MICROSENSOR STUDIES OF OXYGEN AND LIGHT DISTRIBUTION IN THE GREEN MACROALGA CODIUM FRAGILE1

Scalar irradiance, oxygen concentration, and oxygenic photosynthesis were measured at 0.1 mm spatial resolution within the tissue of the siphonous green macroalga Codium fragile subsp. tomentosoides (van Goor) Silva by fiber-optic scalar irradiance microsensors and oxygen microelectrodes. The scalar irradiance of visible light was strongly attenuated in the outer 0.2 mm of the tissue but was nearly constant for the subsequent 1.0 mm of photo-synthetic tissue. Far-red scalar irradiance at 750 nm increased below the tissue surface to a maximum of 200% of incident irradiance at 1.2 mm depth due to multiple scattering in the medullary tissue. The constant intensity of visible light below 0.2 mm was thus a result of the combined effects of absorption and backscattering from the medulla. The oxygen exchange between the alga and the surrounding water was diffusion-limited with a steep O₂gradient inside and around the alga. In darkness, the tissue below 0.6 mm became anoxic, and endophytic extracellular space provided an environment where anoxygenic microbial processes may occur. When illuminated at 160 nmol photons·^?2·^?1, O₂ concentrations exceeded ambient levels throughout the thallus, with a maximum of 250% of air saturation just below the surface. The amplitude of oxygen variation was buffered by gas bubbles formed in the medullary tissue.     

SETTLEMENT AND SURVIVAL OF POLYSIPHONIA LANOSA (CERAMIALES) SPORES ON ASCOPHYLLUM NODOSUM AND FUCUS VESICULOSUS (FUCALES)1

The settlement patterns of spores of Polysiphonia lanosa (L.) Tandy on Ascophyllum nodosum (L.) Le Jolis and Fucus vesiculosus L. were studied using a flow tank. Settlement sites were defined as ‘sheltered’ or ‘exposed.’ Surface area calculations revealed non-random settlement on A. nodosum, with higher than expected spore frequencies on the thallus and lateral pits and lower than expected frequencies on the vesicles. Settlement of F. vesiculosus was random and significantly lower than on A. nodosum. On the shore, survival of sporelings from September (post-sporulation) to May (pre-sporulation) was highly non-random on both basiphytes. On A. nodosum, lateral pits ('sheltered') showed the highest survival frequency. Here the proportion of surviving sporelings increased over the study period, whereas the proportion on open thallus area ('exposed') decreased. On F. vesiculosus also preferential survival occurred on ‘sheltered’ sites such as vesicle/thallus interfaces and wounds. Between September and May, all P. lanosa sporelings were lost from ‘exposed’ areas (thallus surface and vesicles). Overall, frequencies of surviving sporelings were much greater on A. nodosum than on F. vesiculosus. These results are discussed with reference to basiphyte morphology, epiphyte removal mechanisms and the survival stratagy of P. lanosa.     

Seasonal interactive effects of pCO2 and irradiance on the ecophysiology of brown macroalga Fucus vesiculosus L.

ABSTRACT

Stochastic upwelling of seawater in the Baltic Sea from the deep, anoxic bottoms may bring low-pH water rich in CO₂ close to the surface. Such events may become more frequent with climate change and ongoing ocean acidification (OA). Photoautotrophs, such as macroalgae, which are important foundation species, have been proposed to benefit from increased carbon availability due to reduced energetic cost in carbon acquisition. However, the exact effects of CO₂ fertilization may depend on the ambient light environment, as photosynthesis rates depend on available irradiance. In this experimental study, interacting effects of CO₂ addition and irradiance on the habitat-forming macroalga Fucus vesiculosus were investigated during two seasons – winter and summer – in the northern Baltic Sea. Growth rates remained unaffected by CO₂ or irradiance during both seasons, suggesting that direct effects of elevated CO₂ on mature F. vesiculosus are small. Increases in CO₂ affected algal elemental ratios by increasing carbon and decreasing nitrogen content, with resulting changes in the C:N ratio, but only in winter. In summer, chlorophyll a content increased under low irradiance. Increases in CO₂ caused a decline in light-harvesting efficiency (decrease in F_v/F_m and α) under high irradiance in summer, and conversely increased α under low irradiance. High irradiance caused increases in the maximum relative electron transport rate (rETR_max) in summer, but not in winter. Differences between winter and summer indicate that F. vesiculosus responses to CO₂ and irradiance are season-specific. Increases in carbon content during winter could indicate slightly positive effects of CO₂ addition in the long run if the extra carbon gained may be capitalized in growth. The results of this study suggest that increases in CO₂, either through upwelling or OA, may have positive effects on F. vesiculosus, but these effects are probably small.     

THE EFFECT OF GROWTH IRRADIANCE ON THE COUPLING OF CARBON AND NITROGEN METABOLISM IN CHAETOMORPHA LINUM (CHLOROPHYTA)

The influence of growth irradiance on the non-steady-state relationship between photosynthesis and tissue carbon (C) and nitrogen (N) pools in Chaetomorpha linum (Muller) Kutzing in response to abrupt changes in external nitrogen (N) availability was determined in laboratory experiments. For a given thallus N content, algae acclimated to low irradiance consistently had a higher rate of light-saturated photosynthesis (P_max normalized to dry weight) than algae acclimated to saturating irradiance; for both treatments, P_max was correlated to thallus N. Both P_max and the photosynthetic efficiency (α_dw) were correlated in C. linum grown at either saturating or limiting irradiance over the range of experimental conditions, indicating that variations in electron transport were coupled to variations in C-fixation capacity despite the large range of tissue N content from 1.1% to 4.8%. Optimizing both α and P_max and thereby acclimating to an intermediate light level may be a general characteristic of thin-structured opportunistic algae that confers a competitive advantage in estuarine environments in which both light and nutrient conditions are highly variable. Nitrogen-saturated algae had the same photosynthesis–irradiance relationship regardless of light level. When deprived of an external N supply, photosynthetic rates did not change in C. linum acclimated to low irradiance despite a two-fold decrease in tissue N content, suggesting that the active pools of chlorophyll and Rubisco remained constant. Both α and P_max decreased immediately and continuously in algae acclimated to high irradiance on removal of the N supply even though tissue N content was relatively high during most of the N-starvation period, indicating a diversion of energy and reductant away from C fixation to support high growth rates. Carbon and nitrogen assimilation were equally balanced in algae in both light treatments throughout the N-saturation and -depletion phases, except when protein synthesis was limited by the depletion of internal N reserves in severely N-starved high-light algae and excess C accumulated as starch stores. This suggests that the ability for short-term adjustment of internal allocation to acquire N andC in almost constant proportions may be especially beneficial to macroalgae living in environments characterized by high variability in light levels and nutrient supply.     

EFFECT OF TEMPERATURE ON LIGHT-LIMITED GROWTH AND CHEMICAL COMPOSITION OF SKELETONEMA COSTATUM (BACILLARIOPHYCEAE)1

Phytoplankton growth rate in response to irradiance can be approximated by a hyperbola defined by three coefficients: i) initial slope (α); ii) asymptote (μ_m); and, iii) X-axis intercept or compensation irradiance (I_c). To mathematically represent the interaction of temperature and irradiance on growth rate, one must describe the relationship between these constants and temperature. The marine diatom, Skeletonema costatum (Greville) Cleve, was grown in unialgal culture at different levels of irradiance and 2-3 photoperiods at 0, 5, 10, 16 and 22 C. The value of I_c is ca. 1.0 ly·day^?1 or less at all temperatures. The initial slope (div·ly^?1) is a “u-shaped” function of temperature described by the second degree polynomial, α= 0.25–0.02T+0.001T². Within the range 0–10 C, μ_m (div·day^?1) is an exponential function of temperature described by the equation, μ_m= 0.48 exp (0.126T). At each temperature and selected levels of irradiance, cell size and cellular content of C, N and chl a were determined. The C:chl a and N:chl a ratios increased with irradiance because of increases in C and decreases in chl a. At lower temperatures (0, 5, 10 C), the rate of increase of both ratios with irradiance was greater than at the higher temperatures (16, 22 C). Cellular content of N was independent of irradiance and temperature, and the C:N ratio ranged from 5 to 8 with a slight tendency to lower values at low irradiance. Cell volume was not influenced by either temperature or irradiance.     

Microsensor studies on Padina from a natural CO2 seep: implications of morphology on acclimation to low pH

Low seawater pH can be harmful to many calcifying marine organisms, but the calcifying macroalgae Padina spp. flourish at natural submarine carbon dioxide seeps where seawater pH is low. We show that the microenvironment created by the rolled thallus margin of Padina australis facilitates supersaturation of CaCO₃ and calcifi‐cation via photosynthesis‐induced elevated pH. Using microsensors to investigate oxygen and pH dynamics in the microenvironment of P. australis at a shallow CO₂ seep, we found that, under saturating light, the pH inside the microenvironment (pH_ME) was higher than the external seawater (pH_SW) at all pH_SW levels investigated, and the difference (i.e., pH_ME ? pH_SW) increased with decreasing pH_SW (0.9 units at pH_SW 7.0). Gross photosynthesis (P_g) inside the microenvironment increased with decreasing pH_SW, but algae from the control site reached a threshold at pH 6.5. Seep algae showed no pH threshold with respect to P_g within the pH_SW range investigated. The external carbonic anhydrase (CA) inhibitor, acetazolamide, strongly inhibited P_g of P. australis at pH_SW 8.2, but the effect was diminished under low pH_SW (6.4–7.5), suggesting a greater dependence on membrane‐bound CA for the dehydration of HCO₃^? ions during dissolved inorganic carbon uptake at the higher pH_SW. In comparison, a calcifying green alga, Halimeda cuneata f. digitata, was not inhibited by AZ, suggesting efficient bicarbonate transport. The ability of P. australis to elevate pH_ME at the site of calcification and its strong dependence on CA may explain why it can thrive at low pH_SW.     

Heterogeneity of Chlorophyll Fluorescence over Thalli of Several Foliose Macrolichens Exposed to Adverse Environmental Factors: Interspecific Differences as Related to Thallus Hydration and High Irradiance

Spatial heterogeneity of chlorophyll (Chl) fluorescence over thalli of three foliose lichen species was studied using Chl fluorescence imaging (CFI) and slow Chl fluorescence kinetics supplemented with quenching analysis. CFI values indicated species-specific differences in location of the most physiologically active zones within fully hydrated thalli: marginal thallus parts (Hypogymnia physodes), central part and close-to-umbilicus spots (Lasallia pustulata), and irregulary-distributed zones within thallus (Umbilicaria hirsuta). During gradual desiccation of lichen thalli, decrease in Chl fluorescence parameters (F_O - minimum Chl fluorescence at point O, F_P - maximum Chl fluorescence at P point, ₂ - effective quantum yield of photochemical energy conversion in photosystem 2) was observed. Under severe desiccation (>85 % of water saturation deficit), substantial thalli parts lost their apparent physiological activity and the resting parts exhibited only a small Chl fluorescence. Distribution of these active patches was identical with the most active areas found under full hydration. Thus spatial heterogeneity of Chl fluorescence in foliose lichens may reflect location of growth zones (pseudomeristems) within thalli and adjacent newly produced biomass. When exposed to high irradiance, fully-hydrated thalli of L. pustulata and U. hirsuta showed either an increase or no change in F_O, and a decrease in F_P. Distribution of Chl fluorescence after the high irradiance treatment, however, remained the same as before the treatment. After 60 min of recovery in the dark, F_O and F_P did not recover to initial values, which may indicate that the lichen used underwent a photoinhibition. The CFI method is an effective tool in assessing spatial heterogeneity of physiological activity over lichen thalli exposed to a variety of environmental factors. It may be also used to select a representative area at a lichen thallus before application of single-spot fluorometric techniques in lichens.     

FLUORESCENCE EMISSION SPECTRA OF MARINE AND BRACKISH‐WATER ECOTYPES OF FUCUS VESICULOSUS AND FUCUS RADICANS (PHAEOPHYCEAE) REVEAL DIFFERENCES IN LIGHT‐HARVESTING APPARATUS1

The Bothnian Sea in the northerly part of the Baltic Sea is a geologically recent brackish‐water environment, and rapid speciation is occurring in the algal community of the Bothnian Sea. We measured low‐temperature fluorescence emission spectra from the Bothnian Sea and the Norwegian Sea ecotypes of Fucus vesiculosus L., a marine macroalga widespread in the Bothnian Sea. Powdered, frozen thallus was used to obtain undistorted emission spectra. The spectra were compared with spectra measured from the newly identified species Fucus radicans Bergström et L. Kautsky, which is a close relative of F. vesiculosus and endemic to the Bothnian Sea. The spectrum of variable fluorescence was used to identify fluorescence peaks originating in PSI and PSII in this chl c–containing alga. The spectra revealed much higher PSII emission, compared to PSI emission, in the Bothnian Sea ecotype of F. vesiculosus than in F. radicans or in the Norwegian Sea ecotype of F. vesiculosus. The results suggest that more light‐harvesting chl a/c proteins serve PSII in the Bothnian Sea ecotype of F. vesiculosus than in the two other algal strains. Treatment of the Bothnian Sea ecotype of F. vesiculosus in high salinity (10, 20, and 35 practical salinity units) for 1 week did not lead to spectral changes, indicating that the measured features of the Bothnian Sea F. vesiculosus are stable and not simply a direct result of exposure to low salinity.     

Interaction between isopod grazing and wave action: a structuring force in macroalgal communities in the southern Baltic Sea

The macroalgal belt in the southern Baltic Sea may be partly structured by the interaction of physical and biological factors. A field study, spanning the 1990s, describes a rapid decline of the Fucus spp. stands along the wave-exposed Swedish southeast coast. During this period, a relative dominance of Fucus vesiculosus L. shifted to a relative dominance of Fucus serratus L. The decline of F. vesiculosus coincided with observations of large numbers of the grazing isopods Idotea baltica (Pallas) and Idotea granulosa Rathke, or with field observations of frequent grazing marks on Fucus fronds. I. baltica, but not I. granulosa, tended to aggregate in the declining Fucus spp. stands, indicating a strong preference for Fucus spp. In a mesocosm experiment I. baltica, when given a choice, grazed both Fucus species at weak water motion. At strong water motion grazing was concentrated on F. vesiculosus. It is hypothesized that one of the reasons I. baltica preferred F. vesiculosus to F. serratus at strong water motion may have been differences in habitat quality, like width of thallus, influencing the ability to cling to the plant. Smaller thallus, as in F. vesiculosus, thus is the preferred habitat for grazing of I. blatica. We postulate that the existence of F. serratus in the area may be favoured by strong wave action and moderate but not strong grazing by I. baltica, relaxing the interspecific competition from F. vesiculosus.     

Book review

Fucoid macroalgae are important primary producers and habitat modifiers on North Atlantic intertidal rocky shores. With decreasing latitude, western European fucoid populations display reduced levels of abundance, biomass and recruitment, while experiencing higher levels of physical environmental stress during summer months. We hypothesized that such reduction in the south is accompanied by a detectable decline in fucoid reproductive capacity. To test this hypothesis, morphological and reproductive traits of core (Welsh) and marginal (Portuguese) populations of two common fucoid species, Fucus vesiculosus and F. spiralis (Ochrophyta, Fucales), were examined. Morphological measurements showed that for a given thallus length, both fucoid species had smaller thallus volume and lower biomass in the southerly marginal part of the range. Significantly lower biomass of reproductive tissue of F. vesiculosus and a smaller number of receptacles per individual on specimens of both species indicate that levels of reproductive output are probably lower in southern populations. Despite the differences in reproductive traits observed between regions, reproductive effort (measured as the percentage of total dry biomass represented by reproductive tissue) of both species remained similar, as algae from both regions made similar investments in reproduction. The results indicate that stressful conditions reduced growth and number of receptacles of both species and amount of reproductive biomass of F. vesiculosus in the south but do not seem to change the way these algal species invest their energy. The decline in mass and reproductive biomass of specimens from southern shores found in this study, when combined with the lower abundance of adults and lower recruitment levels previously observed, is a strong indication of fucoid populations with lower levels of propagule output. This is an important factor when considering responses of these populations to a changing environment.     

GENETIC AND MORPHOLOGICAL IDENTIFICATION OF FUCUS RADICANS SP. NOV. (FUCALES,PHAEOPHYCEAE) IN THE BRACKISH BALTIC SEA1

Brown seaweeds of the genus Fucus occupy a wide variety of temperate coastal habitats. The genus is evolutionary dynamic with recent radiations to form morphologically distinct taxa. In the brackish Baltic Sea, fucoids are the only perennial canopy‐forming macroalgae. The most northern populations of Fucus occur permanently submerged in extremely low salinity (3–5 psu). These are currently referred to as Fucus vesiculosus L. but are morphologically distinct with a narrow frond without bladders. We report here that a population of this unique morphotype is reproductively isolated from a truly sympatric population of common F. vesiculosus and conclude that the northern morphotype represents a previously undescribed species. We describe Fucus radicans sp. nov., which is attached and dioecious with broadly elliptic receptacles, characterized by a richly branched narrow flat frond (2–5 mm), short thallus (<26 cm), and a high capacity for vegetative recruitment of attached plants. Analysis of five highly polymorphic microsatellite DNA loci showed genetic differentiation between sympatric populations of F. radicans and F. vesiculosus, whereas allopatric populations of the same species revealed a coherent pattern of genetic variation. Sequences of the RUBISCO region in F. radicans were identical to or differing at only one to two dinucleotide positions from those of F. vesiculosus, indicating a recent common origin of the two species.     

SEASONAL VARIATION OF P CONTENT AND MAJOR N POOLS IN PALMARIA PALMATA (RHODOPHYTA)1

The annual variation of major nitrogen pools, phosphorus, carbon, ash, and thallus water content in relation to seasonal environmental changes was studied in two northern Spanish populations of the edible seaweed Palmaria palmata (Linnaeus) Kuntze. Observed patterns were investigated using Spearman rank order correlation coefficients. There were significant relationships between thallus nutrient content and nitrate and orthophosphate seawater concentration, irradiance, temperature, and wave force. The highest levels of total N and P and nitrogenous compounds were observed during autumn and winter because the thallus stored N‐ and P‐rich compounds in response to high nutrient seawater concentration when growth was limited by low light and temperature. Phycoerythrin and other proteins were the main N reserves. Thallus P content was higher in algae from the eutrophic site. During spring, reduced N and P thallus content and increased ash, water, and C content were observed in the growing fronds. N and P seawater concentrations were undetectable during summer when nutrient reserves were low and growth was reduced and eventually suppressed, suggesting nutrient limiting conditions. Palmaria palmata clearly could take advantage of elevated N and P concentrations to create storage reserves in winter to support early summer growth. This storage response reduced the dependence of algal nutrition on the external nutrient supply and supports the use of pulse fertilization to diminish summer nutrient limitation of cultured algae.     

Light Respiratory Processes and Gross Photosynthesis in Two Scleractinian Corals

The light dependency of respiratory activity of two scleractinian corals was examined using O₂ microsensors and CO₂ exchange measurements. Light respiration increased strongly but asymptotically with elevated irradiance in both species. Light respiration in Pocillopora damicornis was higher than in Pavona decussata under low irradiance, indicating species-specific differences in light-dependent metabolic processes. Overall, the coral P. decussata exhibited higher CO₂ uptake rates than P. damicornis over the experimental irradiance range. P. decussata also harboured twice as many algal symbionts and higher total protein biomass compared to P. damicornis, possibly resulting in self-shading of the symbionts and/or changes in host tissue specific light distribution. Differences in light respiration and CO₂ availability could be due to host-specific characteristics that modulate the symbiont microenvironment, its photosynthesis, and hence the overall performance of the coral holobiont.     

DETERMINING THE AFFINITIES OF SALT MARSH FUCOIDS USING MICROSATELLITE MARKERS: EVIDENCE OF HYBRIDIZATION AND INTROGRESSION BETWEEN TWO SPECIES OF FUCUS (PHAEOPHYTA) IN A MAINE ESTUARY1

The high degree of morphological plasticity displayed by species of the brown algal genus Fucus L. is well documented. Such variation is especially pronounced for those estuarine taxa lacking holdfasts (termed ecads) that often bear little resemblance to the attached species from which they are derived. To better understand the systematics of salt marsh fucoids, we developed a suite of four microsatellite‐containing loci capable of distinguishing between F. vesiculosus L. and F. spiralis L. The genetic markers were used to determine the relationships of the fucoid ecads F. vesiculosus ecad volubilis (Hudson) Turner and a muscoides‐like Fucus in the Brave Boat Harbor (ME, USA) estuary. Ecad populations had 2‐ to 3‐fold higher levels of heterozygosity than attached populations of F. vesiculosus and F. spiralis. Further, ecads were “intermediate” between F. vesiculosus and F. spiralis in their allele frequencies and genotype composition. Our data indicate that populations of muscoides‐like Fucus in Brave Boat Harbor mainly consist of F₁ hybrids between F. vesiculosus and F. spiralis, whereas F. vesiculosus ecad volubilis may arise through backcrosses between F. vesiculosus and other fertile hybrids. Finally, our data support the hypothesis that introgression has occurred between attached populations of F. vesiculosus and F. spiralis.     

Carbon dioxide exchange in Cladina lichens from subarctic and temperate habitats

Summary The survival potential of lichens in a given habitat is determined by the response of CO₂ exchange to photosynthetically active radiation (PhAR), thallus temperature, and thallus relative water content (RWC). Therefore morphologically similar lichens from contrasting climatic environments 1) should differ in their CO₂ exchange responses, and 2) these differences should reflect adaptations to their climatic regimes. The CO₂ exchange responses of a subarctic (55°N, 67°W) Cladina stellaris (Opiz) Brodo population and a temperate (29°N, 82°W) Cladina evansii (Abb.) Hale and W. Culb, population were used to test these two related hypotheses.Infrared gas analysis with lichens collected in September–October 1975 established that the two populations differed in their responses to incident PhAR, thallus temperature, and thallus RWC. Net photosynthesis in C. stellaris had an optimum at a lower temperature and a greater relative photosynthetic capacity at low temperatures than did C. evansii. Cladina evansii maintained net photosynthesis above 35°C thallus temperature; C. stellaris did not. In both species the optimum temperature for net photosynthesis increased with increasing irradiance. The C. stellaris light saturation point was consistently lower than that of C. evansii. Both species had maximal rates of net photosynthesis at 70–80% relative water content. In C. evansii the CO₂ exchange rates, expressed as percentages of the maximum rate, declined more rapidly under suboptimal conditions. The absolute CO₂ exchange rates of C. evansii were greater than those of C. stellaris. At 20°C and 90–95% RWC, resaturation respiration occurred in both species and continued until 6–7 h after wetting.Contrasts in the temporal patterns of thallus condition at each collection site suggest that not all differences in the two response surfaces reflect climatic adaptation. The two populations appear well adapted to incident PhAR and thallus temperature regimes but the 70–80% RWC optimum for net photosynthesis common to both species is puzzling since their water regimes differ markedly. The overall adaptedness of the CO₂ exchange responses in the two species cannot be judged without a comprehensive quantitative analysis of carbon balance under differing climatic regimes.     

POROSIMETRIC STUDY OF THE LICHEN FAMILY UMBILICARIACEAE: ANATOMICAL INTERPRETATION AND IMPLICATIONS FOR WATER STORAGE CAPACITY OF THE THALLUS

The pore system within the thalli of 13 lichen taxa belonging to the family Umbilicariaceae has been studied by means of mercury intrusion porosimetry. A characteristic bimodal pore size distribution with a central depression around 0.05 μm of equivalent pore radius was obtained in all lichen samples. However, clear differences were found among the pore size distributions of each lichen taxa. The total thallus porosity was undoubtedly related to the anatomy of the medulla. In general, a radial plectenchymatic medulla conferred larger porosity to the thallus than an arachnoidal one. Maximum thallus water content closely depended on the total thallus porosity in the five lichens possessing rhizinomorphs. The species with a similar type of medulla could be grouped together in a multivariate analysis that considered three porosimetric parameters and the maximum thallus water content. Umbilicaria cinereorufescens was the most distinct species, with the lowest values of total porosity and water storage capacity and the largest value of thallus density, apparently due to its scleroplectenchymatic medulla. The pore size distribution existing inside the thallus of the species studied is discussed in relation to the often opposing problems of CO₂ exchange and optimal water relations. Some results pointed to a large influence of the micropores (<0.05 μm) on the water storage capacity of the thallus, while the macropores would have a more important role in gas exchange.     

Microhabitat and morphometric variation in two species of <Emphasis Type="Italic">Prasiola</Emphasis> (Prasiolales,Chlorophyta) from streams in central Mexico

Prasiolales are characterized by high morphological plasticity. This problem in taxonomic delimitation of the species may be a result of environment heterogeneity. Habitat characteristics and morphological variation of P. mexicana and P. nevadensis, two species of freshwater leaf-like thallus green algae, were investigated in five sites in central México where the morphological traits of these species overlap. Comparisons were made between habitat characteristics among streams and transects with and without Prasiola samples. Although no consistent correlation was found between environmental variables and morphological traits, a significantly wider range of habitat characteristics of current velocity and irradiance and morphometric variation (lamina length, lamina diameter, cell length, surface of the thallus, thickness and number of layers) suggested that P. mexicana was more plastic than P. nevadensis in its physicochemical requirements. Our results suggest that small variation of microhabitat conditions in P. nevadensis (irradiance and current velocity) could explain its very restricted geographic range in streams in central Mexico.     

The Influence of Salinity on the Rate of Dark Respiration and Structure of the Cells of Brown Algae Thalli from the Barents Sea Littoral

We studied the changes in the rate of dark respiration (DR) and structure of the cells in Ascophyllum nodosum and Fucus vesiculosus thalli during the incubation at 40, 34, 20, 10, and 2 salinity for 14 days. The changes in salinity affected the rate of DR and the structure of the thallus apical cells: the organelles swollen and were destroyed later. The effect of hyposalinity on the algae was more pronounced as compared to hypersalinity. The stress intensity directly increased with the degree of desalination. Further adaptation of the algae to low salinity enhanced DR and, hence, was an energy-dependent process. Despite higher DR rates (during the stress and adaptation) in F. vesiculosus as compared to A. nodosum, the seaweeds had similar pattern of adaptation to the variation of salinity. Different primary response of the seaweeds to 20 salinity was an important exception; apparently, the salinity around 20 is the limit of these species distribution in desalination zones.     

Macroalgal photosynthesis near the southern global limit for growth; Cape Evans,Ross Sea,Antarctica

Photosynthetic characteristics of the red macroalgae Phyllophora antarctica and Phymatolithon foecundum collected from under sea ice at Cape Evans, McMurdo Sound (Ross Sea) were determined using in situ fluorometric and lab-based oxygen exchange techniques. Only 0.16% of incident irradiance penetrated the 2.5 m thick ice cover and photosynthetic parameters for both taxa were characteristic of highly shade-adapted plants. Saturation onset parameter (E _k) did not exceed 13 mol photons m^-2 s^-1 in either taxon. For Phyllophora antarctica the light saturated photosynthetic rate at –1^°C was 10 mol O₂ g^-1 FW h^-1 and respiration averaged 3.3 mol O₂ g^-1 FW h^-1 between sampled depths of 10 and 25 m. A light meter deployed at 15 m depth for a year recorded a marked increase in underwater irradiance on the last day of January 2002 coinciding with ice-breakout, and a maximum value for irradiance of 120 mol photons m^-2 s^-1 on 9 February 2002. The 2-month ice-free period was the only time when irradiance consistently exceeded compensation (photosynthesis=respiration) and enabled Phyllophora antarctica to accumulate sufficient carbon to result in a measurable increase in thallus area equivalent to a biomass increment of 1.87 mg (DW) per frond. Near the southern global limit for marine macroalgae, conditions that dictate the availability of underwater irradiance are extremely variable from year to year. Low respiration rates enhance longevity of the Phyllophora antarctica thallus, enabling it to not only survive the winter darkness, but also to retain photosynthetic capacity and thus take advantage of windows of higher irradiance.     

Photosynthetic use of light and inorganic carbon in air and water by the intertidal alga Petalonia fascia (Scytosiphonales, Phaeophyta)

The photosynthetic performance of the intertidal alga Petalonia fascia (0. F. Muller) Kuntze (Scytosiphona-ceae, Phaeophyta) has been investigated, both in air and water, by analyzing the relationship between apparent photosynthesis rate and photon irradiance and inorganic carbon. In relation to the use of photon irradiance, it was found that the net photosynthetic capacity in water was 5.7 times that in air (fully hydrated thallus). The light compensation point was achieved at 5.9 and 3.0 μmol photons m^?2 s^?1 in air and water, respectively. The light onset-saturation parameter and the photosynthetic efficiency were 77% and 25% greater in water than in air, respectively. The dark respiration rate was one-third greater when emersed in comparison to submersion conditions. These data suggest that the photosynthetic response to irradiance in P. fascia is similar to that in infralittoral species rather than the intertidal species. This assessment can be explained by the winter seasonality of the bladed stage of growth, when storms and waves permit a permanent hydrated status of P. fascia that in the intertidal zone. Moreover, the minimum tissue water content that permitted active photosynthesis in the alga was around 20%. The net photosynthetic capacity as a function of inorganic carbon (C) concentration in water was 1.5 times that in air. Photosynthesis was saturated in both media with respect to the availability of inorganic C in natural conditions. The affinity to inorganic C, and the carbon conductance, were two orders of magnitude higher in air than in water. However, the higher photosynthetic capacity when submerged in comparison to emersion conditions suggests that P. fascia can assimilate the external HCO₃,– or the occurrence of a CO₂ concentrating mechanism in this species.