for rapid quantification of Chl
a
in phytoplankton extracts. Microplate sealing combined with ice cooling was proved an effective means for diminishing solvent evaporation during sample loading and minimized the analytical errors involved in Chl
a
measurements with a fluorescence microplate reader. A set of operating parameters (settling time, detector gain, sample volume) were also optimized to further improve the intensity and reproducibility of Chl
a
fluorescence signal. A quadratic regression model provided the best fit (
r
2
= 0.9998) across the entire calibration range (0.05–240 pg μL
−1
). The method offered excellent intra- and interday precision (% RSD 2.2 to 11.2%) and accuracy (% relative error −3.8 to 13.8%), while it presented particularly low limits of detection (0.044 pg μL
−1
) and quantification (0.132 pg μL
−1
). The present assay was successfully applied on marine phytoplankton extracts, and the overall results were consistent (average % relative error −14.8%) with Chl
a
concentrations (including divinyl Chl
a
) measured by high-performance liquid chromatography (HPLC). More importantly, the microplate-based method allowed the analysis of 96 samples/standards within a few minutes, instead of hours or days, when using a traditional cuvette-based fluorometer or an HPLC system.
Graphical abstract
TChl
a
concentrations (i.e. sum of Chl
a
and divinyl Chl
a
in ng L
-1
) measured in seawater samples by HPLC and fluorescence microplate reader.
PDF
