Changes in pH as a measure of photosynthesis by marine macroalgae

An automatically operated method for high precision measurements of steady-state photosynthesis by macroalgae was developed. Changes in pH and oxygen content of seawater passing the algae in a flowthrough system, could be measured with extremely high accuracy over very long periods of time. The method is especially suitable for measurements on flowthrough systems with high rates of water exchanges (i.e. short retention time), and can be used to study exchange processes for marine plants, animals and small ecosystems. Since the same measuring unit is used for several flowthrough chambers, the method is very suitable for comparisons between different species, or between differently pretreated specimens of the same species (e.g. in toxicological studies). The method was used to study the ratio: [oxygen production] to [CO₂+H⁺ uptake] at different light intensities for several macroalgae belonging to different systematic groups and from different habitats. At lower photosynthetic rates this ratio was similar for all of the algae studied (1.17±0.02). For brown algae of the fucacean family, the ratio increased by 0.08 units at higher photosynthetic rates. This increase was thought to be related to the crassulacean acid metabolism (CAM)-like strategies connected to these algae. For all other algae studied, the ratio remained constant or decreased slightly (at most by 0.04 units) at higher photosynthetic rates. The relations between the abovementioned ratio and the photosynthetic quotient are discussed on a theoretical basis.     

Neutral lipids in macroalgal spores and their role in swimming

We followed changes in the neutral lipid content of actively swimming zoospores of the palm kelp Pterygophora californica in a laboratory experiment to investigate the degree to which spore swimming is fueled by endogenous lipid reserves. The neutral lipid content of individual zoospores during the experiment was measured by flow cytometry using Nile Red, a fluorescent stain that is specific for neutral lipid. Results showed that photosynthesis greatly influenced lipid consumption during zoospore swimming. We found no detectable change in the neutral lipid content of zoospores after 30 h of swimming under conditions where light was near the optimum for photosynthesis. By contrast, neutral lipid declined by ≃43% over 30 h in zoospores kept in the dark. To evaluate whether lipid reserves are generally related to spore motility in macroalgae, we surveyed spore lipid-content and composition in species with motile spores and non-motile spores using thin-layer chromatography (TLC), and flame-ionization detection (FID). We observed substantial differences in lipid content and composition among the 20 species examined. Spores high in total lipid (as estimated by the ratio of lipid:carbon) generally had a large amount of neutral lipid; motile spores had significantly more lipid and a significantly larger neutral lipid fraction than non-motile spores. The kelps as a group had the highest total lipid content and the largest neutral lipid fraction, while non-motile spores of red algae were generally low in total lipid and in the proportional abundance of neutral lipid. Phospholipids accounted for more than half of all lipid in 14 of the 20 species examined, while neutral lipid accounted for the majority of lipid in all five species of kelp examined. Triacylglycerols, which function primarily in energy storage, were the primary form of neutral lipid in all but one species of kelp (Agarum fimbriatum), whereas free fatty acids were the dominant form of neutral lipid in most red algae and in brown algae that had a small neutral lipid fraction. Our results are largely consistent with the hypothesis that macroalgae use endogenous lipid reserves to fuel spore-swimming. The small amounts of triacylglycerols observed in the motile spores of several species of brown and green algae, however, indicate that the amount of lipid reserves needed to fuel spore-swimming may be influenced by a variety of factors including swimming behavior, photosynthetic efficiency, and the light environment inhabited by spores. Received: 8 September 1998 / Accepted: 8 January 1999     

Patterns and UV sensitivity of carbon anhydrase and nitrate reductase activities in south Pacific macroalgae

This study describes the activities of the key enzymes involved in carbon incorporation (carbonic anhydrase, CA) and inorganic nitrogen reduction (nitrate reductase, NR) in 25 intertidal macroalgae of southern Chile (39°S). UV radiation as a factor affecting the nutrient metabolism of algae was also examined. The results of the enzyme activities and the UV sensitivity were related to the position of the algae on the shore, species/taxonomic groups and morpho-functional patterns. The CA activity in the studied algae ranged from 42 to 165 REA g⁻¹ FW, and was neither related to growing depth nor to taxonomic or morpho-functional groups. The NR activities ranged from 0.1 to 8.9 μmol NO₂⁻ g⁻¹ FW min⁻¹, with the highest levels observed in red algae. In contrast to CA, the NR activities showed a decreasing tendency from supra/midlittoral to infra/sublittoral. Also, differences between morpho-functional groups were seen. The impact of artificial UV radiation on CA and NR activities was variable as in some species it provoked an increase while in other species a decrease was observed, suggesting species-specific responses and UV sensitivity. The CA activity was the most UV sensitive in the taxonomic group Chlorophyta and in the supralittoral algae. The UV sensitivity of NR activity could not be related to any patterns related to morpho-functional or taxonomic groups and habitat depth.     

Selective consumption and facilitation by mesograzers in adult and colonizing macroalgal assemblages

Grazing pressure on macroalgae in littoral communities may vary with algal species, the age of an algal individual and grazer identity. Previous studies on alga–grazer interactions have shown that grazer preference for an algal species may release another one from interspecific competition. We measured the impacts of four common grazer taxa and the natural grazer guild on macroalgal communities at both their colonization and adult stages, and compared the impacts to grazer exclosures. The grazer effects were stronger on colonizing than on adult macroalgae; grazers did not reduce the total density of adult algae. Grazers both feed on propagules and indirectly facilitate other algae, depending on the grazer or algal species. Hydrobia species increased the settlement of spores of the red alga Ceramium tenuicorne. Similarly, the gastropod Theodoxus fluviatilis tended to facilitate one crustose algal species, but decreased the propagule density of annual filamentous algae, suggesting a preference for one species to the advantage of another. Effects of crustacean mesograzers on the studied macroalgae were weak. These results indicate that northern Baltic macroalgae are limited to grazing mainly during their colonization stage.     

Nitrogen- versus phosphorus-limited growth and sources of nutrients for coral reef macroalgae

Recent investigations of nutrient-limited productivity in coral reef macroalgae have led to the conclusion that phosphorus, rather than nitrogen, is the primary limiting nutrient. In this study, comparison of the dissolved inorganic nitrogen:phosphorus ratio in the water column of Kaneohe Bay, Hawaii, with tissue nitrogen:phosphorus ratios in macroalgae from Kaneohe Bay suggested that nitrogen, rather than phosphorus, generally limits productivity in this system. Results of nutrient-enrichment experiments in a flow-through culture system indicated that inorganic nitrogen limited the growth rates of 8 out of 9 macroalgae species tested. In 6 of the species tested, specific growth rates of thalli cultured in unenriched seawater from the Kaneohe Bay water column were zero or negative after 12 d. These results suggest that, in order to persist in low-nutrient coral reef systems, some macroalgae require high rates of nutrient advection or access to benthic nutrient sources in addition to nutrients in the overlying water column. Nutrient concentrations in water samples collected from the microenvironments inhabited or created by macroalgae were compared to nutrient concentrations in the overlying water column. On protected reef flats, inorganic nitrogen concentrations within dense mats of Gracilaria salicornia and Kappaphycus alvarezii, and inorganic nitrogen and phosphate concentrations in sediment porewater near the rhizophytic algae Caulerpa racemosa and C. sertularioides were significantly higher than in the water column. The sediments associated with these mat-forming and rhizophytic species appear to function as localized nutrient sources, making sustained growth possible despite the oligotrophic water column. In wave-exposed habitats such as the Kaneohe Bay Barrier Reef flat, water motion is higher than at protected sites, sediment nutrient concentrations are low, and zones of high nutrient concentrations do not develop near or beneath macroalgae, including dense Sargassum echinocarpum canopies. Under these conditions, macroalgae evidently depend on rapid advection of low-nutrient water from the water column, rather than benthic nutrient sources, to sustain growth. Received: 1 December 1997 / Accepted: 9 July 1998     

Biomass, diversity and production of rocky shore macroalgae at two nutrient enrichment and wave action levels

The littoral zone of temperate rocky shores is normally dominated by perennial macroalgae (e.g. Fucus, Ascophyllum, Laminaria), but nutrient enrichment and/or permanently decreased wave action may lead to structural community changes from dominance of perennials to increased amounts of annual opportunistic species (mainly green algae). Macroalgal biomass, diversity and production as well as relationships between the two latter were studied using Solbergstrand’s rocky shore mesocosms in SE Norway in connection with a long-term experimental manipulation of nutrient addition and wave action (high and low levels of both factors applied in a crossed way to eight outdoor basins). After more than 2 years of experimental treatment, the total standing stock of macroalgae was larger in low nutrient than in high nutrient treatments as well as in high wave compared to low wave treatments (in autumn only). For macroalgal functional groups, bushy and filamentous brown and filamentous red algae were generally favoured by low nutrient concentrations, while annual filamentous and sheet-like green algae were stimulated by the nutrient enrichment. There was only one significant interaction between nutrient enrichment and wave action (for brown filamentous algae in autumn) and also only one significant main effect of the wave treatment (for bushy brown algae in autumn). Surprisingly, the high nutrient treatments supported a higher diversity of macroalgae, whereas the low nutrient treatments generally showed higher production rates. Moreover, significantly negative correlations were found between macroalgal diversity and primary productivity in both summer and autumn. This study shows that it is the biological components of the communities subjected to external forcing (nutrient addition or decreased wave action) that regulate production and this contradicts the common misperception that resource production in natural systems simply can be fast-forwarded by fertilization. The negative relationships between diversity and productivity, although a consequence of unexpected results for diversity and production, are also novel and hint towards species identities having more important functional consequences than general species dominance patterns and the amount of species per se. These results also emphasise the context dependency of findings within the field of biodiversity and ecosystem functioning.     

Food preferences and related behavior of the browsing sea urchin <Emphasis Type="Italic">Tripneustes gratilla</Emphasis> (Linnaeus) and its potential for use as a biological control agent

The short-spined toxopneustid sea urchin Tripneustes gratilla feeds on a wide variety of algal species and on sea grasses. However, the urchin does show preferences when offered a selection of macroalgal species, which it encounters in nature. Preferences among macroalgae were evident in field-collected urchins exposed to pair-wise tests where the variable was either the consumption rate of the algae or observation of which algal species the urchins chose to touch with their lantern teeth. Exposure of lab-housed urchins to one of five species of macroalgae for 5 months did not seem to alter preferences of urchins in three of the exposure groups, but those exposed to Padina sanctae-crucis seemed to show an enhanced preference for this species when offered a choice of the five species of macroalgae at the end of the exposure period, and those exposed to Gracilaria salicornia seemed to avoid the species when offered the choice of the five species. Perhaps more ecologically important than their preferences were two other observations on these urchins: first, when offered only a single species of algae, the urchins on four of five diets ate the same quantity per day. Second, when simultaneously offered the choice among the five macroalgal species, the urchins consumed more macroalgae per day than when offered only one species. These urchins move about a meter a day. They probably encounter food resources in a relatively coarse-grained fashion and have evolved to eat what is available. Because of their limited movements, their habitat overlap with grazing fishes, their acceptance of a wide variety of macroalgae and their preference for macroalgae, these native urchins are thought to have the potential to serve as biological control agents of alien and invasive macroalgae, which have come to dominate some reef zones normally occupied by corals in Hawaii.     

Dopamine release by the green alga <Emphasis Type="Italic">Ulvaria obscura</Emphasis> after simulated immersion by incoming tides

Ulvaria obscura, a prominent component of green tide blooms in Washington, is unique among macroalgae because it contains dopamine. To examine dopamine release by U. obscura following simulated low tides, we conducted 6 field experiments in which algae were emersed for 75 min and then immersed in filtered seawater (FSW). Dopamine was measured in algal tissues prior to emersion and 3 h after immersion and in seawater for 3 h following immersion. In our experiments, algae released 7–100% of their tissue dopamine, resulting in average seawater concentrations of 3–563 μM. In 5 of 6 experiments, seawater dopamine concentrations were highest immediately after immersion, and then decreased over time. The percentages of dopamine released were not correlated with tissue dopamine concentrations, but were positively correlated with solar radiation during emersion. The release of dopamine, which is both cytotoxic and genotoxic, may explain the negative effects of U. obscura exudates on marine organisms.     

Vertical heterogeneity in physiological characteristics of Ulva spp. mats

In eutrophic areas, green macroalgae are frequently and for long periods arranged in mats, resulting in a steep light gradient. This study investigates the effect of this gradient on physiological characteristics [tissue nitrogen content, maximal photosynthetic efficiency (F_v/F_m), glutathione levels and redox ratio, absorbance and absorption spectra] of the green macroalga Ulva spp. Mats were sampled during the build-up (June), stationary (July), and decomposing (September) phases of a macroalgal bloom in the Veerse Meer, a eutrophic brackish (salinity 15–20 psu) lake in the southwest Netherlands. Water samples were taken for nutrient analyses. At all three sampling dates, the mats were composed almost entirely of Ulva spp.; in September the mats were in decay and covered with silt and epiphytes. In June and July, total dissolved inorganic nitrogen concentration (DIN) of the water within the mat was significantly higher than outside the mat. Pronounced vertical differences were found in tissue N, F_v/F_m values, total glutathione levels, glutathione redox ratios, and absorbance. In June and July, tissue N decreased from over 2.2% dry weight (DW; N-sufficient) in the bottom layers to around 1% DW (minimum level for survival) in the top layers. Wide-band absorption increased with depth in the mat and throughout the season, probably due to higher Chl a and b and lutein contents. The shape of the absorption spectrum was similar for all layers. The absorption of the silt/epiphyte film on the top Ulva layer was highest; its absorption spectrum (high absorption in the 500–560 nm range) indicates that the film on the top layers of the macroalgal mats mainly consisted of diatoms. In June, F_v/F_m and the glutathione redox ratio of the algae increased with depth in the layer, while total glutathione decreased. Low F_v/F_m values in the bottom and middle layers in September reflect the bad condition of the algae; the mats were largely decaying. It is concluded that multiple growth-limiting gradients occur in macroalgal mats: upper layers suffer from nitrogen limitation and photoinhibition while bottom layers are light limited. The algae in the mat acclimatize to low light conditions by increasing their absorption through increased pigment contents and by higher photosynthetic efficiency during the build-up and stationary period. This study qualifies the glutathione redox ratio as a promising candidate for stress indicator in macroalgae and provides suggestions for its further development.Communicated by S.A. Poulet, Roscoff     

Variability in grazer-mediated defensive responses of green and red macroalgae on the south coast of South Africa

Variabilities in the responses of several South African red and green macroalgae to direct grazing and the responses of one green alga to cues from grazers were tested. We used two feeding experiments: (1) testing the induced responses of three red and one green algae to direct grazing by mesograzers and (2) a multi-treatment experiment, in which the direct and indirect effects of one macrograzer species on the green alga Codium platylobium were assessed. Consumption rates were assessed in feeding assays with intact algal pieces and with agar pellets containing non-polar extracts of the test algae. Defensive responses were induced for intact pieces of Galaxaura diessingiana, but were not induced in pellets, suggesting either morphological defence or chemical defence using polar compounds other than polyphenols. In contrast, exposure to grazing stimulated consumption of Gracilaria capensis and Hypnea spicifera by another grazing species. In the multi-treatment experiment, waterborne cues from both grazing and non-grazing snails induced defensive algal traits in C. platylobium. We suggest that inducible defences among macroalgae are not restricted to brown algae, but that both the responses of algae to grazers and of grazers to the defences of macroalgae are intrinsically variable and complex.     

Dietary separation between two blennies and the Pacific gregory in northern Taiwan: evidence from stomach content and stable isotope analyses

Two blennies, Ecsenius lineatus Klausewitz and Ecsenius namiyei (Jordan and Evermann), and a cohabiting territorial damselfish, the Pacific gregory, Stegastes fasciolatus (Ogilby), were collected from shallow reefs in northern Taiwan between September and November 2004, and in October 2005 for stomach content and δ ¹³C and δ ¹⁵N analyses in an effort to study how extensively their food sources overlapped and to delineate the pattern of cohabiting interactions. These analyses showed differences in food use between the Ecsenius blennies and S. fasciolatus. However, there were inconsistencies. Epiphytic algae were their major food items of E. namiyei and E. lineatus. Macroalgae were rarely taken. Nevertheless, δ ¹³C and δ ¹⁵N signatures suggested that E. namiyei and E. lineatus might have assimilated mainly macroalgae-derived detritus instead of epiphytic algae. In contrast, macroalgae were the major food items of S. fasciolatus, followed by epiphytic algae. Differences in both δ ¹³C and ¹⁵N values indicated that for S. fasciolatus, algae (both macroalgae and epiphytic algae) might not be as important as the stomach contents showed. Instead, polychaetes were possibly its major food source. Differences between stomach contents and evidence from the separation of stable isotope signatures between blennies and the Pacific gregory indicate that some of the interspecific interactions derived from exploitative competition may have been alleviated. Moreover, their widespread territory overlap is possibly a sign of mutualism: S. fasciolatus allows territory sharing, while Ecsenius blennies, in return, clean up the algal mat by removing sand and detritus.     

Different patterns of carotenoid composition and photosynthesis acclimation in two tropical red algae

To be able to survive, marine macroalgae in shallow coastal waters need mechanisms for short-term acclimation to fast changes in their environment. Of major importance are mechanisms that regulate the efficiency of photosynthesis by protecting PS II from photo-oxidative damage. Carotenoids, xanthophyll cycles and non-photochemical quenching (NPQ) are central constituents of such protection mechanisms. Red algae as a group do not have a universal carotenoid composition. We screened ten red algal species and selected two species, originating from similar ecological conditions but with different carotenoid compositions, for use in irradiance-acclimation experiments. We selected the tropical intertidal species Gracilaria domingensis and Kappaphycus alvarezii with antheraxanthin and lutein as major xanthophylls, respectively. Simultaneous in vivo fluorescence and O₂ evolution experiments were performed at different irradiance levels, which allowed a direct comparison of overall photosynthetic performance with NPQ. Interconversions of xanthophylls (violaxanthin, zeaxanthin, β-cryptoxanthin and one unidentified carotenoid) did occur in G. domingensis, but not in response to sudden exposure to light. Thus, NPQ was not correlated with any xanthophyll cycle during short-term acclimation to light. G. domingensis had five times higher weight-specific photosynthetic rates than K. alvarezii, which can be explained by the thicker thallus of K. alvarezii. Chlorophyll-specific gross photosynthetic rates were higher in K. alvarezii, but net rates were the same for both species. G. domingensis showed an immediate strong onset of NPQ upon exposure to irradiance, followed by downregulation to the NPQ level required. In K. alvarezii NPQ increased slowly until the required NPQ level was reached. At high irradiance G. domingensis downregulated photosynthesis while K. alvarezii continued to produce O₂ even at 2,000 μmol photons m⁻² s⁻¹ without NPQ increase. The strategy of K. alvarezii may provide short-term gains but with the risk of oxidative damage. The fast onset of NPQ in G. domingensis even at subsaturating irradiance as well as downregulation of photosynthesis when NPQ is saturated might provide this species with a competitive advantage under conditions of changing irradiance in the field.     

Habitat-induced morphological variation influences photosynthesis and drag on the marine macroalga <Emphasis Type="Italic">Pachydictyon coriaceum</Emphasis>

Photosynthesis, growth, distribution, and persistence of macroalgae are determined in part by the physical environment in which they live. Therefore, discerning how macroalgae interact with their physical environment is necessary to better understand their physiological performance. The purpose of this study was to examine what photosynthetic and hydrodynamic costs and benefits the morphology of Pachydictyon coriaceum (Phaeophyta) confers on the thallus in a given environment. Principal components analysis of morphometric measurements of Pachydictyon coriaceum from different flow habitats and depths separated thalli into three distinct morphs: shallow wave-exposed, shallow wave-protected, and deep. To test the hypothesis that thallus morphology affects net photosynthesis (NP), thalli of three morphotypes of P. coriaceum were incubated in an enclosed recirculating flume under three simulated light/water flow environments representing conditions from which the three morphotypes were collected. The wave-protected and deep morphs had significantly higher rates of photosynthesis than the wave-exposed morph for all three simulated environments. The dense, compact shape of the wave-exposed morph readily streamlines with flow and in doing so, potentially shades many of its internal blades likely accounting for its lower biomass-specific NP. Drag coefficients (C _d) were estimated for the three morphotypes over a range of flow velocities between 0.08 and 0.47 m s⁻¹. At lower water flow velocities (0.08–0.21 m s⁻¹), wave-exposed morphs had the lowest C _d among the three morphotypes. But drag coefficients of the three morphotypes converged with increasing flow velocities, and at velocities >0.31 m s⁻¹ there were no differences in C _d among the three morphotypes. The results of this study indicate that the environmentally-shaped morphs influence photosynthesis and, to a lesser degree, hydrodynamic forces acting on P. coriaceum.     

Emerson enhancement effect and quantum yield of photosynthesis for marine macroalgae in simulated underwater light fields

The contribution of enhancement to the total photosynthesis of marine macroalgae in their natural habitats was estimated by comparing the photosynthesis measured by O₂-electrode in five broad-band light fields with that predicted (on the assumption that no enhancement was occurring) from the photosynthetic action spectrum of each plant and the spectral distribution of the light fields. The excess of measured values divided by calculated values provided a measure of enhancement. Although 37% enhancement was observed for red algae in unfiltered quartz-iodine light, and 18% for green and brown algae, substantially lower values were obtained for all species in more natural light fields. In those typical of shallow coastal waters, phycoerythrin-rich red algae exhibited 15 to 20% enhancement, but little enhancement (<5%) was detected in other algae. In a green light field, representing deep coastal water, there was no significant enhancement in any species, and only green and brown algae showed any enhancement (ca 8%) in broad-band blue light, similar to that in deep oceanic waters. Quantum yields of 0.09 to 0.10 O₂ molecules per absorbed photon were recorded in most light fields for green and brown algae with thin thalli, but yields decreased in the blue light field and in species with thicker thalli. All red algae had quantum yields of about 0.08 O₂ molecules per absorbed photon, except in the blue light field, in which quantum yields were reduced by 70%.     

Assessment of AHS (Airborne Hyperspectral Scanner) sensor to map macroalgal communities on the Ría de vigo and Ría de Aldán coast (NW Spain)

Ría de Vigo and Ría de Aldán have high biological richness that is reflected in the number of environmental protection areas like the Atlantic Islands National Park and five places of community interest. Benthic algal communities play an important role in these ecosystems due to their ecological functions and support a great part of this biological richness. We tested by means of bio-optical modelling and Airborne Hyperspectral Scanner (AHS) images to what extent remote sensing could be used to map these communities in Ría de Vigo and Ría de Aldán (NW Spain). Reflectance spectra of dominating macroalgae groups were modelled for different water depths in order to estimate the separability of different bottom types based on their spectral signatures and the spectral characteristics of the AHS. Our results indicate that separation between three macroalgae groups (green, brown and red) as well as sand is possible when the bottoms are emerged during low tide. The spectra differences decrease rapidly with increasing water depth. Two types of classifications were carried out with the three AHS images: maximum likelihood and spectral angle mapper (SAM). Maximum likelihood showed positive results reaching overall accuracy percentages higher than 95?% and kappa coefficients higher than 0.90 for the bottom classes: shallow sand, deep sand, emerged rock, emerged macroalgae and submerged macroalgae. Sand and algae substrates were then separately analysed with SAM. These classifications showed positive results for differentiation between green and brown macroalgae until 5?m depth and high differences between all macroalgae and sandy substrate. However, differences between red and brown macroalgae are only detectable when the algae are emerged.     

Acclimation responses to salinity of three estuarine red algae from New Jersey

The effects of salinity and acclimation time on the net photosynthetic responses of 3 estuarine red algae, Bostrychia radicans Mont., Caloglossa leprieurii (Mont.) J. Ag., and Polysiphonia subtilissima Mont., from Great Bay Estuary, New Jersey, USA, were investigated. The algae were cultured in a series of synthetic seawater media of 5, 15, 25 and 35% S for acclimation periods of 0, 2, 4, 8, and 16 days prior to determining their photosynthetic responses. All species were euryhaline, and demonstrated photosynthesis at all the above salinities. B. radicans, which was more common towards the mouth of the estuary, had a maximum photosynthetic rate at 25% S, whilst C. leprieurii and P. subtilissima, which were more common towards the head of the estuary, had photosynthetic maxima between 15 and 25%, and at 15%, respectively. The curves relating net photosynthesis to salinity were usually similar within a species at different acclimation periods, although statistically significant differences were sometimes noted. The acclimation periods producing maximal net photosynthesis were 0, 2 and 4 days for B. radicans, and 4 days for C. leprieurii, whilst for P. subtilissima there was no significant difference in response for any acclimation period over the range of salinities studied.     

Crustacean epifauna of seagrass and macroalgae in Apalachee Bay,Florida, USA

Epifaunal crustaceans on turtlegrass (Thalassia testudinum) and five dominant macroalgae (Anadyomene stellata, Digenia simplex, Halimeda incrassata, Laurencia poitei and Penicillus lamourouxii) were quantitatively sampled bimonthly over a one-year period from September 1979 to September 1980 in a subtropical seagrass meadow in Apalachee Bay, Florida (northeastern Gulf of Mexico). These plant species exhibited a wide range of morphologies, with surface area-to-biomass ratios differing by over 2.5 times. A similar suite of crustaceans occurred on all macrophytes despite differences in shape or architecture among plant species. Relative abundances of many crustaceans, however, varied among plant hosts. Similarity analysis indicated that the epifaunal associates of T. testudinum were distinct from those of the macroalgae. Species richness was generally higher on turtlegrass than on any of the macroalgae. Abundances of total crustaceans per plant biomass or per plant surface area, on the other hand, were greater on all macroalgal species compared to the seagrass. Abundances (per plant biomass or plant surface area) of 14 of the 16 numerically dominant epifaunal species differed significantly among macrophytes. Twelve of the 16 species had greater abundance on one or more macroalgae, while only two species were more abundant on T. testudinum. Almost half of the dominant species had greatest abundances on the branching red alga L. poitei. Although abundances per plant biomass and plant surface area were greater on macroalgae relative to turtlegrass, densities (individuals per meter square of bottom) of animals associated with T. testudinum were significantly greater than those associated with macroalgae, primarily because of the greater abundance of turtlegrass in the grass bed. Both surface area-to-biomass ratios and degree of branching were poorly correlated with epifaunal abundance and number of species. Neither structural feature is an adequate predictor of faunal abundance and species richness among plant species, especially when macrophytes with very different morphologies are compared.     

Macroalgal morphology mediates particle capture by the corallimorpharian <Emphasis Type="Italic">Corynactis californica</Emphasis>

The shallow kelp forest at Santa Catalina Island, California (33.45 N, −118.49 W) is distinguished by several canopy guilds ranging from a floating canopy (Macrocystis pyrifera), to a stipitate, erect understory canopy (Eisenia arborea), to a short prostrate canopy just above the substratum (Dictyopteris, Gelidium, Laminaria, Plocamium spp.), followed by algal turfs and encrusting coralline algae. The prostrate macroalgae found beneath E. arborea canopies are primarily branching red algae, while those in open habitats are foliose brown algae. Densities of Corynactis californica, are significantly greater under E. arborea canopies than outside (approximately 1,200 versus 300 polyps m⁻² respectively). Morphological differences in macroalgae between these habitats may affect the rate of C. californica particle capture and serve as a mechanism for determining polyp distribution and abundance. Laboratory experiments in a unidirectional flume under low (9.5 cm s⁻¹) and high (21 cm s⁻¹) flow speeds examined the effect of two morphologically distinct macroalgae on the capture rate of Artemia sp. cysts by C. californica polyps. These experiments (January–March 2006) tested the hypothesis that a foliose macroalga, D. undulata, would inhibit particle capture more than a branching alga, G. robustum. G. robustum, found predominantly under the E. arborea canopy did not affect particle capture. However, D. undulata, found predominantly outside of the canopy, inhibited particle capture rates by 40% by redirecting particles around C. californica polyps and causing contraction of the feeding tentacles. These results suggest that the morphology of flexible marine organisms may affect the distribution and abundance of adjacent passive suspension feeders.     

Differences in herbivore preferences, phlorotannin production, and nutritional quality between juvenile and adult tissues from marine brown algae

 Juvenile and adult marine organisms differ in their morphology, chemistry, physiology, behavior, and ecology. Because juvenile algae are thinner, smaller, and have more delicate tissues than adults, they are often assumed to be more susceptible to grazers. We examined within-species food preferences of four common generalist herbivores for juvenile and adult tissues of eight common brown algae in two-choice laboratory food-preference experiments. Our results showed that juvenile algae did not tend to be a preferred food of herbivores. Juvenile tissues were significantly preferred over adult tissues in only four of the 32 combinations of algae and herbivores tested. In 12 experiments, adult tissues were preferred over juvenile tissues, and no choice occurred in the remaining 16 experiments. When sea urchins exhibited a preference, it was always for adult tissues. The other three herbivores, an isopod and two snails, were more variable in their choices, sometimes preferring juveniles, sometimes adults, and sometimes having no preference. We measured nitrogen and phlorotannin concentrations in adult and juvenile seaweeds to see whether these parameters were correlated with herbivore food preferences. Nitrogen levels were similar in juveniles and adults of three algal species and were higher in juveniles of two. Phlorotannin levels were higher in juveniles of four species and lower in juveniles of one. The other three species showed no differences in phlorotannin levels. Phlorotannin concentrations decreased with increasing juvenile size in three species and increased with increasing size in one species. Neither nitrogen nor phlorotannin concentrations explained overall herbivore food preferences for algae of different stages. Our results suggest that preferences of certain grazers for juvenile algae are not as strong as previously assumed and are dependent on herbivore species. Preferences between juveniles and adults are likely to be determined by a combination of morphological and chemical features of the tissues and the unique responses of herbivore species to those features. Received: 10 April 2000 / Accepted: 19 November 2000     

Effects of host-tissue homogenate of the scleractinian coral Plesiastrea versipora on glycerol metabolism in isolated symbiotic dinoflagellates

Symbiotic dinoflagellate algae (Symbiodinium sp.) isolated from the scleractinian coral Plesiastrea versipora and incubated in homogenized host tissue released 4 to 7 times as much glycerol (14 to 46 nmol glycerol/10⁶ algae) as those incubated in seawater (3 to 6 nmol glycerol/10⁶ algae) after 4 h incubation in the light. During this period, no release of triglycerides was detected. Intracellular glycerol increased 2- to 3-fold in algae incubated in host homogenate, but remained unchanged in algae incubated in seawater at a concentration of 0.82 ± 0.47 nmol glycerol/10⁶ algae. In each incubation condition, intracellular triglyceride levels increased. However, in algae incubated in host homogenate, the intracellular levels of triglycerides reached only about 75% of the amount reached in algae incubated in seawater (max. 18.55 ± 2.40 nmol glycerol/10⁶ cells). Host homogenate did not stimulate the release of glycerol from algae during dark incubation. These data show that the glycerol released by algae incubated in host-tissue homogenate was derived from increased synthesis of glycerol or from diversion of some glycerol or other photosynthetic intermediates from incorporation into algal triglyceride stores, and did not come from existing stores. Received: 20 December 1996 / Accepted: 9 January 1997