SETTLEMENT AND SURVIVAL OF POLYSIPHONIA LANOSA (CERAMIALES) SPORES ON ASCOPHYLLUM NODOSUM AND FUCUS VESICULOSUS (FUCALES)1

The settlement patterns of spores of Polysiphonia lanosa (L.) Tandy on Ascophyllum nodosum (L.) Le Jolis and Fucus vesiculosus L. were studied using a flow tank. Settlement sites were defined as ‘sheltered’ or ‘exposed.’ Surface area calculations revealed non-random settlement on A. nodosum, with higher than expected spore frequencies on the thallus and lateral pits and lower than expected frequencies on the vesicles. Settlement of F. vesiculosus was random and significantly lower than on A. nodosum. On the shore, survival of sporelings from September (post-sporulation) to May (pre-sporulation) was highly non-random on both basiphytes. On A. nodosum, lateral pits ('sheltered') showed the highest survival frequency. Here the proportion of surviving sporelings increased over the study period, whereas the proportion on open thallus area ('exposed') decreased. On F. vesiculosus also preferential survival occurred on ‘sheltered’ sites such as vesicle/thallus interfaces and wounds. Between September and May, all P. lanosa sporelings were lost from ‘exposed’ areas (thallus surface and vesicles). Overall, frequencies of surviving sporelings were much greater on A. nodosum than on F. vesiculosus. These results are discussed with reference to basiphyte morphology, epiphyte removal mechanisms and the survival stratagy of P. lanosa.     

ROLE OF SURFACE WOUNDS AND BROWN ALGAL EPIPHYTES IN THE COLONIZATION OF ASCOPHYLLUM NODOSUM (PHAEOPHYCEAE) FRONDS BY VERTEBRATA LANOSA (RHODOPHYTA)1

Ascophyllum nodosum (L.) Le Jol. forms extensive beds in wave‐sheltered, rocky intertidal habitats on the northwestern Atlantic coast. This fucoid seaweed is host to an obligate red algal epiphyte, Vertebrata lanosa (L.) T. A. Chr. [=Polysiphonia lanosa (L.) Tandy], and two facultative brown algal epiphytes, Elachista fucicola (Velley) Aresch. and Pylaiella littoralis (L.) Kjellm. Although V. lanosa can occur throughout most of the length of host fronds, it largely predominates in midfrond segments. The two brown algal epiphytes are restricted to distal segments. Through field experiments conducted in Nova Scotia, Canada, we tested the hypothesis that surface wounds are required for the colonization of distal segments of host fronds by V. lanosa. Distal tissues normally have a smooth surface because of their young age (A. nodosum fronds grow apically). By creating small wounds that mimicked grazing wounds distributed elsewhere on host fronds, we demonstrated that V. lanosa can colonize distal frond segments during the growth and reproductive season (summer and autumn). Approximately half of the artificial wounds were colonized by V. lanosa during this time. The experimental exclusion of both brown algal epiphytes from distal frond segments did not affect colonization by V. lanosa. Thus, we conclude that the absence of surface irregularities on distal segments of host fronds, specifically small wounds, is the main factor explaining the absence of V. lanosa there. We propose that further experimental work clarifying epiphyte distribution in host beds will enhance our ability to understand the functional role of epiphytes in intertidal ecosystems.     

THE DISTRIBUTION OF PHOTOSYNTHATE IN ASCOPHYLLUM NODOSUM AS IT RELATES TO EPIPHYTIC POLYSIPHONIA LANOSA12

The brown alga Ascophyllum nodosum is not a major source of organic carbon for its epiphytic red alga Polysiphonia lanosa. Plants pulse-labeled for 24–25 hr with NaH¹⁴CO₃ were examined for exudation and translocation. The maximum amount of radioactive carbon compounds lost from A. nodosum during this experimental period was less than 0.3% of the total ¹⁴C fixed by the alga, and of this amount, only 5% could, have moved through the frond. The remaining fraction of the ¹⁴C lost from the thallus was released into the water. The total exudate from A. nodosum was collected for 1 week in a series of flasks of filtered seawater changed at 12-hr intervals corresponding with the beginning of the light and dark periods, respectively. During 7 days at 15 C only 1.5% of the total ¹⁴C originally fixed had been released as radioactive organic carbon, whereas at 5 C, 0.6% of the total ¹⁴C fixed was found in the medium. No significant difference in the rate of exudation of organic ¹⁴C was observed in light or darkness. After fractionation of the exudate it was found that only 10% of the radioactivity in these exudates was composed of organic acids or amino acids. P. lanosa, on the other hand, is perfectly capable of fixing its own carbon. The photosynthetic rates measured by H¹⁴CO₃ uptake confirm the observations of Bidwell: 3.96 mg CO₂/g/hr (0.09 m mole/g/hr).     

Sorbitol-1-phosphate and sorbitol-6-phosphate in apricot leaves

Sorbitol-1-phosphate and sorbitol-6-phosphate were isolated from Prunus armeniaca leaves that had been labelled with ¹⁴C by photosynthesis in ¹⁴CO₂. Each hexitol phosphate was present at ca 7 μmol/kg fr. wt in the tissue and formed ca 4% of the hexose monophosphate fraction. ¹⁴C-specific activity measurements suggest that each hexitol monophosphate is formed from a hexose monophosphate, and that one or other could be an intermediate in photosynthesis of sorbitol from CO₂.     

Diel changes in the alkaline phosphatase activity of bacteria and phytoplankton in the hypereutrophic Villerest reservoir (Roanne,France)

The diel changes of the size fractioned alkaline phosphatase activity (APA) were studied in relation to several abiotic and biotic factors in Villerest reservoir (located on the Loire river, near the city of Roanne, France), bihourly during two days in July 1992. The APA measured in this work exceeded considerably those reported in the literature, suggesting that dissolved mineral phosphorus was not available to microorganisms. At 1 m, the APA was primarily due to bacteria which actively assimilated organic P compounds released by photosynthetic algal metabolism. At 5, 10 and 20 m, the APA was predominantly algal. The high concentrations in SRP (soluble reactive phosphorus) would indicate that orthophosphates were not bioavailable. The reverse (i. e availability to phytoplankton) would have resulted in undetectable levels of P-PO _inf4 ^sup3– due to the massive proliferation of algae in Villerest reservoir.     

HOT WATER EXTRACTABLE PHOSPHORUS—AN INDICATOR OF NUTRITIONAL STATUS OF PERIDINIUM CINCTUM (DINOPHYCEAE) FROM LAKE KINNERET (ISRAEL)?1

The intracellular levels of hot water extractable and total phosphorus were determined in the dinoflagellate Peridinium cinctum. f. westii (Lemm.) Lef. for natural samples from the bloom in Lake Kinneret and from laboratory cultures. Amounts of phosphorus (P) in the hot water fraction, relative to total cellular phosphorus, were similar in lake Peridinium and in cells grown in high ambient orthophosphate (P_i) media (3–6 mg P · l^?1). The absolute amounts of hot water extractable P in natural cell and those cultured at lower P_i concentrations (0.02–0.05 mg P · 1^?1) were similar, although average P_i in lake water were 4 μg · l^?1. Under most growth conditions the hot water extract contained approximately equal amounts of molybdate reactive phosphorus (MRP) and non-MRP. Short chain (6–9 units) polyphosphates (mol wt 630–950) probably constituted the bulk of the non-MRP pool, which was hydrolysable by alkaline phosphatase and may serve as a precursor for a more permanent P store. Intracellular P levels and distribution were not directly dependent on external P_i concentrations but may be determined by the N:P atomic ratio or overall external ionic milieu. Peridinium grown in low ambient P_i released significant amounts of non-MRP compounds. In Lake Kinneret, for at least most of the bloom period, Peridinium does not appear to be limited by P supply.     

Changes in 32P-phosphorus compounds during translocation in Laminaria digitata (L.) lamouroux

³²P was applied to a Laminaria digitata thallus and the pattern of ³²P phosphorylated compounds was studied, as a function of time, in the different tissues involved in translocation, i.e. source, pathway and sinks. The results showed that, 3 hours after absorption by the uptake region (lamina), the bulk of the radioactivity was incorporated into organic compounds (70 to 80% of total ³²P taken up), hexose monophosphates being the heaviest labelled. Further change in that region was marked by an accumulation of ³²P in the inorganic pool (65 to 70% after 13 days). Conversely, the ³²P pattern in the medulla of the stipe, which initially showed a similar pattern to the uptake region, did not vary during translocation. The pattern of ³²P distribution into sinks (growing stipe peripheral tissue or hapteron) leads to accumulation of the radioactive element in inorganic and acid-insoluble fractions. These results are discussed in terms of comparative distribution of ³²P in the different parts of the thallus and suggest that phosphate moves as Pi in that alga.Abbreviations TCA trichloroacetic acid - Po organic phosphate - Po sol acid-soluble organic phosphate fraction - Po insol acidinsoluble organic phosphate fraction - Pi morganic phosphate fraction - P lip lipidic phosphate - Np protein nitrogen - ATP adenosine triphosphate - ADP adenosine diphosphate - PEP phosphoenolpyruvic acid - PGA phosphoglyceric acid - G-1-P glucose-1-phosphate - G-6-P glucose-6-phosphate - UDPG uridine diphosphoglucose     

Thymidine Phosphorylation in Arachis hypogaea by Combined Activities of Adenosine Triphosphate Phosphohydrolase and Nucleoside Phosphotransferase

Extracts from hypocotyls of germinating peanuts (Arachis hypogaea L.) stimulated the formation of thymidine monophosphate from thymidine and adenosine triphosphate in the presence of magnesium ions. Such extracts were incapable of incorporating isotopic phosphorus from gamma-labelled adenosine triphosphate into thymidine during the synthesis of thymidine monophosphate but were competent in transferring phosphorus from alpha-labelled adenosine triphosphate to thymidine. The apparent thymidine kinase activity thus appeared to result from the combined activities of an adenosine triphosphatase (adenosine triphosphate pyrophosphohydrolase, E. C. 3.6.1.3) and a nucleoside phosphotransferase (E. C. 2.7.1.77). The latter two enzymes could be largely separated by using DEAE-Sephadex chromatography.     

Research note: Field evaluation of epiphyte recruitment (Vertebrata lanosa,Rhodophyta) in different microsite types on host fronds (Ascophyllum nodosum,Phaeophyceae)

The seaweed Ascophyllum nodosum (Phaeophyceae, Fucales) often forms extensive beds in wave‐sheltered, rocky intertidal habitats on northern Atlantic shores. Since this alga is an ecosystem engineer that influences benthic biodiversity, it is important to understand the factors that regulate its performance. Epiphytism is known to affect the performance of macroalgal hosts. In this study, we investigated the effects of surface irregularities on A. nodosum fronds (wounds, branch axils, and lateral pits resulting from receptacle shedding) on the recruitment rate of its obligate epiphyte Vertebrata lanosa (=Polysiphonia lanosa, Rhodophyta, Ceramiales). For this purpose, we performed a field experiment in Nova Scotia, Canada. In June–July 2007, we created wounds on the surface of host fronds that mimicked the wounds that result from invertebrate grazing. At that time, we also mapped the position of epiphyte‐free lateral pits and branch axils on host fronds. In October 2007, after the reproductive season for V. lanosa, the percentage of microsites colonized by this epiphyte was statistically similar for wounds and lateral pits, but significantly lower for branch axils, although by a small difference (mean recruitment rates ranged between 37 and 50%). Since V. lanosa is known not to colonize smooth frond surfaces, our study suggests that the degree of epiphyte load on A. nodosum beds should largely be affected by the overall amount of surface irregularities, with little influence of the relative availability of each microsite type.     

DISTRIBUTION OF ALGAL EPIPHYTES ACROSS ENVIRONMENTAL GRADIENTS AT DIFFERENT SCALES: INTERTIDAL ELEVATION,HOST CANOPIES,AND HOST FRONDS1

Understanding epiphyte distribution in coastal communities is important because these organisms affect many others directly or indirectly. Yet, their distribution has been considerably less studied than that of their hosts and other primary‐space holders. Identifying major sources of variation in epiphyte abundance is thus still a need. Environmental gradients help predict species distribution and are pervasive on marine shores. In this study, we test the notion that environmental gradients across intertidal elevation, throughout host canopies, and along host fronds explain a large variation in the abundance of sympatric epiphytes. Our model system was the assemblage of Ascophyllum nodosum (L.) Le Jol. and its epiphytes Vertebrata lanosa (L.) T. A. Chr. [= Polysiphonia lanosa (L.) Tandy], Elachista fucicola (Velley) Aresch., and Pylaiella littoralis (L.) Kjellm. On the coast of Nova Scotia (Canada), we found evidence of a spatial segregation among these species at almost all scales. While the red epiphyte V. lanosa was more common at high‐ and midintertidal elevations (peaking at midelevations) and on middle segments of host fronds, the brown epiphytes E. fucicola and P. littoralis were more common at low elevations and restricted to distal segments of host fronds. Canopy habitat affected abundance only for V. lanosa, which was more common within the host canopy than on its periphery at midelevations. Since the studied gradients are related to predictable changes in abiotic factors, the identification of likely causes behind the observed patterns was facilitated. Our study ends by proposing abiotic and biotic factors that deserve priority in the experimental testing of the forces structuring this assemblage.     

SEASONALITY OF MICROBIAL FOULING ON ASCOPHYLLUM NODOSUM (L.) LEJOL., FUCUS VESICULOSUS L., POLYSIPHONIA LANOSA (L.) TANDY AND CHONDRUS CRISPUS STACKH1

The nature and seasonal extent of microbial fouling on Ascophyllum nodosum (L.) LeJol., Fucus vesiculosus L., Polysiphonia lanosa (L.) Tandy and Chondrus crispus Stackh. were observed by scanning electron microscopy. Bacterial filaments and smaller rod and coccoid forms dominated the fouling communities on all species, with pennate diatoms constituting a minor component in contrast to results with plastic substrates on which pennate diatoms dominated and preceded bacterial colonization. The total percent microbial coverage on the surfaces of all four seaweed species was determined by monthly stereological analyses of representative composite micrographs. These showed a simultaneous decline between April and May which could represent the die-off of the cold water bacterial flora when water temperature increased past the threshold for obligate psychrophiles. Microbial colonization patterns were directly correlated (P = 0.005) with maximum coverage in April and November–December and reduced levels from May to October. Significant inverse (P < 0.041) correlations between total percent coverage and water temperature indicate distinct seasonal cycles, however, the patterns of dominance by filamentous bacteria and rod and coccoid forms were markedly different. Total coverage patterns of both rhodophytes showed no apparent seasonal cycle and were not related to water temperature. Rod and coccoid bacteria were apparently suppressed year round on P. lanosa relative to the other species. These interspecific differences in seasonal fouling patterns are discussed in light of possible modes of regulation, especially algal antibiosis.     

Phosphorus cycling in a dense Potamogeton pectinatus L. Bed

Summary An analysis of phosphorus concentrations and standing stocks in the various components of a dense Potamogeton pectinatus L. community in a brackish lake showed that the amounts of P per unit area varied in the following order: Sediments > Above ground macrophytes > Detritus > Benthic Invertebrates > Below ground macrophyte tissue > Epiphytic algae > Water soluble P > Water borne particulate P. Seasonal changes in the variability of these stocks are described, and shown to be controlled by the annual growth and decomposition pattern of the Potamogeton. The sediments were suggested as the major source of P for the plant biomass. Studies using ³²P tracer showed that of a given input of P to the water, 32% went to large epiphytic algae, 17% to the Potamogeton, 16% to the benthic fauna (mostly filter feeding bivalves), 28% to the sediments (almost all incorporated in the top 1 cm), and the remaining 7% was adsorbed on to, or absorbed by microorganisms associated with detritus. Analysis of ³²P uptake curves indicated that of the P absorbed by the Potamogeton, a significant proportion went to the complex adnate periphyton on the leaf surface. We were unable to separate this fraction. Movement of P in the community was shown to be a closed cycle, and any release of P from decaying macrophytes would be rapidly reabsorbed by epiphytic algae. It is unlikely that phosphorus, once cycling in the macrophyte community, would become transferred to the circulation in the open lake.     

Light-Dark Regulation of Starch Metabolism in Chloroplasts: II. Effect of Chloroplastic Metabolite Levels on the Formation of ADP-Glucose by Chloroplast Extracts

The rate of ADP-glucose formation from [¹⁴C]glucose 6-phosphate and ATP by the soluble fraction of lysed chloroplasts is studied as a function of the levels of metabolites (3-phosphoglycerate, orthophosphate, hexose monophosphate, and ATP) as determined in whole chloroplasts of Spinacia oleracea in light and dark.     

Quantitative studies on brown algal phenols. I. Estimation of absolute polyphenol content of Ascophyllum nodosum (L.) Le Jol. and Fucus vesiculosus (L.)

Methods for estimating the absolute polyphenol content of the brown algae Ascophyllum nodosum (L.) Le Jol. and Fucus vesiculosus (L.) have been developed and tested. Polyphenols were extracted almost quantitatively from Ascophyllum nodosum using aqueous acetone, whereas this procedure was somewhat less efficient with Fucus vesiculosus. Colorimetric methods based on the Folin-Denis reagent, Brentamine Fast Red 2G Salt, and vanillin-H₂SO₄ were applied to acetone-free extracts for determination of polyphenol content relative to suitable reference compounds. Gravimetric methods based on hide powder and on haemoglobin were employed to derive ‘estimation factors’ (EFs) which allow calculation of the absolute polyphenol content from the relative polyphenol content. The values calculated for absolute polyphenol content are considered to be reasonably accurate, despite imprecisions in the methods and despite often large standard deviations, and re-emphasize the potential physiological and ecological significance of brown algal polyphenols. Although the precise EFs calculated here are not valid for other brown algae, the methods are considered to be generally applicable to other Phaeophyceae.     

A proposal for a convenient notation for P-chiral nucleotide analogues. Part 3. Compounds with one nucleoside residue and nonnucleosidic derivatives

Recently, we have proposed a new D_P/L_P stereochemical notation for P-chiral dinucleoside monophosphate analogues that permits simple correlation between spatial arrangement of the substituents and the configuration at the phosphorus center. As an extension of this work, we present here applications of the D_P/L_P notation to derivatives containing only one nucleoside unit (e.g., alkyl nucleoside phosphodiesters, nucleoside phosphomonoesters, cyclic phosphate derivatives, nucleoside di-, and triphosphates) and to nonnucleosidic phosphorus compounds.     

Effets du GA3 et de la BAP sur la respiration,l’absorption et le métabolisme du phosphate chez des disques foliaires dePelargonium zonale après survie

Respiration rate of foliarPelargonium discs was insensitive to ageing. The addition of BAP or GA₃ to the ageing medium did not produce any effect. The presence of GA₃ or BAP in the ageing medium induced an increase (27 %) or a decrease (45 %) of the phosphate uptake. The analysis of phosphorylated compound labelling showed that these two hormones decreased³²P incorporation in the non-acid soluble fraction and increased³²P incorporation in the acid soluble organic fraction. GA₃ and BAP had little effect on the distribution of radioactivity between the different acid soluble compounds, but they increased the ATP level. These results suggest that both GA₃ and BAP increase the basal metabolism, but they seem to act differently on the development of the uptake mechanism during ageing.     

Differential uptake of orthophosphate and organic phosphorus substrates by bacteria and algae in Lake Kinneret

The availability of six organic phosphorus substrates (OPS)to serve as sources of this nutrient for natural populationsof phytoplanktonic algae and bacteria was tested by measuringthe degree to which the organic substrates decreased uptakeof [³²P]orthophosphate (³²Pⁱ). When added to samples of LakeKinneret water, six organo-phosphorus compounds usually loweredthe amount of ³²P_i] taken up by microplankton retained on 0.2-µfilters.In contrast, OPS addition generally stimulated ³²P_i uptake intothe mainly algal fraction (>3 µm), indicating thatthe bacteria were mostly responsible for utilising OPS. Thesparing effect of OPS addition on ³²P₁ uptake was very fast,suggestingthat the micro-organisms possessed constitutive or rapidly inducedenzyme systems to exploit the OPS.The results of this studyindicate that a significant flux of phosphorus may pass viaDOP into microbiota, especially bacteria, in some aquatic systems.     

PHOSPHORUS UPTAKE KINETICS OF SPIROGYRA FLUVIATILIS (CHAROPHYCEAE) IN FLOWING WATER1,2

The inorganic phosphorus (P_i) uptake kinetics of Spirogyra fluviatilis Hilse were examined as a function of phosphorus cell quota (Q^P) and flow velocity in a laboratory stream apparatus. Short-term uptake and the acclimation of the uptake mechanism to flow were measured by the disappearance of P_i pulses in a recirculating flow cell. Short-term P_i uptake was biphasic. When the alga was P-deficient, Phase 1 and 2 half-saturation constants and maximum uptake rates were 11.0 and 47.2 μg P·L^?1 and 473 and 803 μg P·g dry wt^?1 h^?1, respectively. Flowing water altered short-term uptake when the alga was P-deficient, but not when it was P-replete. When Q^P was less than 0.21%, increases in flow velocity from 3 to 15 cm·s^?1 enhanced uptake with maximum uptake for any P_i pulse at 12 and 15 cm·s^?1. At 22 and 30 cm·s^?1, uptake was reduced by 12% or more relative to the maxima. If, however, the alga was cultivated at 22 and 30 cm·s^?1 and short-term P_i uptake was measured at 12 cm·s^?1, uptake was on average 33% greater than when the alga was cultivated at the latter velocity. Apparently, the alga could adjust short-term uptake to compensate for the suboptimal conditions of the faster velocities. Long-term P_i uptake and net phosphorus efflux were estimated by a non-steady state application of the Droop equation. Long-term uptake of very low P_i concentrations was not reduced by fast flowing water. Instead, uptake increased proportionately with flow velocity. Maximum phosphorus efflux from S. fluviatilis was 3% of cellular P per hour and occurred when Q^P was greater than 0.2%. At lower Q^P, the hourly efflux rate was typically less than 1%. Flowing water did not greatly enhance efflux, although when P_i was undetectable, efflux did tend to increase slightly with velocity. The data show that the effects of flowing water on P_i uptake were varied and not always beneficial. If the effects of flowing water on nutrient acquisition by other lotic algae are similarly varied and complex, flow may be an important determinant of nutrient partitioning among benthic algae in streams.     

Phosphorus availability and nitrogenase activity in aquatic blue-green algae

Metabolically active phosphorus-starved cultures of blue-green algae assimilate ³²P rapidly in the light and in the dark. The uptake of phosphorus results in a rapid (within 15 min) stimulation in acetylene reduction by Anabaena cylindrica, A. flosaquae, Anabacnopsis circuiaris and Chlorogloea fritschii, with a response being obtained to less than 5 μg/1 of phosphorus. Uptake of phosphorus also causes a rapid increase in respiration in the dark but not in photo respiration, and the size of the cellular ATP pool and the ¹⁴CO₂ fixation rate both increase more slowly. The metabolism of phosphorus-sufficient cells, which assimilate phosphorus more slowly, shows little response when phosphorus is provided. Excess phosphorus is stored in the vegetative cells of blue-green algae as polyphosphate bodies which may form within 60 min of adding phosphorus to phosphorusstarved cells and which serve as a source of phosphorus for the algae when exogenous phosphorus is limiting. Preliminary results from Scottish waters suggest that urban effluents are important sources of available-phosphorus for algal growth and that the levels entering fresh waters from agricultural land are, per unit volume, lower. In both types of water the levels of available-phosphorus are rather similar to the levels of orthophosphate-phosphorus present. Most detergents tested serve as a source of phosphorus for nitrogen-fixing blue-green algae and cause a rapid stimulation in reduction when added to phosphorus-starved cultures. Of the detergents assayed, the biological types were richest in available phosphorus. The addition of detergents may result in a rapid increase in number of polyphosphate bodies present in the algae. Detergents in general also contain an inhibitor of algal metabolism. Whether a stimu-lation or an inhibition occurs depends on the quantities of detergent added and on whether or not the alga is phosphorus-deficient.     

CHLAMYDOMONAS EUGAMETOS (CHLOROPHYTA) STORES PHOSPHATE IN POLYPHOSPHATE BODIES TOGETHER WITH CALCIUM1

When Chlamydomonas eugametos Moewus cells are starved of phosphate, they accumulate ³²P_i much faster than before starvation. Phosphate accumulation is stimulated by calcium. Less than 5% of the ³²P_i taken up by the cell is present in soluble molecules, suggesting that most is in a metabolically inactive, storage form. Nuclear magnetic resonance and X-ray microanalysis data are presented to show that it is stored as polyphosphate in electron-dense bodies hi the cytoplasm. The same bodies accumulate divalent cations, in particular calcium. The P/Ca ratio in the bodies was maintained between 5.4 (1-week-old cells) and 3.3 (5-week-old cells) during cultivation, suggesting that the calcium and phosphorus relations of the bodies are coupled. The possibility that these electron-dense bodies represent calcium stores that can be released to activate calcium signaling is discussed.