Physiological responses of Nodularia harveyana to osmotic stress

The effects of salinity stress on biomass yield, photosynthetic O₂ evolution and nitrogenase activity were investigated using axenic cultures of Nodularia harveyana (Thwaites) Thuret originally isolated from a salt marsh at Gibraltar Point, Lincolnshire, UK in 1971 and studied in this laboratory in 1983. Biomass yields, as chlorophyll a per culture, were highest in the 0 to 100% seawater (0 to 35 sea salt) range with negligible growth in 200% seawater; growth on NH ₄ ⁺ was greater than on N₂ and NO ₃ ^- , which did not differ significantly from each other. In short-term experiments, photosynthetic O₂ evolution remained high at salinities up to 150% seawater (52.5 sea salt); nitrogenase activity remained high at salinities up to 100% seawater (35 sea salt). The major internal low molecular weight carbohydrate which accumulated in response to increased salinity was sucrose, the levels of which fluctuated markedly and rapidly in response to salinity change.     

Determination of photosynthetic rates for the marine algae Fucus vesiculosus and Laminaria digitata

A continuously recording, flow-through oxygen electrode system for the measurement of oxygen exchange is described and applied to an investigation of photosynthetic rates in the marine algae Fucus vesiculosus L. and Laminaria digitata (Huds.) Lam. The photosynthetic rate (mg O₂.g dry weight^-1.h^-1) at 15°C and 21.5 mW.cm^-2 (usually just saturating) ranges in F. vesiculosus from 1.20 in basal portions of the thallus to 9.27 at the apices and in L. digitata from 1.19 mg O₂ at the thallus base to 3.97 mg O₂ at distances of several centimetres behind the upper thallus margin. This variation is reduced when the photosynthetic rate is expressed in terms of fresh weight or surface area.This research was carried out while one of us (R.J.K.) was an Alexander von Humboldt fellow at the University of Kiel, and is part of the programme Sonderforschungsbereich 95, Wechselwirkung Meer-Meeresboden, Universität Kiel.     

Photoadaptation of photosynthesis in Gonyaulax polyedra

Gonyaulax polyedra Stein exhibited a combination of photoadaptive strategies of photosynthesis when only a single environmental variable, the light intensity during growth, was altered. Which of several biochemical/physiological adjustments to the light environment were employed depended on the level of growth irradiance. The photoadaptive strategies employed over any small range of light levels appeared to be those best suited for optimizing photosynthetic performance and not photosynthetic capacity. (Photosynthetic performance, P _i, is defined as the rate of photosynthesis occurring at the level of growth irradiance.) Among all photosynthetic parameters examined, only photosynthetic performance showed a consistent correspondence to growth rates of G. polyedra. Above 3500 to 4000 W cm^-2, where photosynthetic performance was equal to photosynthetic capacity, cells were not considered light-limited in either photosynthesis or growth. At these higher light levels, photosynthetic perfomance, cell volume, growth rates and respiration rates remained maximal; photosynthetic pigment content varied only slightly, while the photosynthetic capacity of the cells declined. At intermediate light levels (3000 to 1500 W cm^-2), photosynthesis, not growth, was light-limited, and photoadaptive strategies were induced which enhance absorption capabilities and energy transfer efficiencies of chlorophyll a to the reaction centers of G. polyedra. Photosynthetic capacity remained constant at about 280 mol O₂ cm^-3 h^-1, while photosynthetic performance ranged from 100 to 130 mol O₂ cm^-3 h^-1. Major increases in photosynthetic pigments, especially peridinin-chlorophyll a-proteins and an unidentified chlorophyll c component, accompanied photoadaptation to low irradiances. Maximal growth rates of 0.3 divisions day^-1 were maintained, as were respiration rates of about-80 mol O₂ cm^-3 h^-1 and cell volumes of about 5.4×10^-8 cm^-3 cell^-1. Below about 1250 W cm^-2, photosynthesis in G. polyedra was so light-limited that photosynthetic performance was unable to support maximal growth rates. Under these conditions, G. polyedra displayed photostress responses rather than photoadaptive strategies. Photostress was manifested as reduced cell volumes, slower growth, and drastic reductions in pigmentation, photosynthetic capacity, and rates of dark respiration.     

Effect of phosphate and ammonium levels on photosynthetic and respiratory responses of the red alga Gracilaria verrucosa

Gracilaria verrucosa (Hudson) Papenfuss exposed to nutrient enriched media (0.1 mM PO₄; 1.0 mM NH ₄ ⁺ ) by pulse feeding 2 h every third day for a period of 5 wk at 20°C and 25–30 salinity showed significantly higher rates of photosynthesis regardless of photon flux density correlated with increased pigment levels. Algae in nonenriched media showed significantly higher levels of soluble carbohydrates and decreased levels of phycoerythrin and chlorophyll a. Photosynthetic and respiratory responses to temperature 15°, 25°, 30°C and salinity (15, 25, 30 S) combinations indicate broad tolerances by both nutrient enriched and non-nutrient enriched algae. Photosynthetic and respiratory rates were highest at the high temperatures. Pulse-fed algae had significantly higher photosynthetic rates than non-nutrient enriched plants at all temperature and salinity combinations. Non-nutrient enriched algae had significantly higher respiratory rates than nutrient enriched algae at only 30°C and 15. The respiratory rates of both nutrient enriched and non-nutrient algae decreased under combinations of higher temperatures and salinities. G. verrucosa, grown without nutrients, has lower tolerances to environmental stresses.     

Effects of salinity fluctuations on the respiration rate of the southern oyster drill Thais haemastoma and the blue crab Callinectes sapidus

Respiration rates of Thais haemastoma and Callinectes sapidus were determined as a function of salinity with a flow-through respirometer at 20°C. Respiration rates were measured at 10, 20 and 30 S for acclimated animals. The effects of 10-5-10, 20-10-20, 30-10-30 and 10-30-10 S semidiurnal cycles (12 h) of fluctuating salinity on the rate of respiration of the oyster drill were studied. During each cycle, salinity was changed from the acclimation salinity over a 4 h interval, held at that salinity for 2 h, returned to the acclimation salinity over 4 h and held at that salinity for 2 h. The effects of diurnal (24.8 h) salinity cycles on respiration in the oyster drill and blue crab were also studied. Salinity was changed from the acclimation salinity over a 10.4 h interval, held at that salinity for 2 h, then returned to the acclimation salinity over 10.4 h and held at that salinity for 2 h. The respiration rate of 30 S acclimated oyster drills (679 l O₂ g dry weight^–1 h^–1) was significantly higher than for individuals acclimated to 10 S (534 l O₂ g dry weight^–1 h^–1). Blue crab respiration was 170 l O₂ g dry weight^–1 h^–1 at 30 S, and was significantly higher at 10 and 20 S than at 30 S. With the exception of the 20-10-20 S semidiurnal cycle, the respiration rate of oyster drills declined as salinity fluctuated in either direction from the acclimation salinity and increased as ambient salinity returned to the acclimation salinity. Semidiurnal cycles (12 h) of fluctuating salinity produced greater changes in the respiration rate of snails than analogous diurnal cycles (24.8 h). A 10-30-10 S pattern of fluctuation caused a greater percentage reduction in the steady state respiration rate of oyster drills than the 30-10-30 S pattern. The respiration rate of blue crabs varied inversely with fluctuating salinity. Relatively minor changes occurred in blue crab respiration rate with fluctuating salinity. Blue crab respiration rate characteristically dropped during the initial phase of declining salinity at a rate directly proportional to the rate of salinity decrease, perhaps representing a metabolic adjustment period by the blue crabs. The respiratory response of T. haemastoma to salinity is consistent with its incomplete volume regulation, while the response of C. sapidus is compatible with its ability to regulate extracellular fluid osmotic and ionic composition.     

Seasonal photosynthetic characteristics of Ascoseira mirabilis (Ascoseirales,Phaeophyceae) from King George Island,Antarctica

Monthly variation in photosynthesis, dark respiration, chlorophyll a content and carbon: nitrogen (C:N) ratios in different lamina sections of adult plants of Ascoseira mirabilis Skottsberg from King George Island, Antarctica, was investigated between September 1993 and February 1994. Light saturated net photosynthesis (P _max) showed maximum values in September (12 to 25 mol O₂ g^-1 fr wt h^-1), and decreased towards the summer to values ranging between 2.0 and 5.0 mol O₂ g^-1. In the distal section, however, a second optimum occurred in December (25 mol O₂ g^-1 fr wt h^-1). Dark respiration rates were also highest in October and November and decreased strongly in December to February (6.0 and 1.0 mol O₂ g^-1 fr wt h^-1, respectively). Gross photosynthesis exhibited high values between September and December. Concomitant with the seasonal decrease of photosynthetic efficiency () from mean values of 1.2 mol O₂ g^-1 fr wt h^-1 (mol photons cm^-2 s^-1)^-1 in September to 0.3 mol O₂ g^-1 fr wt h^-1 (mol photons cm^-2 s^-1)^-1 in January, the initial light saturating point (I _k) gradually increased from 19 to 60 mol photons m^-2 s^-1. Likewise C:N ratios were low in spring (12 to 13) and increased in summer (20). In general, the photosynthetic parameters P _max, gross photosynthesis, and Chl a concentrations were significantly higher in the distal section of the thallus. In contrast, C:N ratios were lower in the distal section of the lamina. The results show that photosynthesis obviously strongly supports growth of the alga in late winter to spring, as it does in some morphologically related brown algae from temperate and polar regions. The question whether growth is additionally powered     

Pigment content and photosynthetic rate of the fronds of Macrocystis pyrifera

The Macrocystis pyrifera (L.) C. Ag. frond is here described in terms of chlorophyll a, fucoxanthin, chlorophyll c and photosynthetic rate. Pigment concentrations increased back from the apical meristem reaching a maximum after 2 to 3 m. Pigment concentrations were then generally constant throughout most of the length of the frond, finally decreasing again in the oldest parts of the frond with the exception of the sporophylls. Pigment ratios remained relatively constant throughout. Maximum net photosynthetic rates on a given frond showed a decrease with tissue age on both an area basis (1040 down to 463 nmol O₂ cm^-2 h^-1) and on a chlorophyll a basis, which was shown as half-saturation constants (quantum irradiance) which dropped on an area basis from 85 mol m^-2 sec^-1 at 4.5 m above the holdfast to 26 mol m^-2 sec^-1 at 15.5 m. Young sporophytes transplanted from the sea floor to the surface (12 m) tended to decrease pigment content, while those transplanted to the bottom tended to increase all pigments, but especially fucoxanthin. Photosynthetic rates, however, changed little on a unit area basis. The results of these data are considered in the light of recent work on photosynthetic units, tissue age effects and general adaptations of the M. pyrifera frond to its light environment.     

Seasonal and depth related variation in the photosynthesis–irradiance response of<Emphasis Type="Italic"> Ecklonia radiata</Emphasis> (Phaeophyta,Laminariales) at West Island,South Australia

The photosynthesis–irradiance response of Ecklonia radiata (C. Agardh) J. Agardh, a common kelp in the temperate southern hemisphere, was investigated in situ throughout the year and across a depth profile at West Island, South Australia. Temperature and irradiance environment altered throughout the year, varying at 3 m between 14–20°C and 279–705 mol photons m^–2 s^–1. Photosynthetic capacity (P_m) varied throughout the year between 177–278 mol O₂ g^–1 dry wt h^–1 at 3 m and 133–348 mol O₂ g^–1 dry wt h^–1 at 10 m. The irradiance required for sub-saturation of photosynthesis (E_k) varied between 97–152 and 81–142 mol photons m^–2 s^–1 for 3 m and 10 m respectively, and the respiration rate varied between 15–36 and 13–20 mol O₂ g^–1 dry wt h^–1 for 3 m and 10 m. A clear seasonal change in photokinetic parameters was detected and provided strong evidence for a seasonal acclimation response. During winter an increase in the efficiency of light utilisation at low irradiance () was accompanied by a decrease in both E_k and that required for photosynthetic compensation. P_m also increased during the winter and autumn months and respiratory requirements decreased. These changes enable E. radiata to display an optimal photosynthetic performance throughout the year despite significant changes in the surrounding environment.Communicated by P.W. Sammarco, Chauvin     

Studies on 14CO2-assimilation in marine rhodophyceae

Experiments on enzymatic (in vitro) and in vivo CO₂-fixation using a variety of marine Rhodophyceae such as Porphyra umbilicalis (L.) J.Ag., Rhodomela confervoides (Huds.) Silva, Corallina officinalis L. and Chondrus crispus Stackh. revealed that carbon assimilation in the Rhodophyceae is almost exclusively performed by photosynthesis via ribulose-1, 5-di-phosphate carboxylase, whereas light-independent CO₂-fixation via -carboxylation by phosphoenolpyruvate carboxykinase scarcely exceeds 1% of the total carbon fixation potential of the plants. Activity of phosphoenolpyruvate carboxylase could not be detected. With respect to the main accumulation products of photosynthetic CO₂-fixation, the Rhodophyceae investigated are not uniform: Corallina officinalis L., Rhodymenia palmata (L.) Grev., and Gigartina stellata (Stackh.) Batt. have been found to accumulate ¹⁴C in the neutral compound floridoside (=2-O-glycerol--D-galactopyranose), whereas Delesseria sanguinea (Huds.) Lamour., Ceramium cubrum (Huds,) C.Ag., and Rhodomela confervoides (Huds.) Silva, representing members of the Delesseriaceae, Ceramiaceae and Rhodomelaceae, respectively, do not photosynthesize floridoside, but show intense ¹⁴C-labelling in an acidic constituent, mannosidoglycerate (= digeneaside). This compound is reported for the first time as a rapidly ¹⁴C-labelled and accumulated photosynthate in a variety of red algal species exclusively belonging to the Ceramiales.     

Gene flow among population of a teleost (painted greenling,Oxylebius pictus) from Puget Sound to southern California

Net photosynthetic oxygen evolution in Amphiroa anceps (Lamarck) Decaisne is inhibited at high oxygen concentrations. Photosynthesis is highest between pH 6.5 and 7.5. At pH 9 to 10 there is still a significant photosynthetic rate, suggesting that this alga can use HCO _- ³ as a substrate for photosynthesis. At pH 7.0 to 8.5, the photosynthetic rate saturates at a total inorganic carbon concentration (C_i) greater than 3 mM. At pH 8.5 and 8.8, calcification rate continues to increase with increasing concentration of C_i. Between pH 7 and 9, the calcification rate in the light in A. foliacea Lamouroux is proportional to the photosynthetic rate, whereas at higher pH where the photosynthetic rate is very low, the calcification rate is stimulated by the higher concentration of CO _2- ³ ion. At all pH values examined, the calcification rate of living plants in the dark and of dead plants is directly proportional to the CO _2- ³ ion concentration, suggesting little metabolic involvement in calcification processes in the dark, whereas calcification in live A. foliacea in the light is influenced both by the photosynthetic rate and the CO _2- ³ ion concentration in the medium.     

Diel variation of TCO2 in the upper layer of oceanic waters reflects microbial composition,variation and possibly methane cycling

Six diel TCO₂ cycles determined by infrared (IR) photometry from five drift stations occupied between 24 February and 16 March 1979 in the mixed layer of the northwestern Caribbean Sea are examined. Comparison of TCO₂ variation with coincident salinity and O₂ variation demonstrated that TCO₂ often co-varied with these independently measured variables. During five diel cycles TCO₂ variation was characterized by nocturnal production and diurnal consumption. The inverse, diurnal production of CO₂, occurred downstream from Misteriosa Bank, whose corals apparently contributed to a water mass having a twofold increase of POC and a sixfold larger population of heterotrophic nanoplankters. For the five diel studies carried out in waters with balanced or nearly blanced heterotrophic and phototrophic components of the nanoplankton, CO₂ consumption at constant salinity always occurred between 06.00 and 09.00 hrs. Net uptake often continued through 15.00 hrs, but not always in the absence of significant salinity changes. At constant salinity net O₂ evolution never exceeded 0.5 mol l^-1 h^-1 while net CO₂ uptake consistently averaged 3 mol l^-1 h^-1 for an apparent net production of 36 mg C m^-3 h^-1, which greatly exceeds the O₂ changes and open ocean ¹⁴C estimates from the literature. Diurnal consumption was apparently balanced by nocturnal production of CO₂ so that no significant net daily change in TCO₂ was observed. Departures from theoretical PQ and RQ and the possibility of nocturnal variations in formaldehyde and carbonate alkalinity imply that chemotrophs, both methane producers and methane oxidizers, play a significant role in CO₂ cycling. This could be through the metabolism of the nonconservative gases CH₄, CO, and H₂, and a link between chemotrophy and phototrophy through these gases is hypothesized. These open system measurements were subject to diffusion and documentable patchiness, but temporal TCO₂ changes appear to indicate the net direction of microbiological activity and join a growing body of literature showing dynamic variation in CO₂ and O₂ that exceeds estimates by ¹⁴C bottle assays of carbon fixation.     

Physiological responses to fluctuation in temperature of salinity in invertebrates. Adaptations ofAlpheus viridari (Decapoda,Crustacea),Terebellides parva (Polychaeta) andGolfinigia cylindrata (Sipunculida) to the mangrove habitat

The snapping shrimpAlpheus viridari (Armstrong, 1949), the polychaeteTerebellides parva Solis-Weiss, Fauchald and Blankensteyn 1990, and the sipunculanGolfingia cylindrata (Keferstein, 1865) are commonly found in the same mangrove habitat, where they experience frequent, acute fluctuations in temperature and salinity. Ecological studies indicate a temporal variation, including occasional absence, in the distribution of bothG. cylindrata andT. parva; this fed us to examine the physiological adaptations of the three species (collected at Western Bay, Twin Cays, Belize in 1985, 1986 and 1988). Each was subjected to acute, repeated exposure to either control (35 S) and decreased (25 S) salinity or to control and increased (45 S) salinity. Ability to regulate water and ion content (g H₂O or mol g^-1 solute free dry wt) was examinedA. viridari behaved as a hyperosmotic conformer at decreased salinity but as an osmoconformer at increased salinity. Regardless of direction of salinity change,A. viridari regulated water content through change in Na⁺, K⁺, and Cl^– contents. In contrast,G. cylindrata behaved as an osmoconformer and did not demonstrate ability to regulate water content.T. parva behaved as an osmoconformer, showed incomplete regulation of water content via change in Na⁺, K⁺, and Cl^– contents but had limited survival following exposure to 45 S. Each species was also exposed to change in temperature. Species were subjected to acute, repeated exposure either to control (28°C) and decreased (21°C) temperature or to control and inereased (35°C) temperatureA. viridari regulated water and ion content under both experimental conditions. In contrast,T. parva did not regulate water and ion content under either experimental temperature.G. cylindrata did not regulate water and ion content during exposure to decreased temperature and did not survive exposure to increased temperature. ForA. viridari, weight specific oxygen uptake rates (mg O₂ g^-1 ash-free dry wt) were determined. Exposure to decreased salinity or to increased temperature resulted in a small sustained elevation in O₂ uptake. It is concluded that, unlikeA. viridari, T. parva andG. cylindrata are only marginally adapted to withstand the salinity and temperature stresses, respectively, of the mangrove habitat. The inability ofT. parva andG. cylindrata to fully adapt to extremes in the mangrove habitat could well explain the temporal variation seen in the distribution of these two species.Contribution number 380. Caribbean Coral Reef Ecosystems, National Museum of Natural History, Smithsonian Institution     

Seasonal effects of light,temperature, nutrient concentration and salinity on the physiology and growth of Caulerpa paspaloides (Chlorophyceae)

Caulerpa paspaloides (Bory) Greville were collected during the winter and summer (1978 to 1979) from the Florida Keys, USA. Thalli collected during the winter photosynthesized more efficiently at low light intensities (I_c<1, I_k=38 Exm^-2xs^-1) than did thalli collected in the summer (I_c=13, I_k=111 Exm^-2xs^-1). Summer thalli exhibited higher P_max values (2.20 mgO₂xg^-1 dry wtxh^-1) than winter thalli (1.70 mg O₂xg^-1 dry wtxh^-1). Rates of rhizome elongation and frond initiation were strongly inhibited by winter temperatures. The maximum lethal temperature for summer thalli was 37° to 38°C as measured by both growth and photosynthesis. No evidence of nitrogen or phosphorus limitation was found. Relatively minor reductions in salinity (3S) resulted in significant increases in rhizome apex motality. Results indicate that low winter temperatures are responsible for reduced winter growth rates previously reported for the Key Largo population. Increased photosynthetic efficiency at low light intensities and preferential maintenance of rhizome elongation over frond initiation appear to allow this tropical macroalga to optimize growth and survival during the winter.     

Coral illumination through an optic glass-fiber: incorporation of 14C photosynthates

The pathways of ¹⁴C incorporation into the three major compartments of the coral body were analysed in colonies of Stylophora pistillata. We used the optic glass-fiber method to carry out two sets of experiments: in the first, 11 different colonies were sampled immediately after incubation; in the second, 3 colonies were returned to the reef at the termination of incubation for a further period of 29 h. Within the tissue compartment, significantly more ¹⁴C labeled products were incorporated into illuminated tips or bases than into unilluminated sections. Tips located above illuminated bases contained amounts of ¹⁴C products similar to unilluminated tips. Within the organic matrix compartment, illumination of tip or base segments again resulted in increased amounts of ¹⁴C fixation, and again unilluminated tips located above the illuminated bases did not accumulate more ¹⁴C photosynthates than other tips on the same branches. The absence of detectable translocation was also confirmed after a post-incubation period of 29 h, and raises questions as to the validity of the widely accepted theory of upward translocation. Within the skeletal carbonate compartment, although the results were associated with a high coefficient of variation, significantly more ¹⁴C accumulated in the tips than in the bases. Illumination of tips or bases did not enhance ¹⁴C uptake. A light-independent carbon assimilation (dark fixation) is significant in S. pistillata within the three tested compartments (the tissue, the organic matrix of the skeleton, and the skeletal carbonate). It is suggested that the dark fixation process in corals in a result of accumulation of respiratory CO₂ and CO₂ from sea water as malate or other titratable acids during the night. During the day these acids are broken down, releasing free CO₂ for C₃ pathway photosynthesis.     

Patterns of photosynthetic carbon metabolism in light-limited phytoplankton

Patterns of phytoplankton carbon (C) metabolism were examined in å combined laboratory and field study to assess the influence of light conditions on ¹⁴C assimilation into photosynthetic end-products. Laboratory studies with three species representing distinct size classes and taxonomic groups tested the influence of low light on patterns of C flow. Prorocentrum mariae-lebouriae (dinoflagellate) and Ditylum brightwellii (diatom) showed decreased movement of photoassimilated ¹⁴C into protein following a shift to low light ¹⁴C assimilation into lipids and photosynthetic pigments increased in low light and was paralleled by increased chl a per cell. The proportion of ¹⁴C fixed into protein returned to the pre-shift level upon return to initial light conditions. Monochrysis lutheri (chrysophyte) did not show this pattern of reduced % ¹⁴C protein. Incubations of 12 and 24 h demonstrated significant rearrangements in labeling patterns at night, wherein ¹⁴C flow into protein in darkness was favored. % ¹⁴C protein at night was lower for M. lutheri than for the other species, suggesting some interspecific differences in the low light response. Measurements of ¹⁴C assimilation in phytoplankton assemblages from Chesapeake Bay demonstrated movement of a higher proportion of photo-assimilated C into protein in samples collected in the surface mixed layer than in those below the pycnocline. In comparison, phytoplankton collected below the pycnocline fixed a higher proportion of ¹⁴C into lipids, photosynthetic pigments, and low molecular weight metabolites, as was observed in low light laboratory cultures. A comparison of 12- and 24-h incubations for measuring patterns of C flow into photosynthetic end-products confirmed the inadequacy of short-term measurements, as significant changes in ¹⁴C allocation occurred in the dark phase of the photocycle. Together, these results suggest that ¹⁴C assimilation into photosynthetic end-products can be a useful measure of adaptive state in changing light conditions, but point out some difficulties in applying this approach in situ.     

Availability of colloidal ferric oxides to coastal marine phytoplankton

Cell growth of a coastal marine diatom, Phaeodactylum tricornutum (stock cultures), and two red tide marine flagellates, Heterosigma akashiwo and Gymnodinium mikimotoi (stock cultures), in the presence of soluble chelated Fe(III)-EDTA (1:2) and of four different phases of ferric oxide colloids were experimentally measured in culture experiments at 20°C under 3000 lux fluorescent light. Soluble Fe(III)-EDTA induced the maximal growth rates and cell yields. The short-term uptake rate of iron by H. akashiwo in Fe(III)-EDTA medium was about eight times faster than that in solid amorphous hydrous ferric oxide (Fe₂O₃·xH₂O) medium. In culture experiments supplied with four different ferric oxide forms, the orders of cell yields are amorphous hydrous ferric oxide>-FeOOH (lepidocrocite)>Fe₅O₇(OH)·4H₂O (hydrated ferric oxyhydroxide polymer >-FeOOH (goethite). The specific growth rates () at logarithmic growth phase in Fe(III)-EDTA, amorphous hydrous ferric oxide and -FeOOH media were significantly greater than those in Fe₅O₇ (OH)·4H₂O and -FeOOH media. The thermodynamically stable forms such as Fe₅O₇(OH)·4H₂O and -FeOOH supported a little or no phytoplankton growth. The iron solublities and/or proton-promoted iron dissolution rates of these colloidal ferric oxides in seawater at 20°C were determined by simple filtration techniques involving -activity measurements of ⁵⁹Fe. The orders of solubilities and estimated dissolution rate constants of these ferric oxides in seawater were consistent with that of cell yields in the culture experiments. These results suggest that the availability of colloidal iron to provide a source of iron for phytoplankton is related to the thermodynamic stability and kinetic lability of the colloidal ferric oxide phases, which probably control the uptake rate of iron by phytoplankton.     

Effects of changes in water salinity upon exercise and cardiac performance in the European seabass (<Emphasis Type="Italic">Dicentrarchus labrax</Emphasis>)

The European seabass is an active euryhaline teleost that migrates and forages in waters of widely differing salinities. Oxygen uptake (M_O2) was measured in seabass (average mass and forklength 510 g and 34 cm, respectively) during exercise at incremental swimming speeds in a tunnel respirometer in seawater (SW) at a salinity of 30 and temperature of 14°C, and their maximal sustainable (critical) swimming speed (U_crit) determined. Cardiac output (Q) was measured via an ultrasound flow probe on their ventral aorta. The fish were then exposed to acute reductions in water salinity, to either SW (control), 10, 5, or freshwater (FW, 0), and their exercise and cardiac performance measured again, 18 h later. Seabass were also acclimated to FW for 3 weeks, and then their exercise performance measured before and at 18 h after acute exposure to SW at 30. In SW, seabass exhibited an exponential increase in M_O2 and Q with increasing swimming speed, to a maximum M_O2 of 339±17 mg kg^–1 h^–1 and maximum Q of 52.0±1.9 ml min^–1 kg^–1 (mean±1 SEM; n=19). Both M_O2 and Q exhibited signs of a plateau as the fish approached a U_crit of 2.25±0.08 bodylengths s^–1. Increases in Q during exercise were almost exclusively due to increased heart rate rather than ventricular stroke volume. There were no significant effects of the changes in salinity upon M_O2 during exercise, U_crit or cardiac performance. This was linked to an exceptional capacity to maintain plasma osmolality and tissue water content unchanged following all salinity challenges. This extraordinary adaptation would allow the seabass to maintain skeletal and cardiac muscle function while migrating through waters of widely differing salinities.Communicated by S.A. Poulet, Roscoff     

Effects of anaerobiosis on root metabolism of Zostera marina (eelgrass): implications for survival in reducing sediments

The temperate seagrass Zostera marina L. typically grows in highly reducing sediments. Photosynthesis-mediated O₂ supplied to below-ground tissues sustains aerobic respiration during photosynthetic periods. Roots, however, experience daily periods of anoxia and/or hypoxia at night and under conditions that reduce photosynthesis. Rhizosphere cores of Z. marina were collected in August 1984 from Great Harbor, Massachusetts, USA. We examined short-term anaerobic metabolism of [U-¹⁴C]sucrose in excised roots and roots of intact plants. Under anaerobic conditions roots showed appreciable labeling of CO₂, ethanol and lactate, and slight labeling of alanine and other metabolites. Over 95% of the ¹⁴C-ethanol was recovered in the root exudate. Release of other metabolites from the roots was minimal. Ethanol was also released from hypoxic/anoxic roots of intact plants and none of this ethanol was transported to the shoot under any experimental conditions. Loss of ethanol from roots prevented tissue levels of this phytotoxin from increasing during anaerobiosis despite increased synthesis of ethanol. Anaerobic metabolism of [U-¹⁴C]glutamate in excised roots led to appreciable labelling of -aminobutyrate, which was known to accumulate in eelgrass roots. Roots recovered to fully aerobic metabolism within 4 h after re-establishment of aerobic conditions. The contributions of these root metabolic responses to the ability of Z. marina to grow in reducing marine sediments are related to light-regulated interactions of shoots and roots that likely dictate depth penetration, distribution and ecological success of eelgrass.     

Long-term acclimation of the teleost Oreochromis mossambicus to various salinities: two different strategies in mastering hypertonic stress

Laboratory-reared tilapia (Oreochromis mossambicus) were long-term acclimated to freshwater (FW), brackish water (BW, 10 salinity), seawater (SW, 35 salinity) and two hypersaline media (45 and 60 salinity). We examined the influence of these ambient salinities on the density (D _cc) and diameter (d _cc) of DASPMI-stained chloride cells and on the capacity for electrogenic Cl^- secretion of the in vitro opercular epithelium. To provide a characterisation of Cl^- secretion, transepithelial potential difference (PD _te), conductance (G _te) and short-circuit current (I _sc) were measured after mounting the respective epithelium in an Ussing-chamber. The cellular electromotive forces (E _c) and conductances (G _c) as well as the leak conductances (G _l) were obtained from G _te: I _sc plots. In the salinity range between FW and SW both D _cc and d _cc increased. All electrophysiological parameters recorded increased in parallel, indicating a strong enhancement of the capacity for Cl^- secretion on the cellular and epithelial level. In the salinity range above SW a further increase of D _cc was observed. However, despite a higher concentration gradient across the body surface of the tilapia during acclimation to hypersaline media, the short-circuit current (I _sc) was not significantly different compared to SW preparations. This reflects proportional decreases of G _c and increases of E _c, respectively. Of particular interest, we found a strong decrease of the leak conductance (G _l) in preparations from tilapia acclimated to hypersaline media compared to those from SW fish, indicating that the tight junctions become less permeable.     

Daily rhythm of O2-evolution in the cyanobacterium Trichodesmium thiebautii under natural and constant conditions

The rate of oxygen evolution by the tropical marine cyanobacterium Trichodesmium thiebautii was recorded at different times during the day in samples collected in 1992 from the Bahama Islands and the NE Caribbean Sea. This cyanobacterium is unique in that it is the only non-heterocystous diazotroph capable of N₂-fixation in daylight. Oxygen evolution was measured under conditions of natural day/night (LD, N=50), constant light (LL, N=14), and constant dark (DD, N=2×14). Photosynthesis vs intensity (P-I) relationships were calculated at various times of day, and the following parameters were used for further evaluation: photosynthesic capacity (P _max, 66 to 91 mg O₂ mg chl a ^-1 h^-1), initial slope of the P-I curve (, 0.23 to 0.27), dark respiration (R, 12 to 27 mg O₂ mg chl a ^-1 h^-1), and the intensity at which O₂ consumption is compensated by O₂ production (I _c, 78 to 160 Em^-2 s^-1). All means showed large standard deviations (for some parameters more than 200%). In some cases, these variations could be explained with a sinusoidal 24-h time course, but only the compensation point showed a significant daily variation (p0.001) in both LD and DD. The fact that the time course of I _c typical for natural conditions remains rhythmic under constant dark conditions strongly suggests a circadian regulation. Few circadian rhythms have been observed in prokaryotes, and this appears to be the first observation of such a rhythm in a cyanobacterium which fixes N₂ in daytime.