Anthropogenic versus mineral aerosols in the stimulation of microbial planktonic communities in coastal waters of the northwestern Mediterranean Sea.

The atmosphere of the northwestern (NW) Mediterranean Sea is affected by continuous inputs of anthropogenic aerosols and episodic Saharan dust events. These atmospheric inputs deliver to the surface waters high amounts of macronutrients and trace metals that can constitute their main source at certain times of the year. The effect of both anthropogenic and crustal particles over the autotrophic and heterotrophic planktonic community assembles was evaluated through three microcosm experiments carried out in the summer of 2013 and in the winter and spring of 2014 at an urban coastal location of the NW Mediterranean (Barcelona, Spain). Particles were added to seawater at a concentration of 0.8mgl-1. The results showed that (i) a greater stimulation of the whole community was observed in summer and spring than in winter; (ii) both kinds of aerosols produced an increase in the growth of phytoplankton, although the stimulation of nanoeukaryotes was significantly larger with anthropogenic aerosols; and (iii) bacterial abundance increased more with mineral dust, whereas bacterial production was more stimulated with anthropogenic inputs. Overall, the effect of atmospheric particles was dependent on their composition and solubility in seawater, as well as on the initial biogeochemical conditions present in the seawater and had the potential to change the net metabolic balance of the microbial planktonic community.

Effects of small-scale turbulence on bacteria: a matter of size.

We examined the influence of small-scale turbulence and its associated shear on bacterioplankton abundance and cell size. We incubated natural microbial assemblages and bacteria-only fractions and subjected them to treatments with turbulence and additions of mineral nutrients and/or organic carbon. Bacterial abundance was not affected directly by turbulence in bacteria-only incubations. In natural microbial assemblage incubations, bacterial concentrations were higher under turbulence than in still-water controls when nutrients were added. In general, in the turbulence treatments bacteria increased significantly in size, mainly due to elongation of cells. The addition of inorganic nutrients had a negative effect on bacterial size, but a significantly positive effect on abundance independently of other factors such as turbulence and the presence of predators. Flagellate grazing did not trigger an increase in bacterial size as a grazing resistance response in unmixed containers. With the addition of organic carbon, bacteria elongated and partly settled to the bottom of the containers, in both the turbulent and still treatment, but bacterial abundance did not further increase. Furthermore, bacteria aggregated in the turbulence treatments after the second day of incubation even in the absence of other components of the microbial community. We found that turbulence and the associated shear increase bacterial size and change bacterial morphology, at least under certain nutrient conditions. This might be due to a physiological response (enhanced growth rate and/or unbalanced growth) or due to the selection of opportunistic strains when organic carbon is in excess compared to mineral nutrients. We suggest that shear associated with turbulent flow enhances the DOM flux to bacteria directly as well as indirectly through enhanced grazing activity and photosynthetic release. The formation of bacterial aggregates and filaments under turbulence might give selective advantage to bacteria in terms of nutrient uptake and grazing resistance.

Seawater-atmosphere O(2) exchange rates in open-top laboratory microcosms: application for continuous estimates of planktonic primary production and respiration.

Seawater-atmosphere O(2) exchange rates were experimentally measured in open-top laboratory microcosms. The objective was to establish the relationships between turbulence and oxygen transfer velocity, and thus correct continuously measured day-night changes in dissolved oxygen as estimates of planktonic primary production and respiration. After saturating 15-l sterile seawater microcosms with an oxygen-poor gas mix (4.9% O(2), 95.1% N(2)), the microcosms were left to equilibrate with the atmosphere under different turbulence conditions. The rate of increase in dissolved O(2) was measured at 15-min intervals with polarographic-pulsed electrodes and the corresponding values of the oxygen transfer velocity (the K(O(2)) constant for the different turbulence conditions) were determined. After pooling these and literature data obtained in similar experimental conditions, the relation between epsilon (turbulent kinetic energy dissipation rates) and K(O(2)) was determined. Theoretical K(O(2)) values were also calculated using semi-empirical models in which oxygen transfer velocity (K(O(2))) is related to wind velocity. Theoretical, wind related K(O(2)) values were significantly higher than the experimental ones, and as a consequence overestimate primary production and underestimate respiration rates, even resulting in nocturnal O(2) increase. The magnitude of the differences between experimentally derived and theoretically calculated oxygen transfer velocity, precludes the use of wind-derived equations to calculate K(O(2)) in meso- and microcosms experiments not affected by wind, while the equation obtained relating experimental epsilon and K(O(2)) provides statistically reliable estimations of primary production and respiration.

The microbial food web along salinity gradients.

The microbial food web was studied along a gradient of salinity in two solar salterns used for the commercial production of salt. The different ponds in the salterns provide a wide range of ecosystems with food webs of different complexities. Abundance of prokaryotes, cell volume, prokaryotic heterotrophic production, chlorophyll a, abundance of heterotrophic flagellates, ciliates and phytoplankton were determined in several ponds in each saltern. Increases in salinity resulted in a progressive reduction in the abundance and number of different groups of eukaryotic microorganisms present, but an increase in biomass of prokaryotes. Maximal activity of both phyto- and bacterioplankton was found at a salinity of around 100 per thousand, where there was also a maximum in chlorophyll a concentration. Growth rates of heterotrophic prokaryotes decreased with increasing salinity. Bacterivory disappeared above 250 per thousand salinity, whereas other loss factors such as viral lysis appeared to be of minor importance throughout the gradient [Guixa-Boixereu et al. (1996) Aquat. Microb. Ecol. 11, 215-227].

Light-dependent phagotrophy in the freshwater mixotrophic chrysophyte Dinobryon cylindricum.

The mixotrophic (bacterivorous), freshwater chrysophyte Dinobryon cylindricum was cultured under a variety of light regimes and in bacterized and axenic cultures to investigate the role of phototrophy and phagotrophy for the growth of this alga. D. cylindricum was found to be an obligate phototroph. The alga was unable to survive in continuous darkness even when cultures were supplemented with high concentrations of bacteria, and bacterivory ceased in cultures placed in the dark for a period longer than one day. Axenic growth of the alga was poor even in an optimal light regime. Live bacteria were required for sustained, vigorous growth of the alga in the light. Carbon (C), nitrogen (N), and phosphorus (P) budgets determined for the alga during growth in bacterized cultures indicated that bacterial biomass ingested by the alga may have contributed up to 25% of the organic carbon budget of the alga. Photosynthesis was the source of most ([Symbol: see text]75%) of the organic carbon of the alga. D. cylindricum populations survived but did not grow when cultured in a continuous low light intensity (30 µE m(-2) sec(-1)), or in a light intensity of 150 µE m(-2) sec(-1) for only two hours each day. Net efficiency of incorporation of bacterial C, N, and P into algal biomass under these two conditions was zero (i.e., no net algal population growth). We conclude that the primary function of bacterivorous behavior in D. cylindricum may be to provide essential growth factor(s) or major nutrients for photosynthetic growth, or to allow for the survival of individuals during periods of very low light intensity or short photoperiod.

Grazing in a turbulent environment: behavioral response of a calanoid copepod, Centropages hamatus.

Models of marine ecosystem productivity rely on estimates of small-scale interactions, particularly those between copepods and their algal food sources. Rothschild and Osborn [Rothschild, B. J. & Osborn, T. R. (1988) J. Plankton Res. 10, 465-474], hypothesized that small-scale turbulence in aquatic systems increases the perceived abundance of prey to predators. We tested this hypothesis by exposing the planktonic copepod Centropages hamatus to turbulent and nonturbulent environments at different prey concentrations. Our results fell into two main categories. First, the response to turbulence was characterized by an initial period having a high number of escape reactions. This period was followed by one of increased foraging. C. hamatus responded to the higher encounter rates due to turbulence as if it were experiencing altered prey concentrations. Second, the termination of turbulence resulted in an increased foraging response, which was not directly related to the encounter rate. Functional response curves do not adequately explain this foraging response because the time course of the foraging response depends on prior encounter experience and foraging motivation.

Grazing in a turbulent environment: energy dissipation, encounter rates, and efficacy of feeding currents in Centropages hamatus.

The creation of feeding currents by calanoid copepods increases encounter rates of copepods with their food and provides and advantage in dilute nutritional environments. Small-scale turbulence has also been hypothesized to increase the encounter rate between planktonic predators and their food. Centropages hamatus was exposed to turbulent and nonturbulent environments at two prey concentrations to quantify the influence of turbulence on feeding current efficacy. Turbulent energy dissipation rates used in the experiment were in the range of 0.05-0.15 cm2. sec-3. In the nonturbulent environments, feeding currents increased the encounter rates of C. hamatus 3-5 times that of control encounter areas. In turbulent environments, encounter rates were not increased by feeding currents, yet C. hamatus continued to create feeding currents. Energetic calculations indicate a tradeoff in the value of turbulence to a copepod feeding on phytoplankton. While turbulence is probably beneficial at low food concentrations, it may be deleterious at high food concentrations.