Productivity and nutrient relationships in psammophytic versus epilithic forms of Bryopsidales (Chlorophyta): comparisons based on a short-term physiological assay

Members of the green algal order Bryopsidales (= Caulerpales) are important calcifying agents of tropical reefs and comprise two fundamentally different life-form groups: (1) epilithic species with limited attachment structures and (2) psammophytic forms that have extensive subterranean rhizoidal systems. Because the shallow-water habitats of the former have relatively low nitrogen (N) to phosphorus (P_i) ratios compared to the pore waters of the sedimentary carbonate-rich substrata in which the latter are anchored, we hypothesized that epilithic forms should tend to be relatively more limited by N, while psammophytic species should tend to show P_i limitation. In partial support of the hypothesis, light-saturated net photosynthesis (P_max) in the epilithic forms, Halimeda opuntia, H. lacrimosa and H. copiosa, tended to be enhanced by N, while P_i was inhibitory or had no effect. In contrast, the psammophytic forms, Udotea sp., U. conglutinata, H. monde, H. tuna and H. simulans, tended to be stimulated more by P_i, whereas N had little effect. The utility of a bioassay to assess macroalgal nutrient limitation, based on a physiological response (net P_max) to short-term nutrient pulses, is demonstrated.     

The deep-water species of Halimeda Lamouroux (Halimedaceae,Chlorophyta) from San Salvador Island,Bahamas: species composition,distribution and depth records

Caribbean species of Halimeda from the steep slopes of San Salvador Island, Bahamas, were collected along vertical transects between 25 and 255 m, on the northeast, northwest, west, southwest and south sides of the island, using the Harbor Branch Oceanographic Institution's submersible Johnson Sea Link I. The characteristics delineating species (segment and utricle size) were assessed for selected species, and the depth and western Atlantic distributions of the species reviewed. Halimeda copiosa was found to show a variation of surface utricle diameter with depth, with the deeper plants having 15% larger diameter utricles than the shallower plants. Of the seven species, one variety and one form of Halimeda studied from our submersible dive sites, H. copiosa, H. cryptica and H. gracilis grew to the greatest depths, from 120 to 150-m depths. H. lacrimosa var. globosa was found growing to 91 m, and H. tuna f. platydisca and H. discoidea to a depth of 73 m. Those of lesser depth, to 61 m, were H. goreauii, H. tuna f. tuna and H. lacrimosa var. lacrimosa. These represent substantial increases in depth distribution of the nine taxa found.     

The impact of Dictyota spp. on Halimeda populations of Conch Reef, Florida Keys

Species of the brown alga Dictyota dominate the reef tract in the Florida Keys. In surveys during summer and fall months between 1994 and 2001, Dictyota occupied as much as 70% of the benthos on Conch Reef. Dictyota spp. were found growing epiphytically on Halimeda tuna, Halimeda opuntia, Lobophora variegata, Galaxura sp., fire coral, hard corals, soft corals, bryozoans and a variety of sponges on Conch Reef. From 1994 to 2001, the percent coverage of Halimeda spp. declined from 15% to 3% on the same reef. In Aug. 1999, 2000 and 2001, on average, 56% of two Halimeda species on Conch Reef had >50% of their thalli covered by Dictyota menstrualis and Dictyota pulchella. To address the impact of Dictyota on Halimeda, short-term growth of unepiphytized and heavily epiphytized (>50% Dictyota cover) H. tuna were compared with unepiphytized H. tuna to which a Dictyota mimic was attached. The number of new segments per plant ranged from 1 to 174 over 9 days. Halimeda thalli with >50% Dicytota cover and thalli covered with Dictyota mimic grew significantly slower than unepiphytized thalli. A second short-term experiment addressed the impact of neighboring Dictyota on the growth and metabolism of unepiphytized H. tuna. Augmenting or clearing epilithic Dictyota around but not in contact with H. tuna had no impact on growth or metabolism of H. tuna. Unepiphytized and heavily epiphytized H. tuna were also collected for studies of metabolism. This work indicated that epiphytic Dictyota negatively impacts metabolic rates of H. tuna in part by shading H. tuna thalli. This negative impact was also in part chemically mediated, as exposure to Dictyota-conditioned water elevated respiration rates in the same manner as seen in the metabolic studies of naturally epiphytized H. tuna.     

Studies on Halimeda

Large areas of the inter-reefal seabed in the Great Barrier Reef are carpeted with vegetation composed almost entirely of the green calcareous alga Halineda. These meadows occur principally in the northern sections between 11°30 and 15°35S at depths of 20 to 40 m, but there are also some in the central and southern sections, where they have been found at depths down to 96 m. The vegetation is dominated by the same sprawling Halimeda species that are common on coral reefs in this region. However, on reefs these species grow on solid substrata, not soft sediments like the Halimeda-rich gravels that underlie the meadows. A total of 12 Halimeda species, together with two Udotea and one Penicillus species, are characteristic components of the shallow meadows. Below 50 m depth, species composition is restricted to only two major components. One, H. copiosa, is also important shallower, but the other is an unusually large and heavily calcified form of H. fragilis, a species that is normally a minor, fragile component of the shallow meadows. The maximum biomass found in these meadows was 4637 gm² of calcareous algae, although the thean for vegetated areas was 525 gm². These meadows are confined to the nutrient-depleted waters of the outer continental shelf just inside the outer barrier reefs, and are usually associated with distinct shoaling of the seabed caused by accumulation of thick deposits of calcareous Halimeda segments. The meadows are probably supported by very localized upwelling of nutrients from the adjacent Coral Sea onto the shelf, where they enrich the otherwise nutrient-depleted waters.Contribution No.367 from the Australian Institute of Marine Science     

Two species of Halimeda, a calcifying genus of tropical macroalgae, are robust to epiphytism by cyanobacteria

Cyanobacteria, an increasingly important epiphyte on macroalgae and seagrass, have been shown to have strong effects on its hosts; this association has been identified as a driving mechanism that maintains algal blooms on coral reefs. We examined both the costs and benefits of epiphytism on 2 algal congeners of Halimeda (H. tuna and H. opuntia), both of which are abundant members of tropical reef communities in the Caribbean. To evaluate potential benefits of an associational defense as well as costs to growth, we manipulated herbivore access to (uncaged/caged) and cyanobacteria presence on (epiphytized/cleaned) 2 species of Halimeda on shallow patch reefs in Belize and measured change in branch length and segment number after 10 (H. tuna) and 5 (H. opuntia) days. Cyanobacterial epiphytes did not serve as an associational defense from herbivory as there were no differences between caged and uncaged treatments for either response variable. The presence of cyanobacterial epiphytes did not affect the growth of branches or net generation of new segments, demonstrating there was also no cost to growth. The robustness of both species of Halimeda to epiphytism contrasts strongly with recent research that found strong effects of epiphytes on several other species of tropical algae. Our results may be attributed to the unique characteristics of Halimeda, a heavily physically and chemically defended algal genus, and the shallow nature of the patch reefs reducing the potential for significant light limitation. These findings suggest that close interactions such as epiphytism may not be as generalizable as originally assumed; studies must consider differences among host species, as this may lead to a better understanding of community-wide effects.     

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.     

VARIABILITY IN THE ECOPHYSIOLOGY OF HALIMEDA SPP. (CHLOROPHYTA,BRYOPSIDALES) ON CONCH REEF,FLORIDA KEYS,USA1

The photosynthetic performance, pigmentation, and growth of a Halimeda community were studied over a depth gradient on Conch Reef, Florida Keys, USA during summer–fall periods of 5 consecutive years. The physiology and growth of H. tuna (Ellis & Solander) Lamouroux and H. opuntia (L.) Lamouroux on this algal dominated reef were highly variable. Maximum rate of net photosynthesis (P_max), respiration rate, and quantum efficiency (α) did not differ between populations of either species at 7 versus 21 m, even though the 21‐m site received a 66% lower photon flux density (PFD). Physiological parameters, as well as levels of photosynthetic pigments, varied temporally. P_max, saturation irradiance, compensation irradiance, and growth were greatest in summer months, whereas α, chl a, chl b, and carotenoid concentrations were elevated each fall. Halimeda tuna growth rates were higher at 7 m compared with 21 m for only two of five growth trials. This may have arisen from variability in light and nutrient availability. Individuals growing at 7 m received a 29% greater PFD in August 2001 than in 1999. In August 1999 and 2001 seawater temperatures were uniform over the 14‐m gradient, whereas in August 2000 cold water regularly intruded upon the 21‐m but not the 7‐m site. These results illustrate the potentially dynamic relationship between nutrients, irradiance, and algal productivity. This suggests the necessity of long‐term monitoring over spatial and temporal gradients to accurately characterize factors that impact productivity.     

Parallel control of the inward-rectifier K+ channel by cytosolic free Ca2+ and pH inVicia guard cells

The influence of cytosolic pH (pH_i) in controlling K⁺-channel activity and its interaction with cytosolic-free Ca²⁺ concentration ([Ca²⁺]_i) was examined in stomatal guard cells ofVicia faba L. Intact guard cells were impaled with multibarrelled microelectrodes and K⁺-channel currents were recorded under voltage clamp while pH_i or [Ca²⁺]_i was monitored concurrently by fluorescence ratio photometry using the fluorescent dyes 2,7-bis (2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF) and Fura-2. In 10 mM external K⁺ concentration, current through inward-rectifying K⁺ channels (I_K,in) was evoked on stepping the membrane from a holding potential of –100 mV to voltages from –120 to –250 mV. Challenge with 0.3-30 mM Na+-butyrate and Na⁺-acetate outside imposed acid loads, lowering pH_i from a mean resting value of 7.64 ± 0.03 (n = 25) to values from 7.5 to 6.7. The effect on pH_i was independent of the weak acid used, and indicated a H⁺-buffering capacity which rose from 90 mM H⁺/pH unit near 7.5 to 160 mM H⁺/pH unit near pH_i 7.0. With acid-going pH_i, (I_K,in) was promoted in scalar fashion, the current increasing in magnitude with the acid load, but without significant effect on the current relaxation kinetics at voltages negative of –150 mV or the voltage-dependence for channel gating. Washout of the weak acid was followed by transient rise in pH_i lasting 3–5 min and was accompanied by a reduction in (I_K,in) before recovery of the initial resting pH_i and current amplitude. The pH_i-sensitivity of the current was consistent with a single, titratable site for H⁺ binding with a pK_a near 6.3. Acid pH_i loads also affected current through the outward-rectifying K⁺ channels (I_K,out) in a manner antiparallel to (I_K,in) The effect on I_K, out was also scalar, but showed an apparent pK_a of 7.4 and was best accommodated by a cooperative binding of two H⁺. Parallel measurements showed that Na⁺-butyrate loads were generally without significant effect on [Ca²⁺]_i, except when pH_i was reduced to 7.0 and below. Extreme acid loads evoked reversible increases in [Ca²⁺]_i in roughly half the cells measured, although the effect was generally delayed with respect to the time course of pH_i changes and K⁺-channel responses. The action on [Ca²⁺]_i coincided with a greater variability in (I_K,in) stimulation evident at pH_i values around 7.0 and below, and with negative displacements in the voltage-dependence of (I_K,in) gating. These results distinguish the actions of pH_i and [Ca²⁺]_i in modulating (I_K,in) they delimit the effect of pH_i to changes in current amplitude without influence on the voltage-dependence of channel gating; and they support a role for pH_i as a second messenger capable of acting in parallel with, but independent of [Ca²⁺]_i in controlling the K⁺ channels.Abbreviations BCECF 2,7-bis (2-carboxyethyl)-5(6)-carboxy fluorescein - [Ca²⁺]_i cytosolic free Ca²⁺ concentration - g_K ensemble (steady-state) K⁺-channel conductance - I_K,out, I_K,in outward-, inward-rectifying K⁺ channel (current) - IN current-voltage (relation) - Mes 2-(N-morpholinolethanesulfonic acid - pH_i cytosolic pH - V membrane potential     

Imidazolium formation from the reaction of N-heterocyclic carbene stabilised group 13 trihydride complexes with organic acids

The reactions of the N-heterocyclic carbene (NHC) stabilised group 13 trihydride complexes [AlH₃(IMeMe)] (1) (IMeMe = 1,3,4,5-tetramethylimidazol-2-ylidene), [AlH₃(IⁱPrMe)] (2) (IⁱPrMe = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) with three molar equivalents of phenol, and [InH₃(IMes)] (3) (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazole-2-ylidene) with one molar equivalent of 1,1,1,5,5,5-hexafluoropentan-2,4-dione (F₆acacH) are presented. These render the imidazolium tetraphenoxyaluminate species; [IMeMe · H][Al(OPh)₄] (4) and [IⁱPrMe · H][Al(OPh)₄] (5), and 1,3-bis(2,4,6-trimethylphenyl)imidazolium 1,1,1,5,5,5-hexafluoropentan-2,4-dionate; [IMes · H][CH{C(O)CF₃}₂] (6), the latter leading to metallohydride decomposition. The molecular structures of 4 and 6 are described.     

HALIMEDA HUMMII SP. NOV., HALIMEDA CRYPTICA V. ACERIFOLIA VAR. NOV. (CAULERPALES,CHLOROPHYTA), AND ADDITIONAL RECORDS OF HALIMEDA SPECIES FROM: PUERTO RICO1

A new species, Halimeda hummii, and a new variety of Halimeda cryptica Colinvaux and Graham both originally collected from the edge of the continental shelf on the southwest coast of Puerto Rico are newly described. The new species is irregular in its segment morphology and is the smallest species of Halimeda presently known. Halimeda cryptica var. acerifolia from deep water possesses distinctive segments resembling maple leaves. Halimeda copiosa Goreau and Graham and typical H. cryptica are also collected in deep water and are newly recorded from Puerto Rico.     

Modifications of current properties by expression of a foreign potassium channel gene in Xenopus embryonic cells

The pH-sensitivity of transepithelial K⁺ transport was studied in vitro in isolated vestibular dark cell epithelium from the gerbil ampulla. The cytosolic pH (pH _iwas measured microfluorometrically with the pH-sensitive dye 2,7-bicarboxyethyl-5(6)-carboxyfluorescein (BCECF) and the equivalent short-circuit current (I _sc), which is a measure for transepithelial K⁺ secretion, was calculated from measurements of the transepithelial voltage (V _t)and the transepithelial resistance (R _t) in a micro-Ussing chamber. All experiments were conducted in virtually HCO ₃ ^– -free solutions. Under control conditions, pH _iwas 7.01±0.04 (n=18), V _twas 9.1±0.5 mV, R _t16.7±0.09 cm², and I _sc was 587±30 A/cm² (n=49). Addition of 20 mm propionate^– caused a biphasic effect involving an initial acidification of pH _i, increase in V _tand I _sc and decrease in R _tand a subsequent alkalinization of pH _i, decrease of V _tand increase of R _t. Removal of propionate^– caused a transient effect involving an alkalinization of pH _i, a decrease of V _tand I _sc and an increase in R _t. pH _iin the presence of propionate^– exceeded pH _iunder control conditions. Effects of propionate ^– on V _t, R _tand I _sc were significantly larger when propionate^– was applied to the basolateral side rather than to the apical side of the epithelium. The pH _i-sensitivityof I _sc between pH 6.8 and 7.5 was –1089 A/(cm² · pH-unit) suggesting that K⁺ secretion ceases at about pH _i7.6. Acidification of the extracellular pH (pH _o)caused an increase of V _tand I _sc and a decrease of R _tmost likely due to acidification of pH _i. Effects were significantly larger when the extracellular acidification was applied to the basolateral side rather than to the apical side of the epithelium. The pH _osensitivity of I _sc between pH 7.4 and 6.4 was –155 A/(cm² · pH unit). These results demonstrate that transepithelial K⁺ transport is sensitive to pH _iand pH _oand that vestibular dark cells contain propionate^– uptake mechanism. Further, the data suggest that cytosolic acidification activates and that cytosolic alkalinization inactivates the slowly activating K⁺ channel (I _sK)in the apical membrane. Whether the effect of pH _ion the I _sK channel is a direct or indirect effect remains to be determined.The authors wish to thank Drs. Daniel C. Marcus, Zhjiun Shen and Hiroshi Sunose for helpful discussions. This work was supported by grants NIH-R29-DC01098 and NIH-R01-DC00212.     

Halimeda biomass,growth rates and sediment generation on reefs in the central great barrier reef province

The average biomass ofHalimeda per m² of solid substratum increased progressively on a series of reefs situated at increasing distances from the shore in the central Great Barrier Reef. There was none on a reef close inshore, increasing to nearly 500 g m^?2 total biomass (?90% calcium carbonate) on an oceanic atoll system in the Coral Sea. The biomass measured contained 13 species ofHalimeda but was dominated by only two species,H. copiosa andH. opuntia, except on the atoll whereH. minima was dominant. Three sand-dwelling species were also present but did not occur anywhere in substantial quantities. Growth rates of the dominant species were measured bv tagging individual branch tips. A mean value of 0.16 segments d^?1 was recorded but 41% of the branch tips did not grow any new segments whilst only 1% grew more than one per day. The number of branch tips per unit biomass was very constant and has been used in conjunction with growth rates and biomass to calculate productivity rates, and thence sedimentation, in the lagoon of one of the reefs. Biomass doubling time of 15 d and production of 6.9 g dry wt m^?2 d^?1 are considerably higher than previously reported values forHalimeda vegetation and there was little seasonal change detected over a whole year. Those values indicate annual accretion of 184.9 g m^?2 year^?1 ofHalimeda segment debris over the entire lagoon floor (5.9 km²) of Davies Reef, equivalent to 0.13 mm year^?1 due toHalimeda alone, or 1 m every 1,892 years when other contributions to that sediment are taken into account.     

Identification and functional reconstitution of phosphate: Sugar phosphate antiport ofStaphylococcus aureus

Summary Resting cells ofStaphylococcus aureus displayed a phosphate (P_i) exchange that was induced by growth with glucose 6-phosphate (G6P) orsn-glycerol 3-phosphate (G3P). P_i-loaded membrane vesicles from these cells accumulated³²P_i, 2-deoxyglucose 6-phosphate (2DG6P) or G3P by an electroneutral exchange that required no external source of energy. On the other hand, when vesicles were loaded with morpholinopropane sulfonic acid (MOPS), only transport of³²P_i (andl-histidine) was observed, and in that case transport depended on addition of an oxidizable substrate (dl-lactate). In such MOPS-loaded vesicles, accumulation of the organic phosphates, 2DG6P and G3P, could not be observed until vesicles were preincubated with both P_i anddl-lactate to establish an internal pool of P_i. Thistrans effect demonstrates that movement of 2DG6P or G3P is based on an antiport (exchange) with internal P_i.Reconstitution of membrane protein allowed a quantitative analysis of P_i-linked exchange. P_i-loaded proteoliposomes and membrane vesicles had comparable activities for the homologous³²P_iP_i exchange (K _i's of 2.2 and 1.4mm;V _max's of 180 and 83 nmol P_i/min per mg protein), indicating that the exchange reaction was recovered intact in the artificial system. Other work showed that heterologous exchange from either G6P- or G3P-grown cells had a preference for 2DG6P (K _i=27 m) over G3P (K _i=1.3mm) and P_i (K _i=2.2mm), suggesting that the same antiporter was induced in both cases. We conclude that³²P_iP_i exchange exhibited by resting cells reflects operation of an antiporter with high specificity for sugar 6-phosphate. In this respect, P_i-linked antiport inS. aureus resembles other examples in a newly described family of bacterial transporters that use anion exchange as the molecular basis of solute transport.     

Na+-independent,electrogenic Cl- uptake across the posterior gills of the Chinese crab (Eriocheir sinensis): Voltage-clamp and microelectrode studies

Summary Single gill lamellae from posterior gills of Chinese crabs (Eriocheir sinensis) were isolated, separated into halves and mounted in a modified Ussing chamber. Area-related short-circuit current (I_sc) and conductance (G_tot) of this preparation were measured. Epithelial cells were impaled with microelectrodes through the basolateral membrane and cellular potentials (V_i under open- and V_sc under short-circuit conditions) as well as the voltage divider ratios (F_i, F_o) were determined.With NaCl salines on both sides an outside positive PD_te (22±2 mV) and an I_sc (-64±13 A·cm^-2) with a polarity corresponding to an uptake of negative charges (inward negative) were obtained. Trough-like potential profiles were recorded across the preparation under open- as well as short-circuit conditions (V_o=-101±5 mV, external bath as reference; V_i=-78±2 mV, internal bath as reference; V_sc=-80±2 mV, extracellular space as reference). The voltage divider ratios of the external (apical membrane plus cuticle) and internal (basolateral membrane) barrier were F_o=0.92±0.01 and F_i=0.08±0.01, respectively. To investigate a Cl^--related contribution to the above parameters, Na⁺-free solutions in the external bath (basolateral NaCl-saline) were used. Inward negative I_sc under these conditions almost completely depended on external Cl^-. Elimination of Cl^- in the external bath reversed I_sc, and G_tot decreased substantially. Concomitantly, V_sc depolarised and F_o increased. Cl^--dependent current and conductance showed saturation kinetics with increasing external [Cl^-]. Addition of 20 mmol·1^-1 thiocyanate to the external bath had similar, although less pronounced, effects as Cl^- substitution. Equally, external SITS (1 mmol·1^-1) inhibited the current and, concomitantly, G_tot decreased substantially. Addition of 1 mmol·1^-1 acetazolamide to, and omission of NaHCO₃ from, the basolateral bath resulted in a decrease of I_sc while G_tot remained unchanged. The Cl^--channel blocker DPC inhibited I_sc almost completely when added to the basolateral saline, whereas G_tot decreased moderately; however, V_sc depolarised without significant change of F_i. Ouabain had no influence on I_sc and G_tot. Increasing the basolateral [K⁺] resulted in a decrease in I_sc, while G_tot was not affected. At the same time V_sc largely depolarised and F_i decreased. Addition of the K⁺-channel blocker Ba⁺⁺ (5 mmol·1^-1) to the basolateral solution resulted in a two-step alteration of the transepithelial (I_sc, Gtot) and cellular (V_sc, F_i) parameters. The results are discussed with regard to (i) the mechanisms responsible for active transbranchial Cl^- uptake, and (ii) the technical improvement of being able to perform transport studies with crab gill preparations in an Ussing chamber.Abbreviations DMSO dimethylsulfoxide - DPC diphenylamine-2-carboxylate - F _{o, i} voltage divider ratio for external (o) and internal (i) barrier, respectively - G _Cl conductance related to the external [Cl^-] - G _tot total tissue conductance - I _Cl short-circuit current related to the external [Cl^-] - I _sc short-circuit current - PD _te transepithelial potential difference - R _ME resistance of the microelectrode - SITS 4-acetamido-4-isothiocyanato-stilbene-2,2-disulfonic acid - V _{o, i} open-circuit voltage across the external (o) and internal (i) barrier, respectively - V _sc intracellular potential under short-circuit conditions     

Asexual propagation in the coral reef macroalga Halimeda (Chlorophyta, Bryopsidales): production, dispersal and attachment of small fragments

Siphonous, green macroalgae of the genus Halimeda are ubiquitous and ecologically important in tropical and subtropical marine environments. It has been hypothesized that the abundance of Halimeda on coral reefs is in part due to the ability of this genus to propagate asexually via vegetative fragmentation. However, vegetative fragmentation has only been documented for H. discoidea in a laboratory setting. To test the hypothesis that vegetative fragmentation contributes to field populations of Halimeda, we examined three aspects of fragmentation by H. tuna (Ellis and Solander) Lamouroux, H. opuntia (Linneaus) Lamouroux and H. goreaui Taylor on Conch Reef in the Florida Keys: (1) short-term (8 days) and long-term (14 weeks) fragment survival and rhizoid production in the laboratory and field (7 and 21 m), (2) size of the fragment pool and (3) influences of herbivory and water motion on production and dispersal of fragments. Although morphologically similar to H. discoidea, only a small percentage of H. tuna fragments survived. Fragments of H. opuntia and H. goreaui were more robust, and survival and rhizoid production were positively correlated with size in short-term trials. In 14-week field trials, one-third or fewer fragments of any species survived at 7 m, potentially because fragments were covered by large amounts of sediment. Survivors included some buried, seemingly dead individuals that turned green when exposed to light, highlighting the remarkable ability of this genus to survive disturbances. There was much less sediment accumulation at 21 m, where more fragments survived. Most (93%) eight-segment fragments of H. opuntia produced attachment rhizoids by the end of the 14-week trial. Overall, a range of 4.7-9.4 fragments of Halimeda m⁻² day⁻¹ were found on Conch Reef; most fragments were generated by H. goreaui. Fish bite marks were evident on 75-85% of the individuals of H. tuna and the number of bites per thallus ranged from 1 to 23. Herbivorous reef fish commonly fed on all three species of Halimeda. Some fish consumed the biomass, while others rejected most bites. For example, 83% of bites were rejected by the blue-striped grunt. Dispersal distances for rejected bites ranged from 0 to 31 m. Water motion was also responsible for fragment dispersal; experimentally produced fragments moved up to 48 cm day⁻¹. Results presented here suggest that asexual propagation of fragments of Halimeda is an important component of the life-history of this genus and vegetative fragmentation contributes to the abundance of this genus on coral reefs.     

Light-Related Photosynthetic Characteristics of Lotic Macroalgae

Photosynthetic characteristics in response to irradiance were analysed in 42 populations of 33 macroalgal species by two distinct techniques (chlorophyll fluorescence and oxygen evolution). Photosynthesis–irradiance (PI) curves based on the two techniques indicated adaptations to low irradiance reflected by low saturation values, high to moderate values of photosynthetic efficiency (α) and photoinhibition (β), for Bacillariophyta and Rhodophyta, which suggests they are typically shade-adapted algae. In contrast, most species of Chlorophyta were reported as sun adapted algae, characterized by high values of I _k and low of α, and lack of or low photoinhibition. Cyanophyta and Xanthophyta were intermediate groups in terms of light adaptations. Photoinhibition was observed in variable degrees in all algal groups, under field and laboratory conditions, which confirms that it is not artificially induced by experimental conditions, but is rather a common and natural phenomenon of the lotic macroalgae. Low values of compensation irradiance (I _c) were found, which indicate that these algae can keep an autotrophic metabolism even under very low irradiances. High ratios (>2) of photosynthesis/respiration were found in most algae, which indicates a considerable net gain. These two physiological characteristics suggest that macroalgae may be important primary producers in lotic ecosystems. Saturation parameters (I _k and I _s) occurred in a relatively narrow range of irradiances (100–400 μmol photons m^?2s^?1), with some exceptions (higher in some filamentous green algae or lower in red algae). These parameters were way below the irradiances measured at collecting sites for most algae, which means that most of the available light energy was not photochemically converted via photosynthesis. Acclimation to ambient PAR was observed, as revealed by lower values of I _k and I _cand higher values of α and quantum yield in algae from shaded streams, and vice versa. Forms living within the boundary layer (crusts) showed responses of shade-adapted species and had the highest values of P _max, α and quantum yield, whereas the opposite trend was observed in gelatinous forms (colonies and filaments). These results suggests adaptation to the light regime rather than functional attributes related to the growth form.     

Identifying populations useful for improving parents of a single cross based on net transfer of alleles

Summary Theory and methods for identifying populations (P _y ) with the highest frequency of favorable dominant alleles not present in an elite single cross (I ₁× I ₂) have been developed recently. During selection, new favorable alleles can be transferred from P _y to either I ₁ or I ₂ only at the risk of losing favorable alleles already present in the single cross. A net improvement (NI) statistic, which estimates the relative number of favorable alleles that can be gained from P _y minus the relative number of favorable alleles that can be lost from I ₁ or I ₂, is presented. NI is calculated as maximum [(I ₁×P _y –I ₁×I ₂)/2,(I ₂×P _y –I ₁×I ₂)/2]. Because I ₁ × I ₂ is constant in an experiment, the method reduces to choosing P _y populations with the best mean performance in combination with either I ₁ or I ₂. For a set of maize (Zea mays L.) grain yield data, NI was highly correlated to three other statistics proposed for choosing populations, namely: (1) minimally biased estimate (l ) of the relative number of favorable dominant alleles present in P _y but not in I ₁ and I ₂; (2) minimum upper bound on l ; and (3) predicted performance of the three-way cross [P _y (I ₁× I ₂)]. While l estimates potential improvement likely to be achieved only through long-term recurrent selection, NI is probably a better predictor of short-term improvement in single-cross performance.A contribution from Lifaco Genetics, a subsidiary of Groupe Limagrain     

31P nuclear magnetic resonance study on changes in phosphocreatine and the intracellular pH in rat skeletal muscle during exercise at various inspired oxygen contents

We measured ATP, phosphocreatine (PCr), inorganic phosphate (P_i), and the intracellular pH in rat hindlimb muscles during submaximal isometric exercise with various O₂ deliveries using³¹P nuclear magnetic resonance spectroscopy (³¹P NMR) to evaluate changes in energy metabolism in relation to O₂ availability. Delivery of O₂ to muscles was altered by controlling the fractional concentration of inspired oxygen (F _IO₂) at 0.50, 0.28, 0.21, 0.11 and 0.08 with monitoring partial pressure of oxygen and carbon dioxide, and bicarbonate at the femoral artery. The steady-state ratio of PCr : (PCr + P_i) during exercise decreased as a function ofF _IO₂ even at 0.21. Significant acidification of the intracellular pH during exercise occurred at 0.08F _IO₂. Change in the PCr : (PCr + P_i) ratio demonstrated that the oxidative capacity, i.e. the maximal rate of the oxidative phosphorylation reaction, in muscle was not limited by O₂ delivery at 0.50F _IO₂, but was significantly limited at 0.21F _IO₂ or below. Change in the intracellular pH at 0.08F _IO₂ could be interpreted as an increase in lactate, suggesting activation of glycolysis. Correlation between the PCr : (PCr + P_i) ratio and the intracellular pH revealed the existence of a critical PCr : (PCr + P_i) ratio and pH for glycolysis activation at around 0.4 and 6.7, respectively.     

Electrogenic 2 Na+/1 H+ exchange in crustanceans

Summary Hepatopancreatic brush border membrane vesicles of the freshwater prawn,Macrobrachium rosenbergii and the marine lobster,Homarus americanus exhibited²²Na uptake which was Cl-independent, amiloride sensitive, and stimulated by a transmembrane H gradient (H _i >H _o ). Sodium influx by vesicles of both species were sigmoidal functions of [Na] _o , yielding Hill coefficients that were not significantly different (P>0.5) than 2.0. Estimations of half-saturation constants (K _Na) were 82.2mm (prawn) and 280.1mm (lobster), suggesting a possible adaptation of this transporter to environmental salinity.Trans-stimulation andcis-inhibition experiments involving variable [H] suggested that the exchangers in both species possessed single internal cation binding sites (pK 6.5–6.7) and two external cation binding sites (prawn, pK 4.0 and 5.7; lobster pK 3.5 and 6.1). Similarcis inhibition studies using amiloride as a competitive inhibitor of Na uptake supported the occurrence of dual external sites (prawn,K _i 50 and 1520 m; lobsterK _i 9 and 340 m). Electrogenic Na/H exchange by vesicles from both crustaceans was demonstrated using equilibrium shift experiments where a transmembrane potential was used as the only driving force for the transport event. Transport stoichiometries of the antiporters were determined using Static Head analysis where driving forces for cation transfer were balanced using a 101 Na gradient, a 1001 H gradient, and a stoichiometry of 2.0. These electrogenic 2 Na/1 H exchangers appear thermodynamically capable of generating sufficient gastric acidification for organismic digestive activities.