Effects of UV-B on motility and photobehavior in the green flagellate, Euglena gracilis

Contrasting effects of high-intensity photosynthetically active radiation on two bloom-forming dinoflagellates

While light limitation can inhibit bloom formation in dinoflagellates, the potential for high-intensity photosynthetically active radiation (PAR) to inhibit blooms by causing stress or damage has not been well-studied. We measured the effects of high-intensity PAR on the bloom-forming dinoflagellates Alexandrium fundyense and Heterocapsa rotundata. Various physiological parameters (photosynthetic efficiency F_v /F_m , cell permeability, dimethylsulfoniopropionate [DMSP], cell volume, and chlorophyll-a content) were measured before and after exposure to high-intensity natural sunlight in short-term light stress experiments. In addition, photosynthesis-irradiance (P-E) responses were compared for cells grown at different light levels to assess the capacity for photophysiological acclimation in each species. Experiments revealed distinct species-specific responses to high PAR. While high light decreased F_v /F_m in both species, A. fundyense showed little additional evidence of light stress in short-term experiments, although increased membrane permeability and intracellular DMSP indicated a response to handling. P-E responses further indicated a high light-adapted species with Chl-a inversely proportional to growth irradiance and no evidence of photoinhibition; reduced maximum per-cell photosynthesis rates suggest a trade-off between photoprotection and C fixation in high light-acclimated cells. Heterocapsa rotundata cells, in contrast, swelled in response to high light and sometimes lysed in short-term experiments, releasing DMSP. P-E responses confirmed a low light-adapted species with high photosynthetic efficiencies associated with trade-offs in the form of substantial photoinhibition and a lack of plasticity in Chl-a content. These contrasting responses illustrate that high light constrains dinoflagellate community composition through species-specific stress effects, with consequences for bloom formation and ecological interactions within the plankton.

Effects of UV-A (320 to 399 Nanometers) on Grazing Pressure of a Marine Heterotrophic Nanoflagellate on Strains of the Unicellular Cyanobacteria Synechococcus spp

In the open ocean, where turbidity is very low, UV radiation may be an important factor regulating interactions among planktonic microorganisms. The effect of exposure to UV radiation on grazing by a commonly isolated marine heterotrophic nanoflagellate, Paraphysomonas bandaiensis , on two strains of the cyanobacteria Synechococcus spp. was investigated. Laboratory cultures were exposed to a range of irradiances of artificially produced UV-B (290 to 319 nm) and UV-A (320 to 399 nm) for up to 10 h. At a UV-B irradiance of 0.19 W m −2 , but not 0.12 W m −2 , grazing mortality of Synechococcus spp. and nanoflagellate-specific grazing rates were reduced compared to mortality and grazing rates with UV-A treatment. Within 6 h of exposure, UV-A alone suppressed grazing mortality at irradiances as low as 3.02 W m −2 . The extent to which grazing mortality and nanoflagellate-specific grazing rates were suppressed by UV-A increased with both irradiance and duration of exposure. Over a 6-h exposure period, differences in grazing mortality were largely attributable to differential survival of nanoflagellates. Over a longer period of exposure, there was impairment by UV-A alone of nanoflagellate-specific grazing rates. Rates of primary productivity of Synechococcus spp. were also reduced by UV-A. The extent to which Synechococcus productivity was reduced, compared to the reduction in Synechococcus grazing mortality, depended on the duration of UV-A exposure. These results support the hypothesis that UV-A alone influences the composition and biomass of marine microbial communities by affecting predator-prey interactions and primary production.

Orientation of the photosynthetic flagellate, Peridinium gatunense, in hypergravity

The photosynthetic freshwater flagellate, Peridinium gatunense, uses both positive phototaxis and negative gravitaxis to move upwards in the water column. At higher fluence rates approaching those at the surface of their habitat, the cells tend to become unoriented and thus stop their upward movement. Orientation and motility of Peridinium gatunense has been studied in the slow rotating centrifuge microscope (NIZEMI), which allows observation of swimming behavior during centrifugation acceleration between 1 g and 5g. The movement vectors were analyzed by real time image analysis capable of tracking many cells simultaneously. At 1 g the orientation was not very precise, but the degree of orientation increased significantly at higher acceleration forces up to about 3 g. Most cells were capable of swimming even against an acceleration vector of 3.8 g; at higher acceleration forces the cells were not able to cope with the centrifugal force. The linear velocity of cells swimming against 1 g was about 20% lower than that of cells moving in other directions. The velocity decreased even more in cells swimming against higher acceleration forces.

Motility and gravitactic orientation of the flagellate, Euglena gracilis, impaired by artificial and solar UV-B radiation

The effects of ultraviolet radiation on the gravitactic orientation of the freshwater flagellate, Euglena gracilis, were determined by a real time image analysis system. Both artificial UV radiation and solar radiation in a temperature-controlled growth chamber were employed. Histograms of gravitaxis showed that the degree of orientation decreased with increasing exposure time; this can be quantified using the Rayleigh test and upper quadrant summation. The effects of artificial UV radiation on the orientation are considerably stronger than those of solar radiation, probably because the radiation source emits higher fluence rates below 300 nm than found in solar radiation. The effects of monochromatic ultraviolet radiation on motility have been determined, and an action spectrum has been calculated.