UVR defense mechanisms in eurytopic and invasive Gracilaria vermiculophylla (Gracilariales, Rhodophyta).

The invasive success of Gracilaria vermiculophylla has been attributed to its wide tolerance range to different abiotic factors, but its response to ultraviolet radiation (UVR) is yet to be investigated. In the laboratory, carpospores and vegetative thalli of an Atlantic population were exposed to different radiation treatments consisting of high PAR (photosynthetically active radiation) only (P), PAR+UV-A (PA) and PAR+UV-A+UV-B (PAB). Photosynthesis of carpospores was photoinhibited under different radiation treatments but photosystem II (PSII) function was restored after 12 h under dim white light. Growth of vegetative thalli was significantly higher under radiation supplemented with UVR. Decrease in chlorophyll a (Chl a) under daily continuous 16-h exposure to 300 µmol photons m(-2) s(-1) of PAR suggests preventive accumulation of excited chlorophyll molecules within the antennae to minimize the generation of dangerous reactive oxygen species. Moreover, an increase in total carotenoids and xanthophyll cycle pigments (i.e. violaxanthin, antheraxanthin and zeaxanthin) further suggests effective photoprotection under UVR. The presence of the ketocarotenoid ß-cryptoxanthin also indicates protection against UVR and oxidative stress. The initial concentration of total mycosporine-like amino acids (MAAs) in freshly-released spores increased approximately four times after 8-h laboratory radiation treatments. On the other hand, initial specific MAAs in vegetative thalli changed in composition after 7-day exposure to laboratory radiation conditions without affecting the total concentration. The above responses suggest that G. vermiculophylla have multiple UVR defense mechanisms to cope with the dynamic variation in light quantity and quality encountered in its habitat. Beside being eurytopic, the UVR photoprotective mechanisms likely contribute to the current invasive success of the species in shallow lagoons and estuaries exposed to high solar radiation.

NH4+ enrichment and UV radiation interact to affect the photosynthesis and nitrogen uptake of Gracilaria lemaneiformis (Rhodophyta).

Solar ultraviolet radiation (UVR, 280-400 nm) is known to inhibit the photosynthesis of macroalgae, whereas nitrogen availability may alter the sensitivity of the algae to UVR. Here, we show that UV-B (280-315 nm) significantly reduced the net photosynthetic rate of Gracilaria lemaneiformis. This inhibition was alleviated by enrichment with ammonia, which also caused a decrease in dark respiration. The presence of both UV-A (315-400 nm) and UV-B stimulated the accumulation of UV-absorbing compounds. However, this stimulation was not affected by enrichment with ammonia. The content of phycoerythrin (PE) was increased by the enrichment of ammonia only in the absence of UVR. Ammonia uptake and the activity of nitrate reductase were repressed by UVR. However, exposure to UVR had an insignificant effect on the rate of nitrate uptake. In conclusion, increased PE content associated with ammonia enrichment played a protective role against UVR in this alga, and UVR differentially affected the uptake of nitrate and ammonia.

Effects of UVB radiation on the agarophyte Gracilaria domingensis (Rhodophyta, Gracilariales): changes in cell organization, growth and photosynthetic performance.

The effects of ultraviolet radiation-B (UVBR) in apical segments of the red macroalgae Gracilaria domingensis (Kützing) Sonder ex Dickie were examined in vitro. Over a period of 21 days, the segments were cultivated and exposed to photosynthetically active radiation (PAR) at 80 µmol photons m(-2)s(-1) and PAR+UVBR at 1.6 Wm(-2) for 3h per day. The samples were processed for electron microscopy, as well as histochemical analysis, and growth rate, photosynthetic pigment contents and photosynthetic performance were measured. Toluidine Blue reaction showed metachromatic granulations in vacuole and lenticular thickness, while Coomassie Brilliant Blue showed a higher concentration of cytoplasmic organelles, and Periodic Acid Schiff stain showed an increase in the number of floridean starch grains. UVBR also caused changes in the ultrastructure of cortical and subcortical cells, which included an increased number of plastoglobuli, changes in mitochondrial organization, destruction of chloroplast internal organization, and the disappearance of phycobilisomes. The algae cultivated under PAR-only showed growth rates of 6.0%day(-1), while algae exposed to PAR+UVBR grew only 2.8%day(-1). Compared with algae cultivated with PAR-only, the contents of photosynthetic pigments, including chlorophyll a, phycoerythrin, phycocyanin and allophycocyanin, decreased after exposure to PAR+UVBR, and significant differences were observed. Finally, analysis of these four photosynthetic parameters also showed reduction after exposure to PAR+UVBR: maximum photosynthetic rate, photosynthetic efficiency, photoinhibition and relative electron transport rate. Taken together, these findings strongly suggested that UVBR negatively affects the agarophyte G. domingensis.

Effect of nutrient supply on photosynthesis and pigmentation to short-term stress (UV radiation) in Gracilaria conferta (Rhodophyta).

The effects of increased photosynthetic active radiation (PAR), UV radiation (UVR), and nutrient supply on photosynthetic activity, pigment content, C:N ratio and biomass yield were studied in tank cultivated Gracilaria conferta (Rhodophyta). Electron transport rate (ETR) and biliprotein content were higher under high nutrient supply (HNS), obtained from fishpond effluents, compared to low nutrient supply (LNS), in contrast to mycosporine-like amino acids (MAAs) dynamic. The high MAA content in LNS-algae could be explained by higher UVR penetration in the thallus and by the competition for the use of nutrients with other processes. Effective quantum yield decreased after short-term exposure to high irradiance whereas full recovery in shade was produced only under slightly heat shock. UVA radiation provoked an additional decrease in photosynthesis under high water temperature. UVB radiation reversed UVA's negative effect mainly with HNS. Results support that nutrient-sufficiency help G. conferta to resist environmental changes as short-term temperature increase.

In vitro assay and light regulation of nitrate reductase in red alga Gracilaria chilensis.

Nitrate reductase (NR) is the first enzyme in the nitrogen assimilation pathway. The in vitro NR activity of Gracilaria chilensis was assayed under different conditions to reveal its stability and biochemical characteristics, and an optimized in vitro assay is described. Maximal NR activities were observed at pH 8.0 and 15 degrees C. The apparent Km value for NADH was 8 microM and for nitrate 680 microM. Crude extracts of G. chilensis stored at 4 degrees C showed a 50% decrease of NR activity after 24 h. The highest NR activity value (253.20+/-2.60 x 10(-3) U g(-1)) was obtained when 100% von Stosch medium (500 microM NO3-) was added before extraction of apical parts. Algae under light:dark cycles of 12:12h exhibited circadian fluctuation of NR activity and photosynthesis with more than 2 times higher levels in the light phase. No evidence of endogenous diel rhythm controlling NR activity or photosynthesis was observed. Light pulses lasting 10 or 60 min during the darkness increased the NR activity by 30% and 45%, respectively. The results indicate that NR and photosynthesis are regulated mainly by light and not by a biological clock.