Comparison of cell wall compositions of the rhizomes of three seagrasses

Cell walls from rhizomes of Halophila ovalis (R. Br.) Hook.f., Halophila stipulacea (Forsk.) Aschers. and Halodule univervis (Forsk.) Aschers. were analysed. The non-cellulosic polysaccharides contained glucose as the most abundant sugar and arabinose as the next most abundant sugar in all cases. Only small amounts of pectin were found. Halodule uninervis differed from the two Halophila species in its large amount of cell wall material per gram fresh weight and in its high proportion of non-cellulosic polysaccharides. The lignin from all three plants contained non-conjugated phenols, with relatively few conjugated phenols.     

Seagrasses in the Zeit Bay area and at Ras Ghârib (Egyptian Red Sea coast)

The distribution and ecology of seagrasses along parts of the Egyptian Red Sea coasts have been investigated. Five of the 10 species ever reported from the Red Sea have been found in this area, i.e. Halodule uninervis (Forssk.) Aschers., Thalassodendron ciliatum (Forssk.) den Hartog, Halophila stipulacea (Forssk.) Aschers., Halophila ovalis (R. Br.) Hook. f. and Halophila ovata Gaud. H. ovata is recorded here for the first time for the west coast of the Red Sea. The finding of Halophila decipiens Ostenfeld represents a new record to the Red Sea; this species has been found at a depth of 30 m, near Ras Ghârib in the Gulf of Suez.     

Chromosome numbers of the European seagrasses

The chromosome numbers of the five European seagrasses have been determined in material from several sites along the coasts of the Atlantic Ocean, the North Sea and the Mediterranean:Zostera marina L., 2n = 12;Z. noltii Hornem., 2n = 12;Posidonia oceanica (L.)Delile, 2n = 20;Cymodocea nodosa (Ucria)Aschers., 2n = 14, 2n = 28;Halophila stipulacea (Forsk.)Aschers., 2n = 18. The difference in chromosome morphology betweenZ. marina andZ. noltii supports the division of the genus into two subgenera.     

Some aspects of decomposition of leaf litter of the seagrass Halophila stipulacea from the gulf of Aqaba (Jordan)

Decomposition of the seagrass Halophila stipulacea (Forsk.) Aschers. followed in situ over 340 days showed that the plant lost 40.8% of its initial weight during the first 85 days and only 21.2% over the remaining 255 days of the incubation period. The observed overall rate of decay of 0.18% day^?1 indicates the high resistance of Halophila tissue to decomposition.An exponential model showing a regularly decreasing decay coefficient was fitted to the data of weight loss. The correlation between observed and calculated weight remaining was highly significant (r = 0.9881, P < 0.0001).The samples of decomposing leaf litter harboured 2.2 × 10¹¹ bacteria g^?1 dry wt. on average and consumed 0.25?1.07 mg O₂^?1 dry wt. h^?1. The early colonizing bacteria of the detritus were mostly non-sporogenous forms. The most prominent genera involved in decomposition were Actinomyces, Arthrobacter, Bacillus and Pseudomonas, all of which are capable of degrading structural carbohydrates.     

Quantitative and dynamic aspects of a mixed seagrass meadow in Papua New Guinea

Aspects of production and biomass were studied from November 1981 to November 1982 in six seagrass species which together from the mixed seagrass meadows in Papua New Guinea. These species, viz. Thalassia hemprichi (Ehrenb.) Aschers., Cymodocea serrulata (R.Br.) Aschers. et Magnus, Cymodocea rotundata Ehrenb. et Hempr. ex Aschers., Syringodium isoetifolium (Aschers.) Dandy, Halodule uninervis (Forssk.) Aschers. and Halophila ovalis (R.Br.) Hook. f. have been previously studied in monospecific seagrass beds. Thalassia hemprichii was the dominant species, followed by Syringodium isoetifolium. These two species were present in all samples and evenly distributed. Cymodocea serrulata and C. rotundata were recorded in 91 and 86%, respectively, of the quadrats sampled. The density, however, varied considerably. Shoots of the remaining two species were found in < 50% of the samples. The percentage presence increased when below-ground plant parts were taken into account.Significant differences in the shoot density were only found in Syringodium isoetifolium. The distribution of the five other species remained unchanged during the year. Annual mean shoot density amounted to 860 for Thalassia hemprichii, 2100 for Syringodium isoetifolium, 200 for Cymodocea serrulata, 250 for C. rotundata and 54 for both Halodule uninervis and Halophila ovalis. All species reached their maximum density from September to November. The mean aboveground production was 3.9 g ash-free dry weight (ADW) m⁻² day⁻¹, of which 64% was contributed by Thalassia hemprichii. Syringodium isoetifolium, which had the highest shoot density, contributed only 17%.The plastochrone interval of the leaves (PIL) was constant in all species and the mean ranged from 10.1 to 11.1 days. The PIL was virtually the same in this mixed meadows as in monospecific seagrass beds. Furthermore, the above-ground relative growth rate was constant during the year. Thalassia hemprichii was the most productive seagrass (mean 0.043 day⁻¹), whereas the lowest mean relative production was observed for Syringodium isoetifolium (0.030 day⁻¹). Total mean production was 6.4 g ADW m⁻² day⁻¹, of which 39% was contributed by the vertical axes, the rhizomes and the roots. The caloric production efficiency of the meadows was 0.58% of the total insolation at the water surface.Thalassia hemprichii was, because of its morphology, the stable element in the meadow. All other species were present at all times and exhibited a continuative process of recolonization.     

Morphological diversity under controlled conditions for the Halophila ovalis-H. minor complex and the Halodule uninervis complex from Shark Bay,Western Australia

The wide range of leaf types that occurs within the populations of Halophila and Halodule in Shark Bay, Western Australia, was studied for plants under laboratory conditions. In the Halophila ovalis-H. minor complex, two types of plants appeared in the cultures, a large-leaved variant that is similar to typical H. ovalis (R.Br.) Hook. f. and a small-leaved variant that is more similar to H. minor (Zoll.) den Hartog. In Shark Bay, the two variants were not clearly separated because both produced leaves that varied in size, degree of waviness of leaf margins and intensity of anthocyanin pigmentation. In the Halodule uninervis (Forsk.) Aschers. complex, two types of plants, wide- and narrow-leaved variants, appeared in the cultures. In Shark Bay, the narrow-leaved plants differed in leaf width in muddy and silty microsites, but were always narrower than the wide-leaved variants. Only the narrow-leaved Halodule plants produced anthocyanin pigmentation both under field and laboratory conditions. The studies under controlled conditions indicate that the highly diverse leaves in Shark Bay result primarily from environmental influences on two types of Halophila and two types of Halodule.     

Expansion and fragment settlement of the non-native seagrass Halophila stipulacea in a Caribbean bay

The non-native seagrass species Halophila stipulacea has spread throughout the Eastern Caribbean since 2002, and could potentially impact the functioning of local seagrass ecosystems. Important characteristics for invasiveness, such as dispersal, recruitment and expansion of H. stipulacea at a local scale, are unknown. We assessed H. stipulacea expansion rates within Lac Bay, Bonaire, Dutch Caribbean (7?km²), since its establishment in 2010 and tested the settlement potential of uprooted vegetative fragments of H. stipulacea. Using 49 fixed locations, we observed that between 2011 and 2015 the occurrence of H. stipulacea in the bay increased significantly from 6% to 20% while native Thalassia testudinum occurrence decreased significantly from 53% to 33%. Free-floating H. stipulacea fragments that were collected and tethered above the sediment rooted within 10 days with a settlement success rate of 100%. The growth of settled fragments was on average 0.91 shoots d^?1. The ongoing shift from native T. testudinum to introduced H. stipulacea dominated meadows may have important consequences for multiple Caribbean seagrass ecosystem functions. Given the large difference in size between the two seagrass species, functions such as coastal protection, habitat structure, food availability, and the stability and resilience of these systems can be altered. The next steps towards modelling future expansion of H. stipulacea throughout the Caribbean and beyond should include the assessment of fragment viability and dispersal distance, and the impacts of natural and anthropogenic disturbance on vegetative fragment density, dispersion and settlement by this species.     

Photosynthetic carbon metabolism in seagrasses C-labeling evidence for the c(3) pathway

The δ¹³C values of several seagrasses were considerably less negative than those of terrestrial C₃ plants and tended toward those of terrestrial C₄ plants. However, for Thalassia hemprichii (Ehrenb.) Aschers and Halophila spinulosa (R. Br.) Aschers, phosphoglycerate and other C₃ cycle intermediates predominated among the early labeled products of photosynthesis in ¹⁴C-labeled seawater (more than 90% at the earliest times) and the labeling pattern at longer times was brought about by the operation of the C₃ pathway. Malate and aspartate together accounted for only a minor fraction of the total fixed label at all times and the kinetic data of this labeling were not at all consistent with these compounds being early intermediates in seagrass photosynthesis. Pulse-chase ¹⁴C-labeling studies further substantiated these conclusions. Significant labeling of photorespiratory intermediates was observed in all experiments. The kinetics of total fixation of label during some steady-state and pulse-chase experiments suggested that there may be an intermediate pool of inorganic carbon of variable size closely associated with the leaves, either externally or internally. Such a pool may be one cause for the C₄-like carbon isotope ratios of seagrasses.     

Lacunal gas discharge as a measure of productivity in the seagrasses Zostera capricorni,Cymodocea serrulata and Syringodium isoetifolium

A new method for estimating the rate of photosynthetic fixation of carbon in seagrasses is described. This method, which is sensitive and simple to apply, is based on the close relationship between photosynthetic rate and the volume of gas discharged through the lacunae.During photosynthesis, the gas discharged from the lacunae was composed primarily of oxygen (32.5%) and nitrogen (67.5%). The rate of discharge was proportional to the rate of photosynthesis and hence was a function of light intensity. There was a linear relationship between gases discharged from the lacunae and oxygen released into the water column. Calibration curves were derived relating volume of gases (oxygen and nitrogen) released from the lacunae to total oxygen produced during photosynthesis for three species of seagrasses (Zostera capricorni Aschers., Cymodocea serrulata (R. Br.) Aschers. & Magnus and Syringodium isoetifolium (Aschers.) Dandy). Molar ratios of ¹⁴C fixed to oxygen produced were close to unity (1.008±0.016; n=8) indicating that measurements of lacunal gas released may be used to measure productivity.     

Qualitative and quantitative aspects of the epiphytic component in a mixed seagrass meadow from Papua New Guinea

During 1982, structural and functional aspects of the epiphytic component in a tropical mixed seagrass meadow, have been investigated for each seagrass species separately. This meadow consisted of the seagrasses Thalassia hemprichii (Ehrenb.) Aschers., Cymodocea serrulata (R.Br.) Aschers. et Magnus, C. rotundata Ehrenb. et Hempr. ex Aschers., Halodule uninervis (Forssk.) Aschers. and Syringodium isoetifolium (Aschers.) Dandy.No significant differences were observed in floristic composition, number of algal species, abundance and diversity of the epiphytic component. On an area basis, annual mean above-ground biomass (seagrass leaves and epiphytes), amounted to 82 g ADW, of which 18% could be ascribed to the epiphytic component. The contribution of the epiphytic component to the annual mean above-ground production ranged from 16% on leaves of Thalassia hemprichii to 33% on leaves of Cymodocea serrulata. Total annual mean epiphyte production was 4.6 g ADW m⁻² sediment surface day⁻¹ (19%).When including the macroalgal component of this mixed seagrass meadow, total annual mean above-ground plant biomass amounted to 93 g ADW (212 g DW) on an area basis, of which the epiphytes contributed 15.5% (28.5% DW), the macroalgal component 12% (32.5% DW) and the seagrass leaves 72.5% (39.5% DW). Aspects of the epiphytic component (e.g., floristic composition, abundance, biomass and production) in monospecific and mixed seagrass communities are discussed.     

Variations in leaf soluble amino acids and ammonium content in subtropical seagrasses related to salinity stress

A survey of leaf soluble amino acids was conducted for four subtropical seagrasses grown at several salinities. Proline functioned as an organic osmoticum in Halodule wrightii Aschers., Thallasia testudinum Banks ex Koenig, and Ruppia maritima L., while alanine functioned in an osmoregulatory capacity in Halophila engelmanni Aschers. When light-and salinity-induced variations in leaf NH₄ and amide-N levels were compared in Halodule, Thalassia, and Halophila, ability to regulate leaf NH₄ levels was correlated with osmoregulatory capacity and maintenance of selected amino acid contents.     

The distribution and expansion of the invasive seagrass Halophila stipulacea in Dominica, West Indies, with a preliminary report from St. Lucia

The seagrass Halophila stipulacea Forsskål, native to the Red Sea, is an invasive species in the Mediterranean that was recently observed offshore Grenada, in the Caribbean. Here, we document the presence of this seagrass in Dominica and St. Lucia, demonstrating it has spread across part of the eastern Caribbean. H. stipulacea in Dominica was present in seven locations along the west coast covering more than 22.9 ha of the benthos, at depths from 2 to 18 m. Populations were concentrated in or adjacent to bays frequented by recreational or commercial boats, likely vectors for the introduction. Morphological features varied from bed to bed, with depth being the predominant driving factor. H. stipulacea had a rapid mean lateral bed expansion rate of 0.5 cm d⁻¹, with a maximum rate of >6 cm d⁻¹. H. stipulacea patches often occurred exclusive of the otherwise dominant seagrasses of the Caribbean. The potential for the expansion of H. stipulacea, combined with its tolerance for a wide spectrum of environmental conditions, positions it as a potential threat to local and regional biodiversity.     

Photosynthetic response of seagrasses to ultraviolet-a radiation and the influence of visible light intensity

Inhibition of photosynthesis by ultraviolet-A radiation (UV-A, 315-380 nanometers) was examined in three marine angiosperms: Halophila engelmannii Aschers, Halodule wrightii Aschers, and Syringodium filiforme Kütz. Sensitivity to UV-A and photosensitization to UV-A by photosynthetically active radiation (PAR, 380-700 nanometers) were characterized.     

The Brazilian seagrasses

Based on a critical review of the literature, and on abundant material recently collected along the coast of Brazil, a synthesis of the occurrence and distribution of the Brazilian species of seagrasses is presented. Two species of Hydrocharitaceae, namely Halophila baillonii Aschers. and H. decipiens Ostenf., and three species of Potomogetonaceae, Halodule emarginata den Hartog, H. wrightii Aschers. and Ruppia maritima L. s.s., were found. Ruppia maritima was found all along the Brazilian coast, from about 3 to 32°S latitude, in brackish water ponds and lakes, with salinities varying from 0.3 to 28?.. The other species are restricted to normal seawater ($\text{±35?.}$), although Halodule wrightii was also found in hypersaline waters (45?.). This last species is the most common seagrass in Brazil. The species of Halophila are restricted to warmer waters and were not found further south than Rio de Janeiro State, being especially common on the NE coast at depths down to 62 m. During the course of these studies, Halophila baillonii was found for the second time in Brazil since its original collection in 1888. A critical comparison of a large number of plants leads us to the conclusion that Halodule brasiliensis Lipkin cannot be distinguished from H. wrightii, and H. lilianae den Hartog cannot be distinguished from H. emarginata. Reproductive structures of H. emarginata are described for the first time.     

Diurnal movements of chloroplasts in Halophila stipulacea and their effect on PAM fluorometric measurements of photosynthetic rates

Since diurnal chloroplast movements in Halophila stipulacea were described by Drew in 1979, this phenomenon has not been studied further for seagrasses. In addition to an apparent photoprotective role, such movements may affect the measurements of photosynthetic rates based on pulse amplitude modulated (PAM) fluorometry. This is because calculations of electron transport rates (ETR) are directly affected by the light absorption of the leaves (or the so-called absorption factor, AF), the latter of which changes with the movements of the chloroplasts. In this work, we therefore determined chloroplast clumping and dispersal, and measured AFs, chlorophyll contents and PAM fluorescence diurnally for H. stipulacea grown under two irradiance regimes. Diurnal chloroplast clumping occurred in high-light grown (HL) plants (∼450 μmol photons m⁻² s⁻¹ during midday), which was accompanied by a decrease in AF values (from 0.56 in the early morning to 0.34 at midday) but not in the chlorophyll content. Also, non-photochemical quenching (measured as NPQ) increased during the day in these plants. No such chloroplast movements and, thus, no diurnal changes in AF values (0.60 ± 0.04 throughout the day), and no changes in NPQ, were found in low-light grown (LL) plants (∼150 μmol photons m⁻² s⁻¹ during midday). As a consequence of the chloroplast clumping in HL plants, and its effect on AF values, maximal ETRs did not differ significantly between HL and LL plants. This finding thus shows the importance of taking into account changing AF values along the day when calculating ETRs of H. stipulacea, and other seagrasses potentially featuring diurnally changing AFs, under high-irradiance conditions.     

Inhibition of seagrass photosynthesis by ultraviolet-B radiation

Effects of ultraviolet-B radiation on the photosynthesis of seagrasses (Halophila engelmanni Aschers, Halodule wrightii Aschers, and Syringodium filiforme Kütz) were examined. The intrinsic tolerance of each seagrass to ultraviolet-B, the presence and effectiveness of photorepair mechanisms to ultraviolet-B-induced photosynthetic inhibition, and the role of epiphytic growth as a shield from ultraviolet-B were investigated.     

Growth patterns in some indo-west-pacific seagrasses

The plastochrone interval of the rhizomes (PIR) was determined in the monopodially branching seagrasses Cymodocea rotundata Ehrenb. et Hempr. ex Aschers. (1981), Cymodocea serrulata (R.Br.) Aschers. et Magnus (1981), Syringodium isoetifolium (Aschers.) Dandy (1982), Halodule uninervis (Forssk.) Aschers. (1982) and Halophila ovalis (R.Br.) Hook.f. (1982). The growth rate during 1981 was in all species, including Thalassia hemprichii (Ehrenb.) Aschers., constant during the year and the PIR amounted to 6.7 days (Cymodocea serrulata) and 3.9 days (Cymodocea rotundata). However, the PIR observed in 1982 showed in all species a unimodal pattern and a decrease during June, July and August. The PIR was 6.3 days in Halodule uninervis while an interval of 5.2 days was calculated in Syringodium isoetifolium.The growth rates of the horizontal and vertical axes (rhizomes and shoots) were correlated. A regression between the number of leaves on the shoot and the number of nodes on the rhizome resulted for most species in a linear correlation. The slopes of the calculated correlation were rather similar in the tree species with a comparable morphology: 0.44 in Cymodocea serrulata, 0.43 in Cymodocea rotundata and 0.49 in Halodule uninervis. In Syringodium isoetifolium, the longevity of the leaves increased during ageing of the shoot.The proliferation ratio (the ratio shoots/rhizome apices) was measured in all species. A maximum of 0.41 was observed in Thalassia hemprichii. This species and Cymodocea rotundata showed a unimodal pattern. In the last species, the ratio ranged from 0.03 to 0.13. The ratio was constant during the year in Cymodocea serrulata and Syringodium isoetifolium. Proliferation by means of monopodially branching of the rhizome was of little importance in Halodule uninervis. The pattern of proliferative and reiterative branching in the Cymodoceoideae is illustrated by an image of 123 days of growth in the fastest growing specimen.Most species showed growth in fronts. The expansion of monospecific seagrass fringes was monitored by repetitive mapping. The pattern of succession in subtidal meadows was determined.     

The distribution of seagrasses of Yap,Micronesia, with relation to low tide conditions

On the shallow coral shelves of Yap, the distribution of Cymodocea rotundata Ehrenb. & Aschers., Thalassia hemprichii (Ehrenb.) Aschers. and Enhalus acoroides (L.) Royle reflects their tolerance of low tide conditions that include mid-day water temperatures exceeding 40°C and water salinity during heavy rains to 2‰. The restriction of Cyamodocea serrulata (R.Br.) Aschers. & Magnus and Syringodium isoetifolium (Aschers.) Dandy to deeper sites reflects less tolerance to low tide conditions.     

Aspects of seagrass ecology along the western Arabian Gulf coast

Results of semi-quantitative observations and quantitative sampling of seagrasses at coastal and offshore sites along the western Arabian Gulf are presented. Overall seagrass cover (all species together) shows significant positive correlation with latitude, but not with salinity, temperature or depth. The same pattern is shown by Halodule uninervis (Forsk.) Aschers., the dominant species. Mean seagrass biomass ranged from 53–235 g m^-2 (dry weight). These values are comparable with biomass estimates from regions in which environmental conditions are generally less extreme than in the Gulf. Seagrass biomass is significantly negatively correlated with depth and sediment grain size. No significant correlation is apparent between seagrass biomass and factors such as season, salinity, or concentrations of nutrients and heavy metals measured. It is pointed out that any correlations observed are not necessarily taken to imply causality.     

Reproduction in Seagrasses: Nature of the Pollen and Receptive Surface of the Stigma in the Hydrocharitaceae

This paper describes the characteristics of the pollen and thereceptive surfaces of the stigmas in the three marine angiospermsincluded in the Hydrocharitaceae. The pollen in Enhalus acoroidesand Thalassia hemprichii is spherical and has an ornamentedexine. An exine layer is not found in Halophila stipulacea wherereniform pollen grains are contained within transparent moniliformtubes. Cytochemical tests show that the pollen wall in the threespecies contains acidic and neutral polysaccharides and acidhydrolase acitivity is detected in the intine of H. stipulaceaand T. hemprichii. In Thalassia, one of the intine enzymes,acid phosphatase, is unambiguously associated with cytoplasmicinclusions. Flowering in Thalassia is coincident with the spring tides andthe pollen is released as a mass suspended in a thecal slimewhich contains approximately 5 per cent by weight carbohydrate,the principal mono-saccharide being mannose. Electrophoreticanalysis of the pollen-free slime shows a single glycoproteincomponent. The stigmas of the three seagrasses are papillate and of the‘dry’ type possessing a continuous protein-aceouspellicle subtended by a cuticle. The stigma pellicle exhibitscytochemically detectable esterase activity and binds the lectinconcanavalin A. Acid phosphatase activity is localized beneaththe cuticle at the tips of the stigma papillae. The discoveries show that the characteristics of the pollenand stigmas in the seagrasses are comparable with those foundin terrestrial flowering plants. The similarity in enzymaticproperites of the pollen wall and stigma pellicle suggests that,intriguingly, a similar mechanism of cuticle erosion might wellfollow compatible pollination both on land and in the sea. Enhalus acoroides, Thalassia hemprichii, Halophila stipulacea, Halophila decipiens, seagrasses pollen wall, stigma surface, hydrolytic enzymes