F.X. Niell

Determination of nitrate reductase activity in Ulva rigida C. Agardh by the in situ method

Assay conditions for determining in situ nitrate reductase (NR) activity in Ulva rigida C. Agardh were studied. The final composition of the incubation medium was: 0.1 M phosphate buffer, 0.5 mM EDTA, 0.1% l-propanol, 30 mM KNO3 and 10 #M glucose. The optimal pH was 8, which is greater than that usually found in higher plants (7.5) and close to the pH of sea water. Samples were incubated in the dark at 30 ° C. A short incubation time (30 min)was more satisfactory than longer ones for determination of the initial rate, and to prevent any limitation of the NO3 reduction rate for the assay conditions themselves. Surfactants such as Tergitol NP-10 and Triton X-100 were less effective than 0.1% l-propanol. Incubation in anaerobic conditions is a critical point in the in situ NR assay in Ulva rigida. The two methods used; vacuum infiltration for 10 min, and N2-flushing 2 min before and 2 min after introducing the sample of tissue, were almost identically effective. The addition of NADH and glucose as external sources of reducing power was studied and the problems associated with NADH interference in NO2 determination are discussed. The addition of DCMU did not overcome the need for darkness as it does in higher plants, even when the photosynthetic O2-production was completely abolished at the same DCMU concentration.

Effects of changes of irradiance on the pigment composition of Gracilaria tenuistipitata var. liui Zhang et Xia

In plants, excess irradiation can damage the photosynthetic apparatus, although some protective mechanisms exist. The excess energy can be dissipated as thermal energy, and pigments (i.e., carotenoids) also play an important role in protecting the photosynthetic apparatus by epoxidating reactions. Chromatographic analysis of pigment extracts of Gracilaria tenuistipitata shows that zeaxanthin is the major carotenoid in this alga, accounting for up to 82% of total carotenoids. Short-term (55 h) and long-term (10 days) response of the pigments shows that Chl a, 13-carotene and zeaxanthin degradation after light increase follows negative exponential trends, while the response of biliproteins is almost linear. Decreasing the irradiance results in a clear saturating response of the synthesis of Chl a and 13-carotene after one to two days. Biliprotein synthesis displays a double linear trend, the first one lasting for four days in the cases of both R-phycoerythrin (RPE) and Rphycocyanin (RPC). The response of zeaxanthin is always faster than that of Chl a or biliproteins to changes of irradiance. Our results might indicate the presence of two pools of zeaxanthin in this alga, with different acclimation responses to the changes in the photon flux density.

Use of light and inorganic carbon acquisition by two morphotypes of Zostera noltii Hornem

The affinity for dissolved inorganic carbon (DIC) and the mechanisms to use HCO3 as a source of DIC for photosynthesis were investigated in two morphotypes of Zostera noltii Hornem. Both morphotypes were collected at Ria Formosa lagoon (Southern Portugal) at two different levels in the intertidal. Affinity for DIC at saturating photon fluence rate (PFR), estimated as photosynthetic conductance for CO2 (gp(CO2)), was reduced by 75% in the Z. noltii plants adapted to shade conditions (lower intertidal) in comparison to the sun morphotype (45 � 10 � 6 and 182 � 10 � 6 ms � 1 , respectively), indicating that the plants acclimated to sun conditions (higher intertidal) had a higher capacity to use HCO3 as DIC source for photosynthesis. Since external carbonic anhydrase activity was negligible and a large inhibitory effect was produced by Tris buffer addition, this HCO3 use was attributed to the operation of H + ATPases creating low pH zones in periplasmic space. The photosynthetic CO2-flux supported for this mechanism was calculated to be 53 Amol O2 m � 2 s � 1 in sun morphotype, about 80% out of maximum photosynthesis rate. In order to determine the possible photosynthetic energy cost of the HCO3 use, the effect of decreasing light on photosynthetic rates and gp(CO2) was estimated. Photosynthetic conductance decreased in both morphotypes at nonsaturating PFR. This dependence of gp(CO2) on PFR indicated the existence of a positive interactive effect between DIC and PFR which was more pronounced in the shade morphotype since the ascending slope of O2 evolution vs. PFR curves at limiting PFRs was reduced from 7.2 to 2.3 mmol O2 mol photon � 1 at 4 and 0.5 mol m � 3 of DIC, respectively.

The effect of photoacclimation on the photosynthetic physiology of Ulva curvata and Ulva rotundata (Ulvales, Chlorophyta)

The effects of photoacclimation on growth, photosynthesis, pigment content and elemental composition of Ulva curvata and Ulva rotundata, which grow together in eutrophic areas of southern Spain, were investigated. Cultures were grown for 6 days at different photon fluence rates (PFR) ranging from darkness up to 200 µmolm-2 s-1 under nutrient-sufficient conditions (artificial seawater supplemented with ammonium and phosphate). Growth rates were not light-saturated (up to 200 µmolm-2s-1), reaching a value close to 0·2d-1. Growth rates based on mass, area or C content were equivalent, except in darkness and very low light levels (2 µmolm-2s-1), where thallus expansion occurred by diluting internal biomass and C. Chlorophyll and absorptance showed a bell-shaped PFR-response curve, with maxima at 30–60 µmolm-2s-1 and lower values at light saturation and under light-limiting conditions. Although net NH4 + incorporation was not affected by growth-PFR, there was inefficient assimilation of N at low light levels, ...

Influence of phosphorus status on the seasonal variation of alkaline phosphatase activity in Porphyra umbilicalis (L.) Kützing

Abstract The influence of internal and external compartments of phosphorus on the alkaline phosphatase activity (APA) of Porphyra umbilicalis during the presence of its gametophytic phase in southern Spain was investigated. The compartments of internal soluble phosphorus were significantly correlated with the percentage cover of Porphyra on the sampling area. APA was inversely correlated with total internal phosphorus and particulate phosphorus. A dynamic model (resembling the cell quota model of Droop) was proposed to relate the enzymatic activity to the internal phosphorus content. According to this model, a maximum APA of 98.5 μmol p -nitrophenol (pNP) released g· dry wt −1 ·h −1 and a minimum APA of 12.05 μmol pNP g· dry wt −1 ·h −1 during the gametophytic cycle were estimated. Similarly, a maximum total phosphorus content of 4.2 mg P g·dry wt −1 and a theoretical minimum total phosphorus content of 0.51 mg P g·dry wt −1 throughout the gametophytic phase were estimated. The latter value might be close to the subsistence quota for phosphorus; the minimum quota for life.

Alkaline phosphatase activity in Porphyra umbilicalis (L.) Kutzing

External alkaline phosphatase activity (APA) has been measured in Porphyra umbilicalis (L.) Kutzing over an ecological range of temperature, pH, salinity, photon irradiance and external phosphate. Temperature, pH, salinity and external phosphate caused significant changes in APA. On the contrary, photon irradiance caused no significant effect. In the range of temperature assayed, maximum APA was 249 μmol pNP g dry wt−1 h−1 at 30 °C. With regard to pH, maximum activity occurred at pH 8.8 (249 μmol pNP g dry wt−1 h−1), whereas varying salinity, maximum enzymatic activity (270 μmol pNP dry g wt−1 h−1) took place at 43.75%. As expected, there was an inverse relationship between APA and external phosphate concentration. There is a minimum constant activity at phosphate concentration over 14 μM and a final inhibition of 71% in enzymatic activity.

Light-dependent uptake, translocation and foliar release of phosphorus by the intertidal seagrass Zostera noltii Hornem.☆

Light-dependent P uptake by root-rhizomes, acropetal translocation and subsequent foliar release by Zostera noltii Hornem. was studied under laboratory conditions in two-compartment chambers using 32P. The uptake by underground parts was unaffected by light conditions but the acropetal translocation proceeded more rapidly in light than in dark, indicating a coupling to the metabolic activity of the plants. The translocated P was mainly accumulated in the youngest leaves (30%), i.e., the most actively growing parts. Foliar release of P might be considered negligible (2–4% of the P taken up by root-rhizomes), indicating that the role of Z. noltii as a “P pump” is of minor importance in the cycling of P between sediment and water. This was calculated for part of the Oosterschelde estuary, Zeeland, The Netherlands.

Photosynthetic acclimation and biochemical responses ofGelidium sesquipedale cultured in chemostats under different qualities of light

The red algaGelidium sesquipedale (Clem.) Born. et Thur. has been cultured in chemostats to assess the effects of light quality and photon-fluence rate (PFR) on growth, photosynthesis and biochemical composition. Plants under blue and red light (BL and RL) showed higher growth rates than under white light (WL) of the same PFR (40 μmol m−2 s−1). The light-saturated rate of photosynthesis was higher for algae grown under BL and RL than for algae grown under WL. When algae were transferred to WL of moderate PFR (100 μmol m−2 s−1), the light-saturated rate of photosynthesis decreased, being higher in previously RL-grown algae than in previously BL- and WL-grown algae. The initial slope of photosynthesis-irradiance (PI) curves (α) was affected by PFR but not by light quality. Pigment content was little affected by light quality. Light-quality treatments also affected the biochemical composition of the alga; previous exposure to various light treatments activate or repress several metabolical pathways that are fully expressed in the subsequent phase of WL of moderate PFR. Thus, phycobiliproteins and soluble proteins increased for previously BL- and RL-grown algae, whereas insoluble carbohydrate concentration was reduced, indicating a change of the C-partitioning between carbon compounds and organic nitrogen compounds. Inorganic nitrogen metabolism was also affected by light: under WL of moderate PFR, NO3− was totally depleted from sea water, and maximal values of NO3− uptake were recorded. In addition, neither NO2− nor NH4+ was released. However, when algae were transferred to a low PFR, there was a drastic reduction of NO3− uptake under WL, which only partially recovered over time. It was accompanied by the release of NO2−, but not NH4+, to the culture medium. Under BL and RL, however, there was a transient enhancement of NO3− uptake that was followed by a net release of NO2− and NH4−. Growth rates were not correlated with PFR. This could be due to the the dynamics of internal carbon mobilization and accumulation in the algae. When algae were exposed to a moderate PFR of WL, carbon requirements for growth were satisfied by photosynthesis. Thus, there was a net accumulation of carbon in the tissue. In contrast, when algae were exposed to low PFRs of either WL, BL or RL, observed growth rates could not be maintained by photosynthesis and carbon was mobilized.

Light quality affects morphology and polysaccharide yield and composition of Gelidium sesquipedale (Rhodophyceae)

Morphology and polysaccharide characterization of Gelidium sesquipedale (Clem.) Bornet et Thuret were studied in cultures grown under various light qualities. White light (WL), blue light (BL) and red light (RL) (all at photon fluence rate of 40 μmol m-2 s-1) were used for the study of morphological characteristics, and in addition yellow light (YL) for polysaccharide characterization. RL and BL induced a proliferating growth, which resulted in bushy plants under RL. Cortical cells of BL-grown plants were smaller and presented a higher density per unit area, whereas those of WL- and RL-grown alga were larger. Medullary cells followed the inverse pattern. Light quality also affected polysaccharide yield and composition, with the yield being higher under BL, RL or YL than WL. Most of the polysaccharide was extracted in distilled water at 100 °C, while a low amount was solubilized at 22 °C and 120 °C. Extracts from BL-grown alga presented the highest galactan content. The starch concentration was lower in extracts from RL-, BL- and YL-cultivated alga than in those from the initial plants. The degree of substitution with methoxyl groups and precursor was very low in all the agar fractions, but fractions extracted from BL- and WL-grown alga were more substituted by precursor. The highest sulfate content was reached under BL (about 9% w/w) and the highest 2-O-methyl-3,6-anhydro-L-galactose and 6-O-methyl-D-galactose content were found in extracts from alga grown under YL.

A comparative study of alkaline phosphatase activity in two species of Gelidium (Gelidiales, Rhodophyta)

A comparative study of alkaline phosphatase activity (APA) was carried out on repeat samples of representative populations of two species of Gelidium (Rhodophyta), G. latifolium and G. sesquipedale, using p-nitrophenyl phosphate (pNPP) as substrate. The APA kinetics of both species exhibited an apparent negative cooperativity for the hydrolysis of pNPP. Differences were found in phosphatase activities of the two species: G. latifolium showed higher APA per unit biomass and a higher proportion of the phosphate cleaved from the model substrate than G. sesquipedale. Temperature, pH, salinity and external phosphate all had significant effects on the APA of both species. The influence of salinity on APA was due in part to the effects of specific ions (Na+ and Mg2+). No correlation between photon irradiance and APA was observed in G. latifolium, but in G. sesquipedale maximum APA was observed at a relatively low photon irradiance. APA of the two species is discussed with respect to a possible relationship with ...