Comparison of different extractive procedures for proteins from the edible seaweeds Ulva rigida and Ulva rotundata

Proteins have been extracted from the edible seaweeds Ulva rigida Agardh and Ulva rotundata Bliding using classical or enzymatic procedures. The protocols using NaOH under reductive conditions or a two-phase system (PEG/K₂CO₃) produced the best protein yields. The cleavage or the limitation of the linkages between proteins and polysaccharides caused by these experimental conditions probably explains the efficiency of these protocols. In SDS PAGE, the protein fraction obtained after NaOH extraction from U. rotundata is characterised by the presence of three major bands with apparent molecular weights of 45 600, 31 800 and 18 600. The protein fraction from U. rigida presents two specific bands with apparent molecular weights of about 27 000 and 12 000. These fractions are mainly rich in aspartic and glutamic acids, alanine, glycine and contain few hydroxyproline residues (0.91–2.44% total amino acid content). The use of cellulase does not significantly improve the extraction of algal proteins in comparison with the blank procedure (without enzymes). The weak accessibility of the substrates in the intact cell wall could explain these experimental data. The improvement of protein yield after the use of the polysaccharidase mixture (-glucanase, hemicellulase, cellulase) partially confirms this hypothesis.     

Chemical and physical-chemical characteristics of dietary fibres from Ulva lactuca (L.) Thuret and Enteromorpha compressa (L.) Grev.

Dietary fibres from Ulva lactuca (L.) Thuret (sea lettuce) and Enteromorpha compressa (L.) Grev. (A.O. nori) were measured according to a ‘standard’ method and a ‘physiological’ protocol simulating the gastric and intestinal environments. U. lactuca contained 15.8–8.0% soluble and 24.2–32.6% insoluble fibres according to the ‘standard’ and ‘physiological’ methods, respectively. For E. compressa, these values were 14.9–15.9 and 21.6–28.7%, respectively. For both algae, the composition suggests that the soluble fibres were xylorhamnoglycuronans sulphates and insoluble fibres were essentially composed of glucans. No marked chemical compositional variation was observed between soluble fractions extracted under the simulated gastric and intestinal conditions. Fibres in both algae are hydrophilic but the water holding capacities were higher after extraction of soluble fibres (5.5–9.5 g g⁻¹ for the dry algae; 14.0–16.0 g g⁻¹ for the standard insoluble fibres). Water soluble fibres demonstrated low intrinsic viscosities at 37 °C in buffers, particularly those from E. compressa (36.0–36.5 ml g⁻¹), and was affected by pH for those of U. lactuca (147.5 ml g⁻¹ at pH 3.0 and 175.0 ml g⁻¹ at pH 7.3).     

Cryptic diversity of Ulva (Ulvales,Chlorophyta) in the Great Bay Estuarine System (Atlantic USA): introduced and indigenous distromatic species

Distromatic foliose blades of the algal genus Ulva are notoriously difficult to identify due to their simple morphologies and few diagnostic characteristics that often exhibit intraspecific variation and interspecific overlap. Hence, species differentiation is difficult and diversity estimates are often inaccurate. Two major goals of this study were to assess the diversity of distromatic Ulva spp. in the Great Bay Estuarine System (GBES) of New Hampshire and Maine, USA, and to compare historical and present day records of these species. Molecular analysis (using ITS sequences) of field-collected specimens revealed four distinct taxa: Ulva lactuca, U. rigida, U. compressa, and U. pertusa. Prior to molecular screening, Ulva lactuca was the only distromatic Ulva species reported for the GBES. Ulva pertusa and the foliose form of U. compressa are newly recorded for the Northwest Atlantic, and the range of U. rigida has been extended. Molecular analysis of historical herbarium voucher specimens indicates that U. rigida, U. pertusa, and the foliose form of U. compressa have been present in the GBES since at least 1966, 1967, and 1972, respectively. The distromatic morphotype of U. compressa is found only in low salinity areas, which suggests that salinity may influence its morphological development. Molecular and morphological evaluations are critical if we are to distinguish between cryptic taxa, accurately assess biodiversity, and effectively monitor the spread of non-indigenous macroalgae.     

Detection of a Phytochrome-like Protein in Macroalgae

A phytochrome-like protein was detected in extracts from the red algae Corallina elongata and Gelidium sp., from the brown algae Cystoseira abiesmarina and Cystoseira tamariscifolia, and from the green algae Ulva rigida, Enteromorpha compressa and Chara hispida. Relative amounts of the photoreversible protein were determined by measurement of Δ (ΔA) values of the crude extract. SDS gel electrophoresis and immunoblotting with monoclonal antibodies directed to phytochrome from etiolated maize and oat seedlings revealed only one phytochrome-related band with apparent molecular weight of 130 kDa. The absorption difference spectrum after partial purification showed a “normal” absorption band (λ_max = 670 nm) for the Pr form but only a very weak band (λ_max = 705 nm) for the “Pfr form”.     

Use of the green alga Ulva rigida C. Agardh as an indicator species to reassess metal pollution in the Thermaikos Gulf, Greece, after 13 years

The concentrations of metals (Mn, Pb, Fe, Zn, Cu, Cd,Co, Ni, Cr, Na, K, Ca, Mg) were determined in thegreen alga Ulva rigida, in the sediment andseawater of Thermaikos Gulf (Greece) during monthlysamplings in 1994–1995. This Gulf is the recipientof domestic and industrial effluents. Pb, Fe, Cu, Coand Cr concentrations in U. rigida at the studyarea were higher than those 13 years earlier andapparently came from different sources than those forZn, Cd and Ni. The relative abundance of metals inthe alga decreased in the order: Mg > Na > K >Ca > Pb > Fe > Mn > Zn > Cr, Cu > Ni >Co > Cd. Only Cu concentrations in the alga fromKalochori and Cd ones from Viamyl showed significantseasonal changes. Cu and Cd concentrations ingeneral followed the same pattern of variation, withminimum values in winter-spring. This pattern isdiscussed in relation to growth dynamics and tissueage. Only Pb concentrations in the alga showed asignificant positive correlation with concentrationsin the seawater. There were both positive andnegative correlations among some metals in the alga. It is concluded that U. rigida can be used as anindicator species, especially for Pb.     

Microbiology and storage life of fresh edible seaweeds

Samples of two edible seaweeds, Palmaria palmata and Ulva rigida, were stored at 4 °C after harvesting, and their microbial flora were measured over a period of two weeks, using food control methods. The results showed a low level of mesophilic aerobes, fungi and yeasts throughout the storage period, whether the seaweeds were unwashed, or washed with seawater, and there was a satisfactory retention of physical characteristics. Seaweeds washed with tap water degraded in less than one week.     

Isolation and substrate specificities of five chitinases from the hepatopancreas of Northern shrimp, Pandalus borealis

Five enzymes designated chitinase I, IIa, IIb, III, and IV have been isolated from the hepatopancreas of Pandalus borealis in a procedure including column chromatography on Q-Sepharose, Sephacryl S-200, phenyl-Superose and Superdex 75. The isolated enzymes were analysed by SDS PAGE. Chitinase I, III, and IV gave only one major band corresponding to 54–55 kDA. Chitinase IIa showed one major band at 61 kDA and two diminutive bands at 17 and 55 kDa, while chitinase IIb gave two major bands at 17 and 44 kDa. Estimated by gel filtration, the native molecular weights of chitinase I, IIa, IIb, III, and IV were 61, 69, 39, 57, and 54 kDa, respectively. The substrate and reaction specificities of the isolated chitinases were investigated, and the results show that the isolated enzymes are true chitinases. They do not hydrolyse N,N′-diacetylchitobiose or p-Nitrophenyl-N-acetyl-β-D-glucosaminide, but express activities when longer chitooligosaccharides or nitrophenylated chitooligosaccharides are used as substrates. Chitinase I and IIa gave an initial random cleavage pattern and might be classified as endochitinases, while chitinase III and IV released dimeric units from the substrates and might be termed chitobiosidases.     

Ploidy Distribution of the Harmful Bloom Forming Macroalgae Ulva spp. in Narragansett Bay,Rhode Island,USA, Using Flow Cytometry Methods

Macroalgal blooms occur worldwide and have the potential to cause severe ecological and economic damage. Narragansett Bay, RI is a eutrophic system that experiences summer macroalgal blooms composed mostly of Ulva compressa and Ulva rigida, which have biphasic life cycles with separate haploid and diploid phases. In this study, we used flow cytometry to assess ploidy levels of U. compressa and U. rigida populations from five sites in Narragansett Bay, RI, USA, to assess the relative contribution of both phases to bloom formation. Both haploid gametophytes and diploid sporophytes were present for both species. Sites ranged from a relative overabundance of gametophytes to a relative overabundance of sporophytes, compared to the null model prediction of √2 gametophytes: 1 sporophyte. We found significant differences in cell area between ploidy levels for each species, with sporophyte cells significantly larger than gametophyte cells in U. compressa and U. rigida. We found no differences in relative growth rate between ploidy levels for each species. Our results indicate the presence of both phases of each of the two dominant bloom forming species throughout the bloom season, and represent one of the first studies of in situ Ulva life cycle dynamics.     

DIURNAL CELL DIVISION REGULATED BY GATING THE G1/S TRANSITION IN ENTEROMORPHA COMPRESSA (CHLOROPHYTA)1

The cell‐cycle progression of Enteromorpha compressa (L.) Nees (=Ulva compressa L.) was diurnally regulated by gating the G₁/S transition. When the gate was open, the cells were able to divide if they had attained a sufficient size. However, the cells were not able to divide while the gate was closed, even if the cells had attained sufficient size. The diurnal rhythm of cell division immediately disappeared when the thalli were transferred to continuous light or darkness. When the thalli were transferred to a shifted photoperiod, the rhythm of cell division immediately and accurately synchronized with the shifted photoperiod. These data support a gating‐system model regulated by light:dark (L:D) cycles rather than an endogenous circadian clock. A dark phase of 6 h or longer was essential for gate closing, and a light phase of 14 h was required to renew cell division after a dark phase of >6 h.     

Free-floating Ulva in the southwest Netherlands: species or morphotypes? a morphological,molecular and ecological comparison

Free-floating Ulva L. biomass in the eutrophic brackish ‘Veerse Meer’ lagoon (southwest Netherlands) consists of four morphologically identified species: U. curvata (Kützing) De Toni, U. lactuca L., U. rigida C. Agardh and U. scandinavica Bliding. U. curvata could be recognized easily because of the characteristic central cavity in the holdfast of the attached plants, the arrangement of cells in rows and the single pyrenoid in each cell. U. rigida was distinguished by the thick thallus and the large number of pyrenoids. The Veerse Meer isolate, however, was slightly different from the isolate from the Oosterschelde estuary (the Netherlands). U. lactuca and U. scandinavica showed a high degree of overlap in thallus thickness and cell size, but U. scandinavica usually possessed more pyrenoids. However, doubts have frequently been expressed about the use of some morphological characters in Ulva taxonomy. To determine the validity of such characters in the identification of Ulva species, the morphological variation within and between morphological species was recorded and a molecular data set generated. To detect possible ecophysiological differences between species, optimum temperatures and salinities for growth were determined experimentally. The sequences of the nuclear ribosomal DNA internal transcribed spacer 2 (ITS2) and flanking regions of U. lactuca, U. rigida and U. scandinavica from the Veerse Meer were all identical, but differed from that of U. rigida from the Oosterschelde estuary. Ulva species from the Veerse Meer were most closely related to U. armoricana and U. rigida from Brittany (2.9% and 3.5% divergence respectively); the difference between U. rigida from the Veerse Meer and from the Oosterschelde estuary was 7.5%. Rooted trees, based on a comparison of these sequences with sequences of other Ulva and Enteromorpha species obtained from the literature, using Monostroma arcticum as outgroup, suggested that Ulva is paraphyletic with respect to Enteromorpha. The optimum temperature for growth of U. curvata was 25?°C; for all other species it was 10?°C. The optimum salinity for growth was 30?°C for all isolates. It is concluded that U. lactuca, U rigida and U. scandinavica from the Veerse Meer are all members of one highly polymorphic species.     

Lipid extracts from different algal species:1H and13C-NMR spectroscopic studies as a new tool to screen differences in the composition of fatty acids,sterols and carotenoids

One- and two-dimensional¹H- and¹³C-NMR spectra of lipid extracts fromUlva rigida, Gracilaria longa, Fucus virsoides andCodium tomentosum collected in the northern Adriatic Sea allowed screening of the content of fatty acid chains, carotenoids, free and acylated cholesterol and chlorophylls. The carotenoid-to-polyunsaturated fatty acid molar ratio was taken as a comparison parameter in samples ofUlva rigida collected in differentloci and seasons; the value was markedly higher in samples from the Lagoon of Venice than from marine coastal waters. The total cholesterol concentration was evaluated by¹H-NMR spectroscopy and similar values were found for all species. Two-dimensional heterocorrelated NMR spectroscopy was shown to give characteristic fingerprints of the lipid extracts from algal samples as regards the content in chlorophylls, unsaturated fatty acids and carotenoids.author for correspondence     

Sterols from some black sea ulvaceae

The free and esterified sterol composition of Ulva rigida and Enteromorpha linza (Ulvaceae) from the Black Sea was investigated. The main sterol component of Ulva rigida was identified as fucosterol. The influence of the season and water pollution on the sterol composition was investigated. The origin of the C₂₆,- and C₂₅-sterols and of saringasterol in these seaweeds is discussed.     

Changes in nitrogen content and protein profiles following <Emphasis Type="Italic">in vitro</Emphasis> selection of NaCl resistant mung bean and tomato

Mung bean and tomato were in vitro selected on media containing 0, 25, 50, 100 and 150 mM NaCl. Two types of media (hormone supplemented media, CB and hormone free media, MS) were used for mung bean using cotyledon explants whereas two types of explants (cotyledons and shoot apices) were used for tomato on MS media. Total-N, protein content, nitrite reductase (NiR) activity and protein protein profiles were checked in selected plants and compared to original non selected ones. NaCl at low concentrations slightly increased total-N in shoots and roots of in vitro selected mung bean and tomato whereas higher concentrations induced significant reductions. Similar increases in protein content were detected at lower concentrations with no significant effects thereover. On the contrary, NaCl gradually inhibited NiR activity. Similar responses of total-N, protein and NiR activity, but with greater magnitudes, were detected in original plants. In addition, NaCl significantly reduced dry weights of shoots and roots of either in vitro selected or, in particular, original intact plants. Moreover, electrophoresis (SDS-PAGE) of protein from shoots of either in vitro selected or intact plants showed that NaCl induced new protein bands while some others were concomitantly disappeared. The induction of one or more of the 86.4, 79, 77.6, 77 and 71.5 kDa bands following in vitro selection and/or the disappearance of the 86 kDa band from intact plants seemed necessary for mung bean resistance. Also, the presence of 86.2 kDa band and/or the loss of the 85.8 and 57.5 kDa bands might be included in tomato resistance. Of these induced bands in mung bean selected on CB media, only two bands were detected in plants selected on MS media. In tomato, two bands lost following selection from cotyledons but only one band lost following selection from shoot apices. These changes in protein pattern therefore might serve as adaptive regulators for resistance to NaCl.     

Allergenic evaluation of Malassezia furfur crude extracts

Crude extracts of the lipophilic yeast Malassezia furfur were obtained from 2, 6, 10 and 28 day old cultures. The in vitro cultivation periods corresponded, respectively, to the lag phase, middle of the log phase, end of log phase and the decline phase of the growth curve, which was based on viable cell counts obtained with a fluorescent viability test. Biochemical analyses showed that the protein and carbohydrate contents were greater in day 10 extracts. Seventy patients with different allergic manifestations and 30 healthy volunteers were skin prick tested using the extracts. Of these, thirteen (18.57%) patients gave positive responses. SDS PAGE gradient electrophoretic profiles of the preparations indicated that the 28 day extracts contained the greatest number of protein bands with molecular weights ranging mostly between 30 and 94 kDa. Immunoblots incubated with individual patient sera showed that four IgE binding M. furfur allergens of approximately 88, 61, 52 and 39 kDa were present in the 28 day extracts. The components identified could be used for detecting IgE mediated responses to M. furfur among individuals affected with different allergic conditions.This revised version was published online in October 2005 with corrections to the Cover Date.     

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.     

Separation and purification of the tonoplast ATPase and pyrophosphatase from plants with constitutive and inducible Crassulacean acid metabolism

Tonoplast vesicles were isolated from Kalanchoe daigremontiana Hamet et Pierrer de la Bâthie and Mesembryanthemum crystallinum L., exhibiting constitutive and inducible crassulacean acid metabolism (CAM), respectively. Membrane-bound proteins were detergent-solubilized with 2% of Triton X-100. During CAM induction in M. crystallinum, ATPase activity increases four-fold, whereas pyrophosphatase activity decreases somewhat. With all plants, ATPase and pyrophosphatase could be separated by size-exclusion chromatography (SEC, Sephacryl S 400), and the ATPase was further purified by diethylaminoethyl-ion-exchange chromatography. Sodium-dodecyl-sulfate electrophoresis of the SEC fractions from K. daigremontiana containing maximum ATPase activity separates several protein bands, indicating subunits of 72, 56, 48, 42, 28, and 16 kDa. Purified ATPase from M. crystallinum in the C₃ and CAM states shows a somewhat different protein pattern. With M. crystallinum, an increase in ATP-hydrolysis and changes in the subunit composition of the native enzyme indicate that the change from the C₃ to the CAM state is accompanied by de-novo synthesis and by structural changes of the tonoplast ATPase.Abbreviations CAM Crassulacean acid metabolism - DTT dithiothreitol - kDa kilodalton - PAGE polyacrylamide gel electrophoresis - PPiase pyrophosphatase - SEC size exclusion chromatography - SDS sodium dodecyl sulfate - Tris 2-amino-2-(hydroxymethyl)-1,3-propanediol     

Analysis of Volatile Constituent by Hydrodistillation and Solid-Phase Microextraction Techniques and Antimicrobial and Scolicidal Activities of Essential Oil and Soxhlet Extracts of Ulva rigida grown in Turkey

In the present study, the volatile composition of Ulva rigida (U. rigida) was elucidated by two different methods. As a result of the identification process of volatile components using the GC/MS-FID instrument, 31 compounds were identified by hydrodistillation (HD) method, and 15 compounds were identified by solid-phase microextraction (SPME) method, elucidating the structure of 99.86 % and 92.65 %, respectively. The most abundant compounds in the essential oil of U. rigida were n-hexadecanoic acid and pentadecanal, while the most abundant compound according to the SPME analysis was heptadecyne, a hydrocarbon compound. In the next step, hexane, dichloromethane, chloroform and methanol solvent extracts of U. rigida were prepared and the antimicrobial activities of the extracts and the essential oil obtained by hydro-distillation as well as the scolicidal activities of the solvent extracts were determined. The results of the antimicrobial activity test of the essential oil showed a high level of activity against Bacillus cereus ATCC 10876 and MRSA. The highest activity was found on the microorganism of Pseudomonas aeruginosa ATCC 9027 in chloroform and methanol extracts of U. rigida. Furthermore, viability detection was performed and the scolicidal effects of the extracts on protoscoleces were assessed. The values of lethal concentration doses (LD₅₀, LD₇₅ and LD₉₀) were calculated using probit analysis.     

Changes in respiration,photosynthesis and protein composition during induced synchronous formation of gametes and zoospores in Ulva mutabilis Føyn

In the marine green alga Ulva mutabilis Føyn, large daily fluctuations in respiratory and photosynthetic rates are found after the induction of synchronous formation of gametes and zoospores. A sharp reduction in photosynthetic capacity preceeds the zooid-forming cell divisions. During the same period, samples for polyacrylamide electrophoresis show an initial increase followed by a decrease in the amount of ribulose-1,5-bisphosphate carboxylase. Some other transient protein changes were also noted. A new protein band with MW 51,400 appears in the 48-h sample and becomes a prominent constituent of gametes and zoospores; it may represent tubulin.Abbreviations PAGE polyacrylamide gel electrophoresis - SDS sodium dodecylsulphate To whom correspondence should be addressed     

MOLECULAR AND MORPHOLOGICAL ANALYSIS OF ENTEROMORPHA INTESTINALIS AND E. COMPRESSA (CHLOROPHYTA) IN THE BRITISH ISLES

The very common green seaweeds Enteromorpha intestinalis (L.) Nees and E. compressa (L.) Nees are important fouling organisms and have commonly been used as indicators of eutrophication, but their taxonomic status is problematic. The genus presents extreme difficulties because there is wide intraspecific variation in morphology, but morphological differences between species are small and difficult to detect. In this study, molecular data were used in parallel with morphological characters to resolve the taxonomic problems. Phylogenetic analysis of sequences of the internal transcribed spacers ITS1 and ITS2 and the 5.8S gene distinguished two groups of samples, which were identified by morphological characters as E. compressa (branched) and E. intestinalis (normally unbranched). There was a low level of sequence divergence within each group of samples, but divergence between groups was as great as that between either of the two species and the outgroup E. prolifera. Clades representing E. compressa and E. intestinalis were also found in analyses of an independent molecular data set, chloroplast DNA restriction fragment length polymorphisms (RFLPs). Enteromorpha intestinalis and E. compressa represent two distinct, genetically divergent species. Reinterpretation of published studies shows that these species are reproductively isolated. However, E. compressa and E. intestinalis are sometimes very difficult to distinguish from each other and could be regarded as cryptic species. The presence or absence of branching was the most useful character distinguishing these two species, but there was an element of ambiguity because low salinity or salinity shock can induce branching in E. intestinalis. If environmental factors such as salinity are taken into account, branching can be used to identify the great majority of thalli correctly. This study therefore provides a basis for identifying the two most important marine fouling macroalgae and for their use in environmental monitoring and experimentation. Typification of these two Linnaean species showed that current usage of the names accords with the lectotype and protologue of both species. Samples that resembled E. usneoides did not form a clade in any of the trees, and constraining the data to support the monophyly of this group incurred a penalty. Enteromorpha usneoides appears to be an ecotype of E. compressa.     

Growth of the seaweed Ulva rigida C. Agardh in relation to biomass densities, internal nutrient pools and external nutrient supply in the Sacca di Goro lagoon (Northern Italy)

Growth of the seaweed Ulva rigida C. Agardh was investigated in relation to biomass densities, internal nutrient pools and external nutrient supply. Research was carried out from 23 March to 5 July 1994 in the Sacca di Goro (Po Delta, Northern Italy), whose south-eastern part was covered by extensive mats of Ulva rigida. Two types of field experiments were conducted by incubating Ulva thalli inside large cages. In the first experiment, beginning on 23 March, 100 g of wet thalli were placed into the cages, allowed to grow for two weeks, then collected and replaced. This procedure was repeated 8 times over the study period. In the second experiment, Ulva thalli were left inside the cages and collected at selected time intervals (14, 27, 41, 64 and 76 days) in order to simulate the effects of increased density on growth and nutrient storage.We recorded specific growth rates (NGR) ranging from 0.025 to 0.081 d^–1 for a period up to two months in the repeated short-term experiments performed at relatively low initial algal densities (300–500 g AFDW m^–3). These NGR resulted significantly related to dissolved inorganic nitrogen (DIN) in the water column. Tissue concentrations of total Kjeldahl nitrogen (TN) were almost constant, while extractable nitrate decreased in a similar manner to DIN in the water column. Total phosphorus showed considerable variation, probably linked to pulsed freshwater inflow.In the long-term incubation experiment, NGR of Ulva was inversely related to density. Internal concentrations of both total P and TN reached maximum values after one month; thereafter P concentration remained almost constant, while TN decreased below 2% w/w (by dry weight). The TN decrease was also accompanied by an abrupt decrease in nitrate tissue concentration. The biomass incubated over the two month period suffered a progressive N limitation as shown by a decreasing NY ratio (49.4 to 14.6). The reciprocal control of Ulva against biogeochemical environment and viceversa is a key factor in explaining both resource competition and successional stages in primary producer communities dominated by Ulva. However, when the biomass exceeds a critical threshold level, approximately 1 kg AFDW m^–3, the macroalgal community switches from active production to rapid decomposition, probably as a result of selfshading, biomass density and development of anaerobic conditions within the macroalgal beds.