Iron uptake mechanism in the chrysophyte microalga Dinobryon.

The mechanism of iron uptake in the chrysophyte microalga Dinobryon was studied. Previous studies have shown that iron is the dominant limiting elements for growth of Dinobryon in the Eshkol reservoir in northern Israel, which control its burst of bloom. It is demonstrated that Dinobryon has a light-stimulated ferrireductase activity, which is sensitive to the photosynthetic electron transport inhibitor DCMU and to the uncoupler CCCP. Iron uptake is also light-dependent, is inhibited by DCMU and by CCCP and also by the ferrous iron chelator BPDS. These results suggest that ferric iron reduction by ferrireductase is involved in iron uptake in Dinobryon and that photosynthesis provides the major reducing power to energize iron acquisition. Iron deprivation does not enhance but rather inhibits iron uptake contrary to observations in other algae.

Evidence for the occurrence of photorespiration in synurophyte algae.

The fluxes of CO(2) and oxygen during photosynthesis by cell suspensions of Tessellaria volvocina and Mallomonas papillosa were monitored mass spectrometrically. There was no rapid uptake of CO(2,) only a slow drawdown to compensation concentrations of 26 µM for T. volvocina and 18 µM for M. papillosa, when O(2) evolution ceased, indicating a lack of active bicarbonate uptake by the cells. Darkening of the cells after a period of photosynthesis did not cause rapid release of CO(2), indicating the absence of an intracellular inorganic carbon pool. However, upon darkening a brief burst of CO(2) was observed similar to the post-illumination burst characteristic of C(3) higher plants. Treatment of the cells of both species with the membrane-permeable carbonic anhydrase inhibitor ethoxyzolamide had no adverse effect on photosynthetic rate, but stimulated the dark CO(2) burst indicating the dark oxidation of a compound formed in the light. In the absence of any active accumulation of inorganic carbon photosynthesis in these species should be inhibited by O(2). This was investigated in four synurophyte species T. volvocina, M. papillosa, Synura petersenii, and Synura uvella: photosynthetic O(2) evolution rates in all four algae, measured by O(2) electrode, were significantly higher (40-50%) in media at low O(2) (4%) than in air-equilibrated (21% O(2)) media, indicating an O(2) inhibition of photosynthesis (Warburg effect) and thus the occurrence of photorespiration in these species.

Direct effects of UV-B radiation on the freshwater heterotrophic nanoflagellate Paraphysomonas sp.

The formation of DNA photoproducts in organisms exposed to ambient levels of UV-B radiation can lead to death and/or reduced population growth in aquatic systems. Dependence on photoenzymatic repair to reverse DNA damage caused by UV-B radiation is demonstrated for Paraphysomonas sp., a member of a widely distributed genus of heterotrophic nanoflagellates. At 20 degrees C, Paraphysomonas sp. was exposed to a range of UV-B intensities encountered in natural systems. Populations of the flagellate survived and grew in a dose-dependent manner, but only when simultaneously exposed to photorepair radiation (PRR). In contrast, flagellates exposed to UV-B at 15 degrees C suffered 100% mortality except at the lowest UV-B level (with PRR) tested, which suggested a photorepair temperature optimum above 15 degrees C. After acute UV-B exposures, DNA damage (measured as the formation of pyrimidine dimers) was reduced only in organisms that underwent subsequent exposure to PRR. Populations kept in the dark after UV-B exposure maintained the initial levels of pyrimidine dimers. These results are the first to demonstrate the reliance of a heterotrophic flagellate on photoenzymatic DNA repair for survival from UV-B exposure.

Effects of temperature on growth rate and gross growth efficiency of an Antarctic bacterivorous protist.

The effects of temperature on the growth rate and gross growth efficiency (GGE) of the heterotrophic nanoflagellate, Paraphysomonas imperforata, cultured from the Ross Sea, Antarctica were investigated using five experimental temperatures (range=0-20 degrees C). This bacterivorous protist exhibited measurable growth over the temperature range examined, although temperature exerted a significant effect on its growth rate. There was no evidence for an effect of temperature on GGE. The growth rates and GGE of our Antarctic P. imperforata isolate were compared to values reported for other cultures of species from this genus. A wide range of growth efficiencies have been reported for different strains of Paraphysomonas spp., but our estimates were comparable to mean/median values reported in the literature. The growth rates of our Antarctic P. imperforata were similar to rates obtained for an Arctic conspecific at low temperatures (0-5 degrees C), among the highest reported rates for any Paraphysomonas species at intermediate temperatures (10-15 degrees C) and similar to rates reported for temperate congeners and conspecifics at 20 degrees C. Q(10) values of 15, 2.2, 3.6 and 0.93 were calculated for growth rates at 5 degrees C intervals between 0 and 20 degrees C, respectively. Results indicated that our Antarctic P. imperforata grew at rates comparable to other polar isolates at ambient polar temperatures, but these low temperatures may be outside the physiological optimum for the isolate.