Eutrophication and macroalgal blooms in temperate and tropical coastal waters: nutrient enrichment experiments with Ulva spp.

Receiving coastal waters and estuaries are among the most nutrient‐enriched environments on earth, and one of the symptoms of the resulting eutrophication is the proliferation of opportunistic, fast‐growing marine seaweeds. Here, we used a widespread macroalga often involved in blooms, Ulva spp., to investigate how supply of nitrogen (N) and phosphorus (P), the two main potential growth‐limiting nutrients, influence macroalgal growth in temperate and tropical coastal waters ranging from low‐ to high‐nutrient supplies. We carried out N and P enrichment field experiments on Ulva spp. in seven coastal systems, with one of these systems represented by three different subestuaries, for a total of nine sites. We showed that rate of growth of Ulva spp. was directly correlated to annual dissolved inorganic nitrogen (DIN) concentrations, where growth increased with increasing DIN concentration. Internal N pools of macroalgal fronds were also linked to increased DIN supply, and algal growth rates were tightly coupled to these internal N pools. The increases in DIN appeared to be related to greater inputs of wastewater to these coastal waters as indicated by high δ¹⁵N signatures of the algae as DIN increased. N and P enrichment experiments showed that rate of macroalgal growth was controlled by supply of DIN where ambient DIN concentrations were low, and by P where DIN concentrations were higher, regardless of latitude or geographic setting. These results suggest that understanding the basis for macroalgal blooms, and management of these harmful phenomena, will require information as to nutrient sources, and actions to reduce supply of N and P in coastal waters concerned.     

Uptake kinetics and assimilation of phosphorus byCatenella nipaeandUlva lactucacan be used to indicate ambient phosphate availability

Uptake, assimilation and compartmentation of phosphate were studied in the opportunist green macroalgaUlva lactucaand the estuarine red algal epiphyteCatenella nipae. The Michaelis–Menten model was used to describe uptake rates of inorganic phosphate (P_i) at different concentrations. Maximum uptake rates (V _max) of P-starved material exceededV _maxof P-enriched material; this difference was greater forC. nipae. Uptake and allocation of phosphorus (P) to internal pools was measured using trichloroacetic acid (TCA) extracts and³²P. Both species demonstrated similar assimilation paths: when P-enriched, most³²P accumulated as free phosphate. When unenriched,³²P was rapidly assimilated into the TCA-insoluble pool.C. nipaeconsistently assimilated more³²P into this pool thanU. lactuca, indicatingC. nipaehas a greater P-storage capacity. In both species,³²P release data showed two internal compartments with very different biological half-lives. The rapidly exchanging compartment had a short half-life of 2 to 12 min, while the slowly exchanging compartment had a much longer half-life of 12 days in P-starvedC. nipaeor 4 days in P-starvedU. lactuca. In both species, the slowly exchanging compartment accounted for more than 90% of total tissue.U. lactucaandC. nipaeresponded differently to high external P_i.U. lactucarapidly took up P_i, transferring this P_iinto tissue phosphate and TCA-soluble P in a few hours (90% of total P).C. nipaetook up P_iat lower rates and stored much of this P in less mobile TCA-insoluble forms. Long-term storage of refractory forms of P makesC. nipaea useful bioindicator of the prevailing conditions of P_iavailability over at least the previous 7 days, whereas the P-status ofU.lactucamay reflect conditions over no more than the previous few hours or days.C. nipaeis a more useful bioindicator for P status of estuarine and marine waters thanU. lactuca.     

苜蓿切叶蜂滞育的诱导因素研究

苜蓿切叶蜂Megachile rotundata(F.)预蛹的滞育主要受光周期和温度的影响。雌蜂感受光周期的变化是决定子代预蛹是否进入滞育的一个主要因素;当代的幼期,特别是高龄幼虫(约3龄)到预蛹这一阶段所感受的温度变化,对预蛹滞育有很大的影响,此期如果环境温度低于(21.59±1.03)℃,可有超过50%的个体进入滞育。此外,放蜂地区的纬度及蜂的代次对预蛹的滞育亦有重要的影响。     

A METHODOLOGICAL COMPARISON OF PHOTOSYNTHETIC OXYGEN EVOLUTION AND ESTIMATED ELECTRON TRANSPORT RATE IN TROPICAL ULVA (CHLOROPHYCEAE) SPECIES UNDER DIFFERENT LIGHT AND INORGANIC CARBON CONDITIONS1

Gross oxygen evolution was compared with the electron transport rate (ETR), estimated from chl a fluorescence parameters on the common tropical green macro alga Ulva fasciata Delile with confirmatory carbon saturation curves from U. reticulata Forskål. Theoretically, the relationship between estimated ETR and gross oxygen evolution should be 4:1, that is, four electrons are transported through PSII for each molecule of oxygen evolved. However, deviations of the 4:1 relationship have previously been reported. Measurements were conducted with two commercially available and portable pulse amplitude modulated (PAM) chl fluorometers. We sought experimental approaches that minimize discrepancies between the two different measuring techniques of photosynthetic rates, both for in situ and laboratory conditions. Using fresh algal tissue for each of the different irradiances gave the best fit of gross oxygen evolution and ETR even at irradiances above light saturation, where large discrepancies between oxygen evolution and ETR are common. With increasing dissolved inorganic carbon (DIC) concentrations, there was a curvilinear response of gross oxygen evolution in relation to ETR. We therefore suggest to establish DIC saturation curves in the laboratory, oxygen evolution is probably the most relevant choice. Photorespiration could not readily explain a curvilinear response of O₂ evolution and proportionally higher ETR at high irradiances. ETRs measured with the rapid light curve function of the PAM were compared with steady‐state rates of gross and net oxygen evolution, and the ETR was found to decrease at higher irradiances whereas oxygen evolution was constant.     

A comparative assessment of molecular marker assays (AFLP, RAPD and SSR) for white yam (Dioscorea rotundata) germplasm characterization

Several DNA‐based marker systems are available for genetic fingerprinting of plants but information on their relative usefulness for yam germplasm characterisation is lacking. The efficiency of RAPD, AFLP and SSR markers for the assessment of genetic relationships, and for cultivar identification and discrimination among 45 West and Central African white yam cultivars belonging to 22 morphotypes/cultivar groups was investigated. Dendrograms were produced based on band pattern scores using the UPGMA method. Results showed that each of the three techniques could unequivocably identify each cultivar, but that techniques differed in the mean number of profiles generated per primer (or primer pair) per cultivar, referred to as genotype index (GI). The order of merit based on this criterion in this study was AFLPs (GI = 2.56), SSRs (GI = 0.39) and RAPDs (GI = 0.35). Yam genotypes classified in the same cultivar group based on morphology were often genetically different, emphasising the need for molecular fingerprinting in yam germplasm characterisation. AFLPs showed the highest efficiency in detecting polymorphism and revealed genetic relationships that most closely reflected morphological classification.     

THE EFFECTS OF PHOSPHITE ON PHOSPHATE STARVATION RESPONSES OF ULVA LACTUCA (ULVALES,CHLOROPHYTA)1

Effects of phosphite (Phi) on phosphate (Pi) starvation responses were determined in Ulva lactuca L. by incubation in Pi‐limited (1 μM NaH₂PO₄) or Pi‐sufficient (100 μM NaH₂PO₄) seawater containing 0–3 mM Phi. Exposure to 1 μM NaH₂PO₄ decreased the growth rate and the content of free Pi and esterified‐P but increased the activities of extracellular alkaline phosphatase (EC 3.1.2.1) and intracellular acid phosphatase (ACP; EC 3.1.2.2); two ACP isozymes observed by activity staining on isoelectric focussing (IEF) gel were induced. The K_m value of Pi uptake rate was decreased by incubation with 1 μM NaH₂PO₄ and the decrease in K_m value was inhibited by 2 mM Phi, reflecting the operation of a high‐affinity Pi uptake system at low Pi concentrations. In the presence of Phi, the growth rate of Pi‐sufficient and Pi‐starved thalli decreased as Phi concentrations were increased from 0 to 2 mM. As Phi concentrations were increased from 0 to 2 mM, the free Pi contents in both Pi‐sufficient and Pi‐starved thalli decreased, but the esterified‐P contents in Pi‐starved thalli increased, whereas those in Pi‐sufficient thalli increased at 1 mM Phi and decreased at 2 mM Phi. Cell wall localized AP activity in both Pi‐sufficient and Pi‐starved thalli decreased as Phi concentrations were increased from 0 to 2 mM. Intracellular ACP activity in Pi‐starved thalli decreased as Phi concentrations were increased from 0 to 2 mM but was not affected in Pi‐sufficient thalli. The induction of ACP isozyme activity and high‐affinity Pi uptake system in Pi‐starved thalli was inhibited by Phi. The present investigation shows that Phi interrupts the sensing mechanisms of U. lactuca to Pi‐limiting conditions.     

UV‐A/BLUE LIGHT–INDUCED REACTIVATION OF SPORE GERMINATION IN UV‐B IRRADIATED ULVA PERTUSA (CHLOROPHYTA)1

Recent reduction in the ozone shield due to manufactured chlorofluorocarbons raised considerable interest in the ecological and physiological consequences of UV‐B radiation (λ=280–315 nm) in macroalgae. However, early life stages of macroalgae have received little attention in regard to their UV‐B sensitivity and UV‐B defensive mechanisms. Germination of UV‐B irradiated spores of the intertidal green alga Ulva pertusa Kjellman was significantly lower than in unexposed controls, and the degree of reduction correlated with the UV doses. After exposure to moderate levels of UV‐B irradiation, subsequent exposure to visible light caused differential germination in an irradiance‐ and wavelength‐dependent manner. Significantly higher germination was found at higher photon irradiances and in blue light compared with white and red light. The action spectrum for photoreactivation of germination in UV‐B irradiated U. pertusa spores shows a major peak at 435 nm with a smaller but significant peak at 385 nm. When exposed to December sunlight, the germination percentage of U. pertusa spores exposed to 1 h of solar radiation reached 100% regardless of the irradiation treatment conditions. After a 2‐h exposure to sunlight, however, there was complete inhibition of germination in PAR+UV‐A+UV‐B in contrast to 100% germination in PAR or PAR+UV‐A. In addition to mat‐forming characteristics that would act as a selective UV‐B filter for settled spores under the parental canopy, light‐driven repair of germination after UV‐B exposure could explain successful continuation of U. pertusa spore germination in intertidal settings possibly affected by intense solar UV‐B radiation.     

HETEROTROPHIC GROWTH OF ULVA LACTUCA (CHLOROPHYCEAE)1

The effect of external glucose (51 mM) and acetate (13 mM) on growth and photosynthetic capacity of Ulva lactuca L. was tested in laboratory cultures over 41 days in the dark and in dim light (0.9 μmol photons·m^?2·s^?1) at 7–8° C. Glucose and acetate had a significant positive effect on growth rate, chlorophyll content, and quantum yield for discs grown in the dark and in dim light. The carbon gain from heterotrophic uptake was low and only allowed U. lactuca to maintain a specific uptake was low and only allowed U. lactuca to maintain a specific growth rate of 0.005 day^?1 compared to 0.06–0.1 day^?1 at higher light intensities. However, plants with added organic substrate maintained a normal chlorophyll content and were able to photosynthesize whereas control plants lost pigmentation and photosynthetic capability after 41 days in both dim light and darkness, probably because of disorganization of the photosynthetic apparatus. This suggest that the ecological significance of heterotrophic uptake is to allow U. lactuca to survive during prolonged low light conditions with an intact photosynthetic apparatus.     

EFFECT OF ROCKY INTERTIDAL AMPHIPODS ON ALGAL RECRUITMENT: A FIELD STUDY1

Evidence about the potential of mobile marine invertebrates to act as algal spore dispersal agents is presently circumstantial. Using a field correlational and experimental protocol, our study tested the hypothesis that amphipods can increase the spore recruitment of the red alga Iridaea laminarioides Bory. Iridaea laminarioides spore recruitment onto glass slides was measured at a site with high amphipod abundance and a site with low density of amphipods. To evaluate the effect of an Ulva canopy on recruitment, replicated glass slides with and without a surrounding Ulva canopy were installed at both sites. The number of I. laminarioides spores recruited on the glass slides was four to eight times higher at the high amphipod abundance site than at the low density site. However, the presence of an Ulva canopy covering the glass slides did not significantly increase the recruitment of I. laminarioides. Because the abundance of I. laminarioides, the proportion of cystocarpic plants, and the percentage of open cystocarps only differed slightly between the low and high abundance amphipod sites, we suggest that the variation in recruitment between the sites is due to the differences in amphipod abundance (and their movements) and not to differences in spore production. Moreover, the presence of I. laminarioides cystocarps showing amphipod grazing scars was significantly higher at the high amphipod density site than at the low density site.     

SEASONAL FLUCTUATIONS IN TISSUE NITROGEN,PHOSPHORUS, AND N:P FOR FIVE MACROALGAL SPECIES COMMON TO THE PACIFIC NORTHWEST COAST1

Variations in tissue nitrogen (N) and phosphorus (P) were examined over a complete seasonal cycle for five macroalgae common in Oregon coastal water. Tissue N ranged from 2.0 to 5.5% dry weight (dry wt) in leafy macroalgae (Enteromorpha intestinalis (Linnaeus) Link. Ulva fenestrata Postels et Ruprecht, and Porphyra sp.) and from 0.9 to 2.6% drt wt in branched macroalgae (Codium fragile (Suringar) Hariot and Pelvetiopsis limitata Setchell). Tissue P ranged from 0.32 to 0.86% dry wt in leafy macroalgae and from 0.27 to 0.50% dry wt in branched macroalgae, Ulva fenestrata, C. fragile, and P. limitata appeared to be N limited during part of the year based on tissue N levels. Variations in N:P showed a more distinct seasonal pattern than either tissue N and tissue P. All macroalgae examined appeared to be N limited at least part of the year based on N:P composition, and P limitation occurred for all macroalgae examined except C. fragile. Our results suggest that tissue N:P ratio for macroalgae may be a good index for evaluating in situ nutrient status.