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

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

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

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

Host-plant specialization by a non-herbivorous amphipod: advantages for the amphipod and costs for the seaweed

Studies of factors affecting host plant specialization by herbivores commonly highlight the value of the plant as both food and habitat, but often cannot distinguish the relative importance of these plant traits. A different approach is to study non-herbivorous animals that specialize on particular plants but do not feed on tissue from these plants. Such animals will not be affected directly by the nutritional, chemical, or morphological traits that determine the value of the plant as a food. This study reports on a filter-feeding amphipod, Ericthoniusbrasiliensis, that lives in domiciles it constructs by curling terminal segments of the green, calcified, and chemically defended seaweed Halimedatuna. We examined the temporal (1850s–1990s) and spatial (Caribbean, Mediterranean, and Pacific regions) scale of the association, the factors that may select for specialization on H. tuna, and the effect of the amphipod on growth of its host. Sampling along 125 km of coral reefs in the Florida Keys (USA) indicated that almost all populations of H. tuna had been colonized by this amphipod. Infested plants occurred on nine of ten reefs that supported H. tuna populations, with between 8 and 75% of the plants on those reefs colonized by the amphipod. For infested plants, 2–23% of all segments on each plant had been curled by the amphipod. Common co-occurring congeners of H. tuna (H. opuntia and H. goreaui) were never used for domicile construction. A survey of 1498 Halimeda specimens collected during the last 140 years and archived in the U.S. National Museum of Natural History (Smithsonian Institution, Washington, D.C.) indicated that the association has existed for >100 years and occurs throughout the Caribbean region, never in the Indo-Pacific or Mediterranean, and only on H. tuna. Predation by fishes could select for amphipod specialization on H. tuna. Laboratory experiments demonstrated that amphipods inhabiting curled segments of H. tuna were relatively immune from fish predation while those on the exterior surface of the plant or in open water were rapidly eaten. Segments of H. tuna are large enough to provide full protection from predators, while those of the co-occurring congeners H. goreaui and H. opuntia are of a size that may provide only partial protection. Experimental addition of E. brasiliensis to H. tuna plants in the field significantly decreased segment accumulation on infested relative to uninfested control plants. Whether this negative effect was a direct or indirect consequence of amphipod occupancy is unclear. Rolling plant portions into domiciles could directly decrease host growth by increasing shading and decreasing exposure of plant surface area to water column nutrient flux. Amphipod occupancy could indirectly slow net host growth if fishes selectively feed on plant sections occupied by amphipods. Underwater video showed that herbivorous fishes did not graze infested plants more than uninfested plants, but small predatory fishes did prefer feeding from infested plants. These non-herbivorous fishes may slow host growth by damaging the terminal meristematic tissues of plants during attacks on amphipods. This study demonstrates that habitat specialists can negatively impact hosts without consuming them and that specialization on a plant can occur due to its habitat value alone (as opposed to its value as a food). Received: 24 March 1998 / Accepted: 1 November 1998     

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

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

Species diversity and distribution of the calcareous green macroalgae Halimeda in Taiwan,Spratly Island,and Dongsha Atoll,with the proposal of Halimeda taiwanensis sp. nov.

The calcified green algal genus Halimeda is one of the most ecologically important but morphologically diverse seaweeds in sub-tropical and tropical waters. Because of its high morphological plasticity, the identification of Halimeda species based on morphological characters is challenging without the assistance of molecular analysis. To date, the species diversity of Halimeda in Taiwan and its overseas territories has not been investigated with the assistance of DNA sequencing, and this taxonomic knowledge gap should be filled. The present study initiates a systematic examination of the species diversity and distribution of Halimeda in Taiwan, Spratly Island, and Dongsha Atoll in the South China Sea, using DNA sequence data (plastid tufA gene and rbcL) and morphological data. Our DNA analyses revealed the presence of 10 Halimeda species (Halimeda borneensis, Halimeda cylindracea, Halimeda discoidea, Halimeda distorta, Halimeda macroloba, Halimeda minima, Halimeda opuntia, Halimeda renschii, Halimeda taiwanensis sp. nov., and Halimeda velasquezii) in the waters around Taiwan, Spratly Island, and Dongsha Atoll. The majority of the species could be readily distinguished by their morphological and anatomical characters. The proposed new species, H. taiwanensis, was differentiated not only by our algorithmic species delimitation analyses (statistical parsimony network analysis and automated barcode gap discovery), but also by its morphological features. The proposed new species differs from two externally resembled species, H. cuneata and H. discoidea, in having an undulated segment margin, the complete fusion of medullary siphons at the node, the lack of segment stalk, and the presence of a large primary utricle. Here, we present the up-to-date taxonomic account, molecular diversity, and geographical distribution of Halimeda spp. in Taiwan and associated areas of the South China Sea. Environmental factors that might drive the occurrence and latitudinal distribution of the species are also discussed.     

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

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

Chemical defense and chemical variation in some tropical Pacific species of Halimeda (Halimedaceae; Chlorophyta)

Over a dozen species of the genus Halimeda have been chemically investigated and found to produce the diterpenoid metabolites halimedatrial (1) and halimedatetraacetate (2) in varying concentrations. These meabolites have been proposed to play a role in chemical defense against herbivores based on their chemical structures and their demonstrated biological activities in laboratory and aquarium assays. We examined and compared the feeding deterrent effects of these two compounds tovard herbivorous fishes in field experiments on Guam reefs. Halimedatrial is a more effective feeding deterrent than halimedatetraacetate. It is the major secondary metabolite in young Halimeda macroloba and in the newly produced segments of growing plants. The organic extracts from young plants and new segments were significantly more deterrent than extracts from mature plant tissue. Some populations of Halimeda growing in reef-slope habitats, where herbivory is intense, also have high concentrations of halimedatrial. We compared extracts between reef slope and reef flat collections of Halimeda opuntia on Guam and Pohnpei (= Ponape), and H. discoidea and H. macroloba on Guam. We found that halimedtrial was the major metabolite in reef-slope collections of H. opuntia from Pohnpei and Pago Bay, Guam, and that halimedatetraacetate was the major metabolite a non-reef slope populations. In the cases examined, chemical defenses were greatest in (1) plant parts and (2) populations that were at greatest risk to herbivores.     

Distributional patterns of mobile fauna associated with Halimeda on the Tiahura coral-reef complex (Moorea,French Polynesia)

This study deals with the mobile fauna living associated with the turfs of three Halimeda species [H. incrassata, H. opuntia (three forms) and H. macroloba] from the Tiahura Reef complex. Ten 0.05 m² test areas of each Halimeda form have been randomly sampled from each geomorphological reef unit in order to obtain the specific abundance and biomass of different faunistic groups and species living within Halimeda populations. Then the raw data have been classified using the correspondence factor analysis to highlight the relationships between faunistic groups, or species, and the different Halimeda forms studied. The distributional patterns of the faunal communities seems to be controlled by the morphological features of the host-alga. The complex arborescent structure of the rhyzophytic H. incrassata species shelters fauna three times higher in abundance than other Halimeda heads. This fauna is mostly composed of a highly rich community of microcrustaceans and a very diversified community of Syllidae polychaetes. H. macroloba living on the outer reef flat retains a lot of small Nereidae polychaetes and a rich microgastropod assemblage. Dense H. opuntia tussocks (type A) on the fringing-reef and isolated H. opuntia fronds of pendulous chains (type B) on the barrier reef as well as beds of scattered H. opuntia (type C) on the outer reef flat provide environment for larger animals, and according to their zone of life, they retain very different faunal associations. Some ophiuroids, and some fish, collected in H. opuntia A and C, are newly reported from the Society Islands, French Polynesia, and the Pacific province. One Brachyurid species is presumably undescribed.     

The effects of phosphate on the biomineralization of the green alga, Halimeda incrassata (Ellis) Lam.

Field surveys indicated that individuals of Halimeda incrassata (Ellis) Lamouroux, a rhizophytic alga, were significantly more mineralized when collected from phosphate-limited carbonate sediments of the Florida Keys than those collected from siliciclastic sediments at Tarpon Springs on the west coast of Florida. Results from field experiments in Tarpon Springs, which compared growth of H. incrassata in enriched conditions to unmanipulated controls, indicated that biomineralization of new growth was significantly lower when phosphate was added to plots. After 18 days in a tank culture experiment, newly produced segments of H. incrassata grown in 20 µM phosphate showed a significant decrease in mineralization compared to the new growth in control tanks. Phosphate enrichment also led to a non-significant increase in segment mortality and a significant decrease in amount of new growth. Results from the survey and experimental studies suggest that biomineralization of H. incrassata is negatively impacted by elevated phosphate levels.     

Field biology of Halimeda tuna (Bryopsidales, Chlorophyta) across a depth gradient: comparative growth, survivorship, recruitment, and reproduction

Growth, survivorship, recruitment, and reproduction of Halimeda tuna, a dominant green alga in many reef systems of the Florida Keys, were monitored at a shallow back reef (4–7m) and deep reef slope (15–22 m) on Conch Reef. Despite lower light intensities and similar grazing pressures, amphipod infestations, and epiphyte loads at both sites, the deeper site exhibited significantly higher growth rates in summer months over a 4-year period than found for the shallow population, possibly because of higher nutrient levels at depth and photoinhibition of shallow plants. Sexual reproductive events occurred simultaneously across the entire reef, with up to 5% of the population at both sites developing gametangia. New upright axes formed from zygotes, asexual fragmentation, or vegetative runners. Plants appear to have persistent basal stumps that survive harsh environmental conditions, even if upright, photosynthetic axes are removed. Sexual reproduction and smothering by epiphyte overgrowth are hypothesized to be two causes of death for individuals.     

Fragment generation, survival, and attachment of Dictyota spp. at Conch Reef in the Florida Keys, USA

During the past decade, the relative abundance of the brown macroalgae Dictyota spp. has been high in the Florida Keys. Recent studies have shown that members of this genus successfully reproduce via vegetative fragmentation. To investigate the importance of fragmentation on the reef community, this study examined: (1) the degree of epiphytism on benthic organisms, (2) the rate of fragment production through fish foraging activities, (3) the likelihood of fragment entanglement, and (4) the fragment attachment and success rate. It was found that reef fish contributed substantially to the fragment pool; furthermore, most fish-produced fragments produced rhizoids and attached to sand grains within 24 h in the field. Fragments of Dictyota spp. most commonly became entangled around and then attached themselves to the green alga Halimeda tuna, and other Dictyota spp. These results suggest that vegetative fragmentation of Dictyota spp. plays an important role in the changing community structure on the Florida Keys reef tract.     

The dispersal of Halimeda in northern hemisphere mid-latitudes: Palaeobiogeographical insights

The bryopsidalean alga Halimeda gained an important role as carbonate producer in Cenozoic tropical coral reefs and became a significant constituent of the modern Mediterranean seaweed flora. There are, however, open questions at which time the thermophile alga appeared in the cooler Mediterranean Sea and why it is not detected in coral reefs of the modern Persian Gulf. To unravel the biogeography and ecology of Halimeda at its northern margin of distribution, we use fossil Halimeda records of the Central Paratethys/Medditerranean for comparison of the geological, (palaeo)ecological and evolutionary dispersal constraints of the alga in the Miocene and Holocene Persian Gulf. The revealed spatial and temporal distribution patterns of Halimeda in the regions of the Mediterranean and Arabian seas identify water temperature as the major ecological constraint and the extreme Plio-Pleistocene climate changes as the motor for the dispersal and evolution of Halimeda in higher latitudes. Generally, the distribution of tropical species in higher latitudes was related to warm climate intervals during the Neogene. Accordingly, the available (palaeo)biogeographic data implies that the warm-adapted ancestors of the present-day Mediterranean H. tuna population possibly entered the Mediterranean Sea during the mid-Pliocene global warmth and became isolated during subsequent cooling. It also implies that the warm Persian Gulf water is probably unsuitable for the cool-adapted H. discoidea population in the Gulf of Oman and that its tropical ancestors could have reached the Gulf of Oman only during a Pleistocene glacial phase when monsoon-induced upwelling of cold water in the Arabian Sea was reduced and the Persian Gulf fell dry. This example demonstrates the limitation of the actualistic palaeontological approach when using biota at the edges of their distribution range as palaeoclimate proxy.     

Irradiance and nutrient limitation of Dicytota spp. populations on Conch Reef, Florida Keys, USA

The photosynthetic performance, pigmentation and response to nutrient enrichment of a Dictyota community were studied over a 32 m depth gradient on Conch Reef, Florida Keys, USA. Dictyota spp. was the dominant space occupier on Conch Reef. During summer months from 1994 to 2001, mean percent cover was 43% at 7 and 21 m depths. Percent cover of Dictyota spp. was markedly lower at 32 m relative to shallower sites. The metabolism of Dictyota menstrualis and Dictyota pulchella were negatively impacted by attenuation of irradiance with increased depth such that ETR_max and P/R ratio decreased by 85% and 47%, respectively in samples from 7 to 32 m. Decreased cover of Dicytota spp. at 32 m relative to shallower sites may be the result of the inability of this species complex to acclimate to low irradiance levels as indicated by a lack of change in α and photosynthetic pigment content with increased depth.The response of D. menstrualis populations on Conch Reef to nutrient enrichment was variable. During August 2000, a natural enrichment experiment was conducted as D. menstrualis photosynthetic performance was surveyed both during and for days after a period of cool, nutrient-rich internal tidal bores bathed the 21 m site. No changes in in situ photosynthetic performance were observed either during or for 2 to 4 days after this natural event suggesting no nutrient limitation. In August 2001, a manipulative nutrient enrichment experiment was conducted with D. menstrualis from 7, 21 and 32 m. Increases in Φ_II were found in individuals exposed to nutrient enrichment from all depths, indicating at that time D. menstrualis was nutrient-limited on Conch Reef. Variation in the physiological response to nutrient enrichment may be the result of the frequency of internal tidal bores on Conch Reef in the months prior to our experiments. Variation in the responses by Dictyota spp. to irradiance and nutrient enrichment suggests that long-term monitoring over relevant temporal and spatial scales is necessary to accurately characterize limits on productivity and spread of this weedy species assemblage.     

The Physiological Response of Two Green Calcifying Algae from the Great Barrier Reef towards High Dissolved Inorganic and Organic Carbon (DIC and DOC) Availability

Increasing dissolved inorganic carbon (DIC) concentrations associated with ocean acidification can affect marine calcifiers, but local factors, such as high dissolved organic carbon (DOC) concentrations through sewage and algal blooms, may interact with this global factor. For calcifying green algae of the genus Halimeda, a key tropical carbonate producer that often occurs in coral reefs, no studies on these interactions have been reported. These data are however urgently needed to understand future carbonate production. Thus, we investigated the independent and combined effects of DIC (pCO₂ 402 μatm/ pH_tot 8.0 and 996 μatm/ pH_tot 7.7) and DOC (added as glucose in 0 and 294 μmol L^-1) on growth, calcification and photosynthesis of H. macroloba and H. opuntia from the Great Barrier Reef in an incubation experiment over 16 days. High DIC concentrations significantly reduced dark calcification of H. opuntia by 130 % and led to net dissolution, but did not affect H. macroloba. High DOC concentrations significantly reduced daily oxygen production of H. opuntia and H. macroloba by 78 % and 43 %, respectively, and significantly reduced dark calcification of H. opuntia by 70%. Combined high DIC and DOC did not show any interactive effects for both algae, but revealed additive effects for H. opuntia where the combination of both factors reduced dark calcification by 162 % compared to controls. Such species-specific differences in treatment responses indicate H. opuntia is more susceptible to a combination of high DIC and DOC than H. macroloba. From an ecological perspective, results further suggest a reduction of primary production for Halimeda-dominated benthic reef communities under high DOC concentrations and additional decreases of carbonate accretion under elevated DIC concentrations, where H. opuntia dominates the benthic community. This may reduce biogenic carbonate sedimentation rates and hence the buffering capacity against further ocean acidification.     

Calcification in the Green Alga Halimeda: IV. THE ACTION OF METABOLIC INHIBITORS ON PHOTOSYNTHESIS AND CALCIFICATION

The effects of a number of metabolic inhibitors on calcificationand photosynthesis in Halimeda tuna, H. discoidea, and H. macrolobaare described. The inhibitors used are CCCP, DNP, DCMU, azide,cyanide, chloramphenicol, cycloheximide, and Diamox. The effectsof these inhibitors, although complex, are consistent with ourmodel of calcification in Halimeda. Inhibition of photosyntheticCO₂ uptake inhibits calcification as does stimulation of respiratoryCO₂ evolution (i.e. uncoupling). There is also indirect evidencefor the presence of a possible light stimulated H⁺ efflux whichinhibits calcification. The observed calcification rate is thereforethe result of a number of factors which affect the concentrationof CO^–and the pH in the intercellular space of the Halimedathallus. The results obtained with the carbonic anhydrase inhibitor Diamoxprovide further evidence for the effective separation of theintercellular space from the external medium by the appressedperipheral utricles.     

Production of rhizoids by Caulerpa prolifera in culture

Studies on Caulerpa prolifera rhizoids were carried out to determine the possibility of mass culture, because the rhizoids produce a bio-adhesive. Rhizoids can be induced by cutting the base of a blade and floating it in a media or planting it in sand. Measurement of rhizoid production included determination of number, length, and the weight of attached sand grains. The growth experiments were for 1–2 weeks and fronds growth was compared to rhizoid production. Optimal conditions for rhizoid growth included low levels of nitrogen and phosphate (less than 5 and 2 μM, respectively), low irradiance (30 μmol photon m⁻² s⁻¹), moderate temperature (22–28°), continuous shaking, addition of microelements and auxin (1 ppm) and initially detached fronds followed by attachment. Under these optimal conditions maximal weekly growth reached 70–170 rhizoids per blade, 7–11 mm length and maximal attachment to sand grains. Blade growth of C. prolifera responded similarly to rhizoid production and reached a weekly growth rate of 30–130%.     

Structural characterization of trace stilbene glycosides in Lysidice brevicalyx Wei using liquid chromatography/diode-array detection/electrospray ionization tandem mass spectrometry

The mass fragmentation patterns of stilbene glycosides isolated from the genus Lysidice were investigated by negative ion electrospray ionization tandem mass spectrometry, and the influence of collision energy on their fragmentation behavior is discussed. It is found that the presence of the Y₀⁻ and B₀⁻ ions in the MS² spectra is characteristic for 1 → 6 linked diglycosyl stilbenes, while the Y₀⁻, Y₁⁻, and Z₁⁻ ions are representative ions of 1 → 2 linked diglycosyl stilbenes. These results indicate that ESI-MSⁿ in the negative ion mode can be used to differentiate 1 → 6 and 1 → 2 linked diglycosyl stilbenes. Based on the fragmentation rules, 9 new trace constituents were identified or tentatively characterized in a fraction of Lysidice brevicalyx by using HPLC/HRMS and HPLC-DAD/ESI-MSⁿ. The results of the present study can assist in on-line structural identification of analogous constituents and targeted isolation of novel compounds from crude plant extracts.