Species-specific acclimatization capacity of key traits explains global vertical distribution of seagrass species

Aim

The global vertical depth distribution of seagrass species remains poorly understood. Locally, the abundance and distribution of seagrasses is determined by light penetration, but at global levels each seagrass species has very distinct maximum distributional depth ranges, indicating that plant-associated traits must also influence their specific depth ranges. Seagrass-specific attributes, such as plant size or architecture, growth or reproductive strategy and their physiological and/or morphological acclimatization potential, have been suggested to be responsible for this variety of vertical distributions. We investigate here whether these species-specific traits drive differences in the global maximum vertical distribution of seagrasses.

Location

Global.

Time period

Publications between 1982 and 2020.

Major taxa studied

Seagrasses (order Alismatales).

Methods

We tested whether the species-specific maximum vertical distribution of seagrasses can be predicted by (1) their rhizome diameter (a proxy for plant size); (2) their functional resilience (growth/reproductive strategy); or (3) their acclimatization capacity. For the last aspect, we used a systematic review followed by meta-analytical approaches to select key seagrass traits that could potentially acclimatize to extreme light ranges across different seagrasses.

Results

We found that vertical distribution is best explained by the species-specific acclimatization capacity of various seagrass traits, including saturation irradiance (physiological trait), leaves per shoot (morphological trait) and above-ground biomass (structural trait). In contrast, our results indicate no predictive power of seagrass size or growth/reproductive strategy on the vertical distribution of seagrasses.

Main conclusions

Across the globe, the ability of seagrass species to thrive at a wide range of depths is strongly linked to the species-specific acclimatization capacity of key traits at different organizational levels.     

Effects of irradiance, temperature, and nutrients on growth dynamics of seagrasses: A review

Productivity of seagrasses can be controlled by physiological processes, as well as various biotic and abiotic factors that influence plant metabolism. Light, temperature, and inorganic nutrients affect biochemical processes of organisms, and are considered as major factors controlling seagrass growth. Minimum light requirements for seagrass growth vary among species due to unique physiological and morphological adaptations of each species, and within species due to photo-acclimation to local light regimes. Seagrasses can enhance light harvesting efficiencies through photo-acclimation during low light conditions, and thus plants growing near their depth limit may have higher photosynthetic efficiencies. Annual temperatures, which are highly predictable in aquatic systems, play an important role in controlling site specific seasonal seagrass growth. Furthermore, both thermal adaptation and thermal tolerance contribute greatly to seagrass global distributions. The optimal growth temperature for temperate species range between 11.5 °C and 26 °C, whereas the optimal growth temperature for tropical/subtropical species is between 23 °C and 32 °C. However, productivity in persistent seagrasses is likely controlled by nutrient availability, including both water column and sediment nutrients. It has been demonstrated that seagrasses can assimilate nutrients through both leaf and root tissues, often with equal uptake contributions from water column and sediment nutrients. Seagrasses use HCO₃⁻ inefficiently as a carbon source, thus photosynthesis is not always saturated with respect to DIC at natural seawater concentrations leading to carbon limitation for seagrass growth. Our understanding of growth dynamics in seagrasses, as it relates to main environmental factors such as light, temperature, and nutrient availability, is critical for effective conservation and management of seagrass habitats.     

Plant–animal associations in two species of seagrasses in Western Australia

Relationships between algal epiphytes and epifaunal invertebrates (amphipods, molluscs and polychaetes) occurring within meadows of the seagrasses Posidonia sinuosa and Amphibolis griffithii were compared along the south west coast of Western Australia. Although the seagrasses are very different structurally, many species of algal epiphytes and epifaunal grazers were common to both. However, meadows of Amphibolis supported a greater number of both algal epiphyte and epifaunal species. The long-lived stems of Amphibolis supported a larger biomass of algal epiphytes and grazers than did the leaves of either Posidonia or Amphibolis. The densities and biomass of epifauna were variable but on a comparison adjusted to the biomass of seagrass, both the density and biomass of the taxonomic groups were similar between seagrass species except that the density of grazing gastropods and the biomass of polychaetes were greater in Amphibolis (by 238% and 252%, respectively). Nested analyses of variance (ANOVA) indicated that variations in plant and animal biomass differed at all spatial scales (sites, meadows within sites and replicates) and the pattern was inconsistent amongst biota. However, a significant proportion of the variability occurred between replicate samples. Canonical correlation and multiple regression analyses indicated that associations between algal epiphytes and epifauna were also inconsistent and differed between seagrass species. These patterns highlight the importance of seagrass species and structural complexity in affecting both the epiphytic and grazer community. The importance of spatial scales at which seagrasses and their associated communities are sampled are equally important because of the differing levels of spatial patchiness.     

Distribution of seagrasses along the Indian coast

Seagrass environments, from the main coast of India, Lakshadweep and Andaman Islands, were surveyed for seagrass and marine algal composition. Extensive seagrass meadows and the maximum number of species (seven genera and 12 species) occurred along the Tamil Nadu coast. Seagrasses were observed from intertidal to subtidal regions down to 8 m depth. Thalassia hemprichii (Ehrenberg) Aschers. and Cymodocea serrulata (R. Brown) Aschers. & Magnus were the dominant seagrasses in the subtidal zones. Halophila beccarii Aschers. was restricted to the intertidal mudflats in association with mangroves. The rich growth of seagrasses along the Tamil Nadu coast and Lakshadweep can be attributed mainly to high salinity, clarity of the water and sandy substratum. One hundred species of marine algae were recorded from the seagrass environments of India.     

Biomass seasonality of Caulerpa taxifolia in the Mediterranean Sea

The biomass of the introduced and invasive alga Caulerpa taxifolia was measured monthly over one year at four different sites along the French Mediterranean coast at depths of 5 and 20 m in a sheltered and an exposed area. At the 5 m depth, C. taxifolia mean biomass ranged from 203 to 518 g dry wt. m⁻², while at the 20 m depth, it ranged from 62 to 466 g dry wt. m⁻². The study clearly shows that a major characteristic of C. taxifolia is its perennial life cycle with relatively high biomass values throughout the year, in different biotopes. This could be a factor in the broad ecological impact of C. taxifolia.     

Temporal changes in the abundance, leaf growth and photosynthesis of three co-occurring Philippine seagrasses

The analysis of the temporal changes in shoot density, areal leaf biomass, leaf growth and parameters of the photosynthesis–irradiance relationship of three tropical seagrass species (Enhalus acoroides, Thalassia hemprichii and Cymodocea rotundata), co-existing in a shallow subtidal meadow in Cape Bolinao, Philippines, shows that species-specific traits are significant sources of temporal variability, and indicates that these seagrass species respond differently to a common environmental forcing. Species-specific differences are much less important as source of variability of the temporal change in chlorophyll concentration of seagrass leaves. The results indicate that the temporal changes in photosynthetic performance of these seagrasses were driven by environmental forcing and their specific responses to it mostly, but the temporal change in their abundance and leaf growth was also controlled by other factors. The significant contribution of species-specific factors in the temporal changes of biomass, growth and photosynthetic performance of co-occurring seagrass species in Cape Bolinao should contribute to the maintenance of the multispecific, highly productive meadows characteristic of pristine coastal ecosystems in Southeast (SE) Asia.     

Detection of inconspicuous epiphytic algae supporting food webs in seagrass meadows

Summary Detritus from common seagrasses and other marine angiosperms may often be a less important basis for estuarine food webs than previously believed. In NW Gulf of Mexico seagrass meadows, epiphytic algae have high productivities, palatability, and a more important trophic role than common large plants have. Interdisciplinary field experiments show (1) intensive night-time ingestion of epiphytes by various invertebrate detritivores, (2) very high productivity of epiphytic algae on seagrasses, and (3) assimilation of epiphytes rather than seagrasses, as measured by ¹³C comparisons. These combined data show that many naturally concentrated and potentially competing invertebrates in Gulf of Mexico seagrass meadows feed largely on the algal overgrowth on seagrass blades, even when such algae appear to be sparse. Primary productivity of these epiphytic algae can equal that of the seagrasses, per blade or per unit biomass. Animal ¹³C values tracked epiphytic values rather than seagrass values when comparisons were made over six sites. These measurements reinforce the view that epiphytic algae can be the primary basis of the food web in seagrass meadows.Contribution No. 608 of The University of Texas Marine Science Institute     

Seagrass–sediment interactions,positive feedbacks and critical thresholds for occurrence: a review

This literature review summarizes the limiting factors for seagrass occurrence, and the effect positive feedbacks in seagrass systems have on these threshold levels. Minimum water depth is mainly determined by wave orbital velocity, tide and wave energy; and maximum depth by light availability. Besides these, other limiting factors occur, such as an upper current velocity threshold, above which seagrasses are eroded, or a lower water current velocity threshold below which carbon exchange is limiting. In some locations organic matter content, sulphide concentration or nutrient availability are limiting. N-limitation is mainly reported from temperate terrigenous sediments, and P-limitation from tropical carbonate sediments. However, limiting factors sometimes change over the year, switching from light limiting to N- or P-limiting, and show at times regional variation. The effect seagrasses have on current reduction, trapping sediment and decreasing resuspension can lead to several changes in both the sediment and the water column. In the sediment, an increase in nutrient availability has been reported, and increases in organic matter, sediment height increases, and burial of the seagrasses. In the water column the effect is a reduction of the turbidity through a decrease of the sediment load, decreasing the attenuation coefficient, thereby increasing light availability. Due to the large effect light availability has on seagrass occurrence, the effect of an improvement of the light conditions by a reduction of the turbidity by seagrasses is probably the most important positive feedback in seagrass systems. The latter effect should therefore be incorporated in models that try to understand or predict seagrass changes. Generalization are difficult due a lack of studies that try to find relationships between seagrass architecture and sediment trapping (studying both turbidity reduction and nutrient increase) on a global level under a variety of different conditions. Areas for research priorities are identified.     

Dihydroxamic acid complexes of iron (III): ligand pKa and coordinated water hydrolysis constants

A series of dihydroxamic acid ligands of the formula [RN(OH)C(O)]₂(CH₂)_n, (n = 2, 4, 6, 7, 8; R = CH₃, H) has been studied in 2.0 M aqueous sodium perchlorate at 25.0 °C. These ligands may be considered as synthetic analogs to the siderophore rhodotorulic acid. Acid dissociation constants (pK_a) have been determined for the ligands and for N-methylacetohydroxamic acid (NMHA). The pK_a1 and pK_a2 values are: n = 2, R = CH₃ (8.72, 9.37); N = 4, R = CH₃ (8.79, 9.37); N = 6, R = CH₃; N = 7, R = CH₃ (8.95, 9.47); N = 8, R = CH₃ (8.93, 9.45); N = 8, R = H (9.05, 9.58). Equilibrium constants for the hydrolysis of coordinated water (log K) have been estimated for the 1:1 feeric complexes of the ligands n = 2, 4, 8; R = CH₃. The N = 8 ligand forms a monomeric complex with Fe(III) while the n = 2 and 4 ligands form dimeric complexes. For hydrolysis of the n = 8 monomeric complex, log K₁ = −6.36 and log K₂ = −9.84. Analysis of the spectrophotometric data for the dimeric complexes indicates deprotonation of all four coordinated waters. The successive hydrolysis constants, log K_1–4, for the dimeric complexes are as follows: n = 2 (−6.37, −5.77, −10.73, −11.8); n = 4 (−5.54, −5.07, −11.57, −10.17). The log K₂ values for the dimers are unexpectedly high, higher in fact than log K₁, inconsistent with the formation of simple ternary hydroxo complexes. A scheme is proposed for the hydrolysis of the ferric dihydroxamate dimers, which includes the possible formation of μ-hydroxo and μ-oxo bridges.     

Photosynthetic responses of seven tropical seagrasses to elevated seawater temperature

This study uses chlorophyll a fluorescence to examine the effect of environmentally relevant (1–4 h) exposures of thermal stress (35–45 °C) on seagrass photosynthetic yield in seven tropical species of seagrasses. Acute response of each tropical seagrass species to thermal stress was characterised, and the capacity of each species to tolerate and recover from thermal stress was assessed. Two fundamental characteristics of heat stress were observed. The first effect was a decrease in photosynthetic yield (F_v / F_m) characterised by reductions in F and F_m′. The dramatic decline in F_v / F_m ratio, due to chronic inhibition of photosynthesis, indicates an intolerance of Halophila ovalis, Zostera capricorni and Syringodium isoetifolium to ecologically relevant exposures of thermal stress and structural alterations to the PhotoSystem II (PSII) reaction centres. The decline in F_m′ represents heat-induced photoinhibition related to closure of PSII reaction centres and chloroplast dysfunction. The key finding was that Cymodocea rotundata, Cymodocea serrulata, Halodule uninervis and Thalassia hemprichii were more tolerant to thermal stress than H. ovalis, Z. capricorni and S. isoetifolium. After 3 days of 4 h temperature treatments ranging from 25 to 40 °C, C. rotundata, C. serrulata and H. uninervis demonstrated a wide tolerance to temperature with no detrimental effect on F_v / F_m′ qN or qP responses. These three species are restricted to subtropical and tropical waters and their tolerance to seawater temperatures up to 40 °C is likely to be an adaptive response to high temperatures commonly occurring at low tides and peak solar irradiance. The results of temperature experiments suggest that the photosynthetic condition of all seagrass species tested are likely to suffer irreparable effects from short-term or episodic changes in seawater temperatures as high as 40–45 °C. Acute stress responses of seagrasses to elevated seawater temperatures are consistent with observed reductions in above-ground biomass during a recent El Niño event.     

Study of the interaction of artemisinin with bovine serum albumin

The study on the interaction of artemisinin with bovine serum albumin (BSA) has been undertaken at three temperatures, 289, 296 and 303 K and investigated the effect of common ions and UV C (253.7 nm) irradiation on the binding of artemisinin with BSA. The binding mode, the binding constant and the protein structure changes in the presence of artemisinin in aqueous solution at pH 7.40 have been evaluated using fluorescence, UV–vis and Fourier transform infrared (FT-IR) spectroscopy. The quenching constant K_q, K_sv and the association constant K were calculated according to Stern–Volmer equation based on the quenching of the fluorescence of BSA. The thermodynamic parameters, the enthalpy (ΔH) and the entropy change (ΔS) were estimated to be −3.625 kJ mol⁻¹ and 107.419 J mol⁻¹ K⁻¹ using the van’t Hoff equation. The displacement experiment shows that artemisinin can bind to the subdomain IIA. The distance between the tryptophan residues in BSA and artemisinin bound to site I was estimated to be 2.22 nm using Föster's equation on the basis of fluorescence energy transfer. The decreased binding constant in the presence of enough common ions and UV C exposure, indicates that common ions and UV C irradiation have effect on artemisinin binding to BSA.     

Trophic Transfers from Seagrass Meadows Subsidize Diverse Marine and Terrestrial Consumers

In many coastal locations, seagrass meadows are part of a greater seascape that includes both marine and terrestrial elements, each linked to the other via the foraging patterns of consumers (both predators and herbivores), and the passive drift of seagrass propagules, leaves, roots and rhizomes, and seagrass-associated macroalgal detritus. With seagrasses declining in many regions, the linkages between seagrass meadows and other habitats are being altered and diminished. Thus, it is timely to summarize what is known about the prevalence and magnitude of cross-habitat exchanges of seagrass-derived energy and materials, and to increase awareness of the importance of seagrasses to adjacent and even distant habitats. To do so we examined the literature on the extent and importance of exchanges of biomass between seagrass meadows and other habitats, both in the form of exported seagrass biomass as well as transfers of animal biomass via migration. Data were most abundant for Caribbean coral reefs and Australian beaches, and organisms for which there were quantitative estimates included Caribbean fishes and North American migratory waterfowl. Overall, data from the studies we reviewed clearly showed that seagrass ecosystems provide a large subsidy to both near and distant locations through the export of particulate organic matter and living plant and animal biomass. The consequences of continuing seagrass decline thus extend far beyond the areas where seagrasses grow.     

Annual biomass and production of epiphytes in three monospecific seagrass communities of Thalassia hemprichii (Ehrenb.) Aschers

The biomass of epiphytes and seagrasses has been measured in relation to leaf age in three monospecific seagrass stands of Thalassia hemprichii (Ehrenb.) Aschers. in Papua New Guinea. From June 1981 through August 1982, biomass values for epiphytes at the three sites ranged from 5 to 70 g ADW m⁻² sediment surface at site 1, from 5 to 14 g ADW m⁻² at site 2, and from 3.5 to 7.0 g ADW m⁻² at the site 3. Annual mean epiphyte biomass values for the different sites were 1.3 g ADW m⁻² leaf surface at site 1, 1.7 g ADW m⁻² leaf surface at site 2, and 1.5 g ADW m⁻² leaf surface at site 3.

The annual mean standing crop of T. hemprichii leaves was highest at site 1 (103 g ADW m⁻². Values for site 2 and site 3 were 60 g ADW m⁻² and 41 g ADW m⁻², respectively.

Production of epiphytes was calculated in three different ways: firstly, by using biomass values for each specific leaf-age group, with corrections for colonization; secondly, by fitting the biomass values with a specific growth curve; and thirdly, by estimated the rate of biomass accumulation. On an area basis, production of epiphytes on leaves of T. hemprichii ranged from 0.55 to 3.97 g ADW m⁻² day⁻¹ at site 1, from 0.17 to 0.73 g ADW m⁻² day⁻¹ at site 2, and from 0.24 to 0.68 g ADW m⁻² day⁻¹ at site 3.     

Global seagrass distribution and diversity: A bioregional model

Seagrasses, marine flowering plants, are widely distributed along temperate and tropical coastlines of the world. Seagrasses have key ecological roles in coastal ecosystems and can form extensive meadows supporting high biodiversity. The global species diversity of seagrasses is low (< 60 species), but species can have ranges that extend for thousands of kilometers of coastline. Seagrass bioregions are defined here, based on species assemblages, species distributional ranges, and tropical and temperate influences. Six global bioregions are presented: four temperate and two tropical. The temperate bioregions include the Temperate North Atlantic, the Temperate North Pacific, the Mediterranean, and the Temperate Southern Oceans. The Temperate North Atlantic has low seagrass diversity, the major species being Zostera marina, typically occurring in estuaries and lagoons. The Temperate North Pacific has high seagrass diversity with Zostera spp. in estuaries and lagoons as well as Phyllospadix spp. in the surf zone. The Mediterranean region has clear water with vast meadows of moderate diversity of both temperate and tropical seagrasses, dominated by deep-growing Posidonia oceanica. The Temperate Southern Oceans bioregion includes the temperate southern coastlines of Australia, Africa and South America. Extensive meadows of low-to-high diversity temperate seagrasses are found in this bioregion, dominated by various species of Posidonia and Zostera. The tropical bioregions are the Tropical Atlantic and the Tropical Indo-Pacific, both supporting mega-herbivore grazers, including sea turtles and sirenia. The Tropical Atlantic bioregion has clear water with a high diversity of seagrasses on reefs and shallow banks, dominated by Thalassia testudinum. The vast Tropical Indo-Pacific has the highest seagrass diversity in the world, with as many as 14 species growing together on reef flats although seagrasses also occur in very deep waters. The global distribution of seagrass genera is remarkably consistent north and south of the equator; the northern and southern hemispheres share ten seagrass genera and only have one unique genus each. Some genera are much more speciose than others, with the genus Halophila having the most seagrass species. There are roughly the same number of temperate and tropical seagrass genera as well as species. The most widely distributed seagrass is Ruppia maritima, which occurs in tropical and temperate zones in a wide variety of habitats. Seagrass bioregions at the scale of ocean basins are identified based on species distributions which are supported by genetic patterns of diversity. Seagrass bioregions provide a useful framework for interpreting ecological, physiological and genetic results collected in specific locations or from particular species.     

Structure of major seagrass beds from three coral reef atolls of Lakshadweep, Arabian Sea, India

Detritus-based marine ecosystems such as mangrove and seagrass are of immense ecological importance. Major seagrass meadows from three coral atolls of the Lakshadweep group (Arabian Sea) were studied for their floral components. Seagrass beds were heterogeneous, comprising mainly of Thalassia hemprichii and Cymodocea rotundata, in Agatti and Kavaratti and it was observed to be monospecific (T. hemprichii) in the Kalpeni lagoon. Maximum (0.34 km²) and minimum (0.005 km²) extent of seagrass beds were observed in Kavaratti and Agatti lagoons, respectively. Seagrass weight (dry) of 43.97, 30.88 and 0.74 t were estimated from Kavaratti, Kalpeni, and Agatti, respectively. Maximum biomass occurred from 0–2 m depth, mainly contributed by the aboveground shoots, and was found to be negatively correlated with depth (r=0.71, p<0.05). Sediments were devoid of seed reserves indicating seagrass growth mainly by vegetative propagation. Epiphytes, on an average, contributed 7.5% of the seagrass biomass and were dominated by algae such as Melobesia spp., Microcoleus lyngbyaceus and Ceramium spp. Epiphytic biomass, too, decreased with increasing depth. Associated marine algae were represented by 66 species, dominated by rhodophytes.     

The distribution of seagrasses in Dominica, Lesser Antilles

Seagrass beds are the largest organism-built marine habitat in Dominica, yet have only been surveyed since 2007. Standardized examinations along a depth gradient between 0 and 24 m, focusing on magnoliophyte species composition and benthic cover of shoots at 17 seagrass bed sites, were carried out between September 10 and December 7, 2008. The Cymodoceaceae Syringodium filiforme (Kuetzing 1860) and Halodule wrightii (Ascherson 1868), as well as the Hydrocharitaceae Halophila decipiens (Ostenfeld 1902), H. stipulacea (Fosskal & Ascherson 1867) and Thalassia testudinum (Banks ex K?nig 1805) displayed distinct regional and horizontal distribution patterns. Syringodium filiforme is the island's dominant seagrass along the western and northern coasts, occurring at depths between 2 and 18 m and with a mean benthic cover ranging from 0.9-10% along the West coast. Along the North coast it grew between 0.2 and 1 m depth with a mean maximum benthic cover of 48.9%. Halodule wrightii grew along the North and West coasts, in depths between 1 and 14m in areas of recent and chronic disturbances. Its delicate morphology and sparse benthic cover (< 0.1%) did not constitute seagrass beds. Halophila decipiens grew along the deep, shallow and lateral margins of west coast S. filiforme beds and monospecifically in depths between 3 and 24m. Halophila stipulacea, an invasive species, was widespread along 45km of the West coast and was found in depths between 5 and 24m. Both Halophila species formed extensive beds at depths beyond the survey limit of 24m thus playing a potentially important role in the resettlement of shallow areas after storms. H. decipiens and H. stipulacea are currently the second and third most common seagrasses on the island respectively, despite their absence along the North coast. T. testudinum was confined to North coast's sheltered reef flats at depths Im or less with mean a benthic cover ranging from 2 to 76%. It grew monospecifically in the most turbulent and in the calmest locations, yet intermixed with S. filiforme in areas of moderate turbulence. Strong surge along the West coast (October 15-16, 2008), associated with Hurricane Omar, caused uprooting and burial of seagrass beds in varying degrees, in particular along the shallow margins between 2 and 10m depth. This event also demonstrated the dynamic nature of Dominica's shallow seagrass bed margins and the resistance level of individual beds to storm disturbances.     

Effects of water depth on clonal characteristics and biomass allocation of Halophila ovalis (Hydrocharitaceae)

Aims Halophila ovalis is a dioecious seagrass with a wide geographical and water depth range. The objective of this study was to understand its plasticity in clonal characteristics and biomass and also its allocation between above- and belowground in seagrass beds at different water depths.Methods Four monospecific H. ovalis beds, Shabei, Xialongwei, Beimu and Yingluo, which have different water depths at maximum tide level (MTL) but otherwise similar environmental conditions, were studied. We measured main clonal characteristics, i.e. horizontal internodal length, branching angle, shoot height, leaf length and width, and rhizome diameter. Above- and belowground biomasses of H. ovalis were also estimated using a harvest method.Important findings We found no significant differences in coverage, leaf pair density or number of stem nodes per square meter between the four study sites. However, horizontal internodal length, leaf length, width, rhizome diameter and shoot height all increased significantly with the increases in water depth from 2- to 9-m MTL and decreased when the water depths were greater than 9-m MTL. No significant difference in above- or belowground biomass between the seagrass beds was found. However, the ratio of above- to belowground biomass was significantly higher in the shallowest site compared to the other three seagrass beds, indicating that more biomass was stored belowground in deeper water. The results demonstrated plastic responses in clonal characteristics and biomass allocation in H. ovalis across the water depth gradient.     

A nitrite reductase with cytochrome oxidase activity from Micrococcus denitrificans

The formation of nitrite reductase and cytochrome c in Micrococcus denitrificans was repressed by O₂. The purified nitrite reductase utilized reduced forms of cytochrome c, phenazine methosulphate, benzyl viologen and methyl viologen, respectively, as electron donors. The enzyme was inhibited by KCN, NaN₃ and NH₂OH each at 1 mM, whereas CO and bathocuproin, diethyl dithiocarbamate, o-phenanthroline and ,'-dipyridyl at 1 mM concentrations were relatively ineffective. The purified enzyme contains cytochromes, probably of the c and a₂ types, in one complex. A K_m of 46 μM for NO₂⁻ and a pH optimum of 6.7 were recorded for the enzyme. The molecular weight of the enzyme was estimated to be around 130000, and its anodic mobility was 6.8·10⁻⁶ cm²·sec⁻¹·V⁻¹ at pH 4.55.

The most highly purified nitrite reductase still exhibited cytochrome c oxidase activity with a K_m of 27 μM for O₂. This activity was also inhibited by KCN, NaN₃ and NH₂OH and by NO₂⁻.

A constitutive cytochrome oxidase associated with membranes was also isolated from cells grown anaerobically with NO₂⁻. It was inhibited by smaller amounts of KCN, NaN₃ and NH₂OH than the cytochrome oxidase activity of the nitrite reductase enzyme and also differed in having a pH optimum of about 8 and a K_m for O₂ of less than 0.1 μM. Spectroscopically, cytochromes b and c were found to be associated with the constitutive oxidase in the particulate preparation. Its activity was also inhibited by NO₂⁻.

The physiological role of the cytochrome oxidase activity associated with the purified nitrite reductase is likely to be of secondary importance for the following reasons: (a) it accounts for less than 10% of total cytochrome c oxidase activity of cell extracts; (b) the constitutive cytochrome c oxidase has a smaller K_m for O₂ and would therefore be expected to function more efficiently especially at low concentrations of O₂.     

Macrograzers strongly influence patterns of epiphytic assemblages in seagrass meadows

The influence of factors, both abiotic (light and nutrients) and biotic (meadow structure, herbivory and seagrass shoot length) in the abundance and distribution of epiphytes on the Mediterranean seagrass Posidonia oceanica was investigated by means of a correlational approach over a spatial scale of ca. 500 km and at two different depths (5 and 15 m). Variability was examined from the double perspective of integrative community measures (biomass, species richness and alpha-diversity) and species composition. We assessed the influence of biotic and abiotic factors on integrative community measures using multiple correlation analyses. The influence of these factors in the structure of the whole community was investigated using a distance-based RDA for a linear model. A total of 129 taxa, mostly epiphytic algae, were recorded across all sites and depths. A large part of the variability in species composition (51%) was explained by the variables investigated. In particular, variability caused by differences in grazing pressure was the most important (25%), followed by variables related to nutrient availability (11%), meadow structure (6%), light (5%) and seagrass shoot length (4%). Among integrative community variables investigated, species richness was also best explained by grazing and nutrients.