Effect of inhibitors of carbonic anhydrase activity on photosynthesis in the red alga Soliera filiformis (Gigartinales: Rhodophyta)

The effect of light intensity, pH and carbonic anhydrase (CA) inhibitors on photosynthesis of the red marine macroalgae Solieria filiformis (Kützing) Gabrielson, collected from Taliarte (Gran Canaria, Canary Islands) in 1991, has been investigated. Plants taken from the sea (wild phenotype) developed spherical morphology (ball phenotype) after 2 mo culture in aerated tanks. The photosynthetic oxygen evolution in the wild phenotype was saturated at 100 mol photons m^-2s^-1, while the ball phenotype displayed saturation at 200 mol photons m^-2s^-1. The inhibitors of total CA activity (6-ethoxizolamide) and extracellular CA activity (dextran-bound sulfonamide) inhibited photosynthesis at pH 8.2, to 90 and 50% respectively, in both phenotypes. No inhibition of the photosynthetic oxygen evolution was detected at pH 6.5. CA activity was associated with both supernatant and pellet fractions of crude extracts of S. filiformis. The rate of alkalization of the medium by the algae was dependent on light intensity. We suggest that carbon dioxide is the general form of inorganic carbon transported across the plasmamembrane in S. filiformis. HCO₃ transport into the cell takes place simultaneously by an indirect mechanism (dehydration to CO₂ catalyzed by CAext) and by direct uptake. Extracellular (CAext) and intracellular (CAint) CAs are involved in the mechanisms of inorganic carbon assimilation by S. filiformis.     

Light absorption,photosynthesis and growth of Nannochloris atomus in nutrient-saturated cultures

Nannochloris atomus was maintained in exponential growth at photon flux densities (PFD) from 400 to 700 nm, ranging from 10 to 200 mol m^-2 s^-1. Growth was lightsaturated at PFDs in excess of 100 mol m^-2 s^-1, with a mean light-saturated growth rate at 23 °C of 1.5×10^-5s^-1 (1.2 d^-1). The light-limited growth rates extrapolated to a compensation PFD for growth that was not significantly different from zero, although no changes in cell numbers were observed in a single culture incubated at a PFD of 1.0 mol m^-2s^-1. Dark-respiration rates were independent of PFD, averaging 1.7×10^-6 mol O₂ mol^-1 C s^-1 (0.14 mol O₂ mol^-1 C d^-1). The maximum photon (quantum) efficiency of photosynthesis was also independent of PFD, with a mean value of 0.12 mol O₂ mol^-1 photon. The chlorophyll a-specific light absorption cross-section ranged from 3 to 6×10^-3 m² mg^-1 chl a and was lowest at low PFDs due to intracellular self-shading of pigments associated with high cell-chlorophyll a contents. The C:chl a ratio increased from 10 to 40 mg C mg^-1 chl a between PFDs of 14 and 200 mol m^-2 s^-1. These new observations for N. atomus are compared with our previous observations for the diatom Phaeodactylum tricornutum in terms of an energy budget for microalgal growth.     

Effects of nitrogen and phosphorus enrichement on in vivo symbiotic zooxanthellae of Pocillopora damicornis

The reef coral Pocillopora damicornis (Linnaeus) was grown for 8 wk in four nutrient treatments: control, consisting of ambient, unfiltered Kaneohe Bay seawater [dissolved inorganic nitrogen (DIN, 1.0 M) and dissolved inorganic phosphate (DIP, 0.3 M)]; nitrogen enrichment (15 M DIN as ammonium); phosphorus enrichment (1.2 M DIP as inorganic phosphate); and 15 M DIN+1.2 M DIP. Analyses of zooxanthellae for C, N, P and chlorophyll a after the 8 wk experiment indicated that DIN enrichment increased the cellular chlorophyll a and excess nitrogen fraction of the algae, but did not affect C cell^-1. DIP enrichment decreased both C and P cell^-1, but the decrease was proportionally less for C cell^-1. the response of cellular P to both DIN and DIP enrichment appeared to be in the same direction and could not be explained as a primary effect of external nutrient enrichment. The observed response of cellular P might be a consequence of in situ CO₂ limitation. DIN enrichment could increase the CO₂ (aq) demand by increasing the net production per unit area. DIP enrichment could slow down calcification, thus decreasing the availability of CO₂ (aq) in the coral tissue.Hawaii Institute of Marine Biology Contribution No. 920     

Physiological ecology of picoplankton in the North Sea

The distribution of cyanobacteria in the surface waters of the North Sea was measured during July 1987. Numbers of cyanobacteria ranged from 2.5x10⁶ to 1.7x10⁸ cells 1^-1. In the majority of stations, cyanobacterial numbers were highest in the near-surface water and a subsurface maximum was found at only one station. The distribution of ¹⁴C among the end-products of photosynthesis was determined for picoplankton (<1 m) and other phytoplankton >1 m throughout the North Sea. The majority of label was found in the protein fraction of both picoplankton and >1 m phytoplankton; incorporation into lipids and polysaccharides plus nucleic acids was much lower. We interpret the large incorporation into protein to be a consequence of nutrient limitation of these natural assemblages. Photosynthetic parameters of the two size fractions were also determined. Assimilation number (P _m ^B ) and initial slope were greater for the picoplankton fraction than for phytoplankton >1 m but there was no evidence of significant photoinhibition of either fraction at irradiances up to 1 000 E m^-2 s^-1.     

Bioavailable iron species in seawater measured by macroalga (Laminaria japonica) uptake

The bioavailability of iron in seawater filtered through a 0.025-m filter was investigated using ⁵⁹Fe-labeled iron uptake by the macroalga Laminaria japonica (Areschoug: Phaeophyta) (collected in the northern Japan Sea 1993) as an assay. About 80% of the iron in the 0.025-m filtered coastal seawater was soluble and/or small colloidal organically bound iron, associated with natural organic ligands forming complexes with ferric ion. After decomposition of the organic matter by ultraviolet (UV) irradiation, 55% of the iron addition [or 0.6 nM, nearly the concentration of Fe(OH) ₂ ⁺ in equilibrium with amorphous hydrous ferric oxide in seawater at pH 8.0] in the 0.025-m filtered coastal seawater was taken up by the macroalga. Since the iron concentrations in the 0.025-m filtered coastal seawater are 0.1 to 2.0 nM and only 0.6 nM of the iron is likely available to biota over 1 to 2 d, we suggest that only small amounts of bioavailable iron exist in coastal seawater not affected by inflow from land and that a significant fraction of dissolved (<0.025 ) iron occurs in forms, such as organic iron complexes, other than the simple hydroxo-complex species predicted by thermodynamic models.     

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     

Primary production in Dumbell Bay in the Arctic Ocean

Photosynthesis, chlorophyll, inorganic nutrients, and related factors were measured throughout a productive season in a small coastal inlet of the Arctic Ocean. Significant production was confined to 5 m for a very limited time period beacause of both light and nutrient limitations. Maximum chlorophyll was 8.2 mg Chl a m^-3 and maximum gross and net photosynthesis rates were 830 and 550 mg C m^-2 d^-1. Annual gross and net photosynthesis is estimated at about 12 and 9 g C m^-2 respectively. The effect of light stimulation on assimilation is described, sources of coastal zone nutrient enrichment are considered, and discrepancies between gross and net photosynthesis are discussed with reference to the growth characteristics of the phytoplankton population.Bigelow Laboratory for Ocean Sciences. Contribution No. 76008     

Calcification in the maerl coralline alga Phymatolithon calcareum: Effects of salinity and temperature

The coralline alga Phymatolithon calcareum was dredged from 13 m in the Kattegatt, Baltic Sea, in December, 1980, and its rate of calcification was measured by ⁴⁵Ca⁺⁺-uptake methods. Light-saturated calcification rates at 5°C ranged from 15.8 g CaCO₃ g^-1 dry wt h^-1 for the basal parts of the plants to 38.7 g CaCO₃ g^-1 dry wt h^-1 for the tips. These age gradients were not apparent when calcification rates were expressed on the basis of surface area. Experiments with salinity (10, 20, 30) and temperature (0°, 5°, 10°, 20°C) indicated that optimum conditions for calcification were at 30 S and at temperatures above 10°C. Salinity had a greater influence on calcification rate than did temperature, and there was a positive relationship between salinity and calcification rate at all temperatures. In 6 mo old cultures, salinity was again the important factor, with all plants remaining healthy at 30 except those at the highest temperature (20°C). These trends, and the low calcification rates at 10S (4.6 g CaCO₃ g^-1 dry wt h^-1 at 5°C to 8.6 g CaCO₃g^-1 dry wt h^-1 at 20°C) suggest that low salinity may be the explanation for the general absence of P. calcareum from the brackish waters of the Baltic Sea. Short-term experiments in which salinity was kept constant while Ca⁺⁺ concentration was altered, and experiments in which salinity was varied and Ca⁺⁺ concentration kept constant, suggest that it is the calcium ion concentration and not salinity per se which affects calcification rates.     

Effect of heavy metals (Zn,Hg, Cu,Cd, Pb,Ni) on the length growth of Mytilus edulis

The shortterm (10–22 d) effect of Zn, Hg, Cu, Cd, Pb, and Ni on the length growth of Mytilus edulis is studied. Significant reductions of growth rate was found at 0.3 g Hgl^-1, 3 g Cul^-1, 10 g Znl^-1, and 10 g Cdl^-1 added to the local sea water, while concentrations of up to 200 gl^-1 of Pb and Ni had no effect on the growth. With exposure to Cu and Zn, there was a linear reduction in growth rate with increasing metal concentration up to about 6 g Cul^-1 and 100 g Znl^-1. Above these levels, growth stopped with Cu, while with Zn it was stabilized at about 20% of control growth. When Hg and Cd were added, a curvilinear relationship between growth and metal concentration is indicated. With Hg, growth rate is nearly zero above 3–4 g Hgl^-1, while the growth rate was 50% of control after 10 d of exposure to 100 g Cdl^-1. At 2 g Cdl^-1 there was a significant stimulation of length increase. Observed EC₅₀-values for growth were 0.3–0.4 g Hgl^-1, 3–4 g Cul^-1, 60 g Znl^-1, and 100 g Cdl^-1.     

Effect of pentachlorophenol (PCP) on meiobenthic communities established in an experimental system

Aquaria containing clean sand received a continuous supply of seawater from Santa Rosa Sound, Florida, USA, mixed with known concentrations (7, 76 and 622 g l^-1) of pentachlorophenol (PCP). After 9 weeks, nematodes accounted for 87% of the total meiofauna. Nematode biomass and densities were greatest in aquaria exposed to 76 g PCP l^-1 and were least in aquaria exposed to 622 g PCP l^-1. Epistrate feeders were abundant in control aquaria and aquaria exposed to 7 and 76 g PCP l^-1, but not in aquaria exposed to 622 g PCP l^-1. Selective deposit feeders were not abundant in the control aquaria and aquaria exposed to 7 g PCP l^-1, but comprised 19% of the nematodes in aquaria exposed to 76 g PCP l^-1 and 61% in aquaria exposed to 622 g PCP l^-1.     

Elemental composition (C,N,H) in larval and crab-1 stages of Pagurus bernhardus (Decapoda,Paguridae) and Carcinus maenas (Decapoda,Portunidae)

Analyses of individual content of carbon (C), nitrogen (N), and hydrogen (H) were carried out for all larval stages of Pagurus bernhardus and Carcinus maenas, and for newly metamorphosed crabs. Maximum range in total larval development is 12.8 to 165.8 g C, 3.2 to 35.1 g N, and 1.9 to 24.9 g H in P. bernhardus and 3.1 to 43.2 g C, 0.7 to 10.1 g N, and 0.4 to 6.3 g H in C. maenas. From these data energy equivalents were calculated. Maximum range in total larval life is 0.43 to 6.38 J ind. ^-1 in P. bernhardus and 0.1 to 1.49 J ind. ^-1 in C. maenas. There is a 32.4% mean loss of energy in P. bernhardus megalopa development; this seems to describe the normal developmental pattern in this stage. Biomass was determined as fresh and dry weight respectively. Individual dry weight is about 3.6 to 5.6 times higher in P. bernhardus (44 to 340 g) than in C. maenas (12 to 93 g) larvae.Contribution to research project Experimentelle marine Ökosystemanalyse sponsored by Bundesministerium für Forschung und Technologie, Bonn (Grant No. MFU-0328/1)     

Carotenoids versus retinoids (Vitamin A) as essential growth factors in penaeid prawns (Penaeus semisulcatus)

Penaeus semisulcatus de Haan adult gravid females, eggs and non-feeding early larvae, including Protozoea I, were used to test the hypothesis that retinoid (Vitamin A) is not required in early decapod crustacean development. In the adult gravid females, retinoids were detected only in the eyes (1.56±0.23 g g^-1 wet mass), whereas there were up to 97 g g^-1 wet mass total carotenoid in digestive gland and epidermis. This was mostly esterified, except in the ovaries, where free astaxanthin predominated (30 g g^-1). No retinoids could be detected in the eggs, the naupliar stages or Protozoea I, but free astaxanthin was metabolised exponentially, falling from 19 g g^-1 in the eggs to 4 g g^-1 in Protozoea I. This suggests that retinoid is not essential in early development and that carotenoid could be taking its place. Also, including retinoids in artificial diets appears to be unnecessary, provided adequate carotenoid is supplied.     

Effects of light intensity on nitrate and nitrite uptake and excretion by Chaetoceros curvisetus

Michaelis-Menten uptake kinetics were observed at all light intensities. With constant illumination, the V_max and K₁ in nitrate uptake over the natural light intensity range of 0 to 2000 E were 0.343 g-at NO₃–N(g)^-1 at protein-N h^-1 and 26 E, respectively. Nitrate uptake was inhibited at higher light intensities. The K_s for nitrate uptake did not vary as a function of light intensity remaining relatively constant at 0.62 g-at NO₃–N 1^-1. With intermittent illumination, the V_mzx for light intensity in nitrate uptake over a light intensity range of 0 to 5000 E was 0.341 g-at NO₃–N(g)^-1-at protein-N h^-1. No inhibition of nitrate uptake was observed at higher than natural light intensities. Chaetoceros curvisetus will probably never experience light inhibition of nitrate uptake under natural conditions.     

Cadmium accumulation,LC50 and oxygen consumption in the tropical marine amphipod Elasmopus rapax

Adult Elasmopus rapax, collected from the eastern coast of Venezuela in 1990, were exposed to seawater containing various CdCl₂ concentrations ranging from 0.25 to 5.5 mol l^-1. The 48-h and 96-h LC₅₀ values obtained were 4.0 and 1.6 mol Cd l^-1, respectively. In amphipods exposed to 1 mol Cd l^-1 for up to 240 h, the apparent rate of cadmium uptake was higher in dead animals (most of which had molted during the preceding 24 to 48 h) than in those which survived throughout the treatments without molting. Thus, whole-body cadmium content reached 1.74 mol g^-1 dry weight (dw) in the former and only 0.85 mol g^-1 dw in the latter; the higher body Cd-load may have caused the increased mortality observed in molters. On exposure to cadmium levels above 0.5 mol l^-1 the oxygen consumption rate of non-molters decreased from 2.2 to about 1.5 ml O₂ g^-1 dw h^-1 over the first 24 h, remaining unchanged thereafter. The results place E. rapax among the most sensitive marine organisms yet studied concerning cadmium toxicity, and emphasize the usefulness of the Amphipoda as bioindicators and research tools for bioassays.     

Effect of cadmium on survival and osmoregulation of various developmental stages of the shrimp Penaeus japonicus (Crustacea: Decapoda)

The object of this study was to evaluate the acute toxicity of cadmium in different post-embryonic stages of the penaeid shrimp Penaeus japonicus (Bate, 1888) and to determine the effect of sublethal cadmium on the osmoregulatory capacity used as an indicator of physiological condition. Tolerance to cadmium increases with the developmental stage. The least tolerant stages are the nauplii (48 h LC₅₀: 124 g Cd l^-1) and the zoeae (96 h LC₅₀: 10 to 30 g Cd l^-1). The most tolerant stages are the postlarvae (96 h LC₅₀: 200 to 3500 Cd l^-1) and juveniles (96 h LC₅₀: 5500 g Cd l^-1). In juvenile shrimp, 2000 g Cd l^-1 significantly reduce hypo- and hyper-OC. The effect of cadmium on hypo- and hyper-osmoregulatory capacity illustrates a dose- and time-dependent response. Surviving shrimp recover their hypo-osmoregulatory capacity after 6 d of readaptation in cadmium-free seawater.     

Growth response of Chaetoceros calcitrans (Bacillariophyceae) in batch culture to a range of initial silica concentrations

Batch cultures of the marine unicellular centric diatom Chaetoceros calcitrans (Paulsen) Takano were maintained by serial subculturing every 4 d into nutrient-enriched natural sea-water medium supplemented with 350, 950 and 1 400 g-at Si l^-1. The diatom cultures removed initial silica concentrations of 350 and 950 g-at l^-1 from the medium within 2 and 3 d, respectively. About 30 g-at l^-1 of the highest initial concentration remained in the medium after 4 d. The mean final cell density with an enrichment of 350 g-at Si l^-1 was 3.43±0.26×10⁴ cells l^-1 (median cell volume = 77.5±5.0 m³); with 950 g-at Si l^-1, 8.55±0.55×10⁴ cells l^-1 (50.0±4.5 m³); and with 1 400 g-at Si l^-1, 9.72±0.48×10⁴ cells l^-1 (37.3±5.0 m³). There was no significant difference in the final total organic weight of cells produced, which was in the range of 170 to 190 mg per 250 ml culture. This consisted of proportionately more lipid and carbohydrate and less protein from the treatment with 350 g-at Si l^-1 than from the 1 400 g-at Si l^-1 treatment.     

Influence of food concentration on the physiological energetics and growth ofOstrea edulis larvae

Feeding, respiration and growth rates of oyster (Ostrea edulis L.) larvae reared at five food levels were measured throughout the entire larval period. Energy budgets were derived as a function of alga (Isochrysis galbana Parke) food concentration. Ingestion rate (IR, cells h^-1) and oxygen consumption rate ( , nl h^-1) were almost isometric functions of larval size [ash-free dry weight, (AFDW, g)], characterized by the equations: IR=803.9 AFDW^1.13 and =4.85 AFDW^1.09. Ingested ration was directly correlated to cell concentration up to a maximum at 200 cells l^-1, with further increases failing to support higher ingestion rates. Likewise, growth rate linearly increased with food ration up to 100 cells l^-1 (max. growth efficiency,K ₁=25%) and reached a maximum at 200 cells l^-1 (growth rate=5.6 m d^-1), with further increases in food not supporting significantly faster growth. Maintenance ration was 2 to 3% daily dry weight (DW); optimum ration increased during larval development from 5 to 20% DW; maximum ration was 20% DW. During larval rearing, an increasing feeding schedule of 50, 100 and 200 cells l^-1 from Days 0, 5 and 10, respectively, is recommended.     

Diurnal patterns in photosynthetic capacity and depth-dependent photosynthesis-irradiance relationships inSynechococcus spp. and larger phytoplankton in three water masses in the Northwest Atlantic Ocean

The photosynthetic characteristics of prokaryotic phycoerythrin-rich populations of cyanobacteriaSynechococcus spp. and larger eukaryotic algae were compared at a neritic frontal station (Pl), in a warm-core eddy (P2), and at Wilkinson's Basin (P3) during a cruise in the Northwest Atlantic Ocean in the summer of 1984.Synechococcus spp. numerically dominated the 0.6 to 1 m fraction, and to a lesser extent the 1 to 5 m size fractions, at most depths at all stations. At P2 and P3, all three size categories of phytoplankton (0.6 to 1 m, 1 to 5 m, and >5 m) exhibited similar depth-dependent chages in both the timing and amplitude of diurnal periodicities of chlorophyllbased and cell-based photosynthetic capacity. Midday maxima in photosynthesis were observed in the upper watercolumn which damped-out in all size fractions sampled just below the thermocline. For all size fractions sampled near the bottom of the euphotic zone, the highest photosynthetic capacity was observed at dawn. At all depths, theSynechococcus spp.-dominated size fractions had lower assimilation rates than larger phytoplankton size fractions. This observation takes exception with the view that there is an inverse size-dependency in algal photosynthesis. Results also indicated that the size-specific contribution to potential primary production in surface waters did not vary appreciably over the day. However, estimates of the percent contribution ofSynechococcus spp. to total primary productivity in surface waters at the neritic front were significantly higher when derived from short-term incubator measurements of photosynthetic capacity rather than from dawn-to-duskin situ measurements of carbon fixation. The discrepancy was not due to photoinhibitory effects on photosynthesis, but appeared to reflect increased selective grazing pressure onSynechococcus spp. in dawn-to-dusk samples. Low-light photoadaptation was evident in analyses of the depth-dependency ofP-I parameters (photosynthetic capacity,P _max; light-limited slope, alpha;P _max alpha,I _k ; light-intensity beyond which photoinhibition occurs,I _b ) of the > 0.6 m communities at all three stations and was attributable to stratification of the water column. There was a decrease in assimilation rates andI _k with depth that was associated with increases in light-limited rates of photosynthesis. No midday photoinhibition ofP _max orI _b was observed in any surface station. Marked photoinhibition was detected only in the chlorophyll maximum at the neritic front and below the surface mixed-layer at Wilkinson's Basin, where susceptibility to photoinhibition increased with the depth of the collected sample. The 0.6 to 1 m fraction always had lower light requirements for light-saturated photosynthesis than the > 5 m size fraction within the same sample. Saturation intensities for the 1 to 5 m and 0.6 to 1 m size fractions were more similar whenSynechococcus spp. abundances were high in the 1 to 5 m fraction. The > 5 m fraction appeared to be the prime contributor to photoinhibitory features displayed in mixed samples (> 0.6 m) taken from the chlorophyll maxima. InSynechococcus spp.-dominated 0.6 to 1 and 1 to 5 m size fractions, cellular chlorophylla content increased 50- to 100-fold with depth and could be related to increases in maximum daytime rates of cellularP _max at the base of the euphotic zone. Furthermore, the 0.6 to 1 m and > 5 m fractions sampled at the chlorophyll maximum in the warm-core eddy had lower light requirements for photosynthesis than comparable surface samples from the same station. Results suggest that photoadaptation in natural populations ofSynechococcus spp. is accomplished primarily by changing photosynthetic unit number, occuring in conjuction with other accommodations in the efficiency of photosynthetic light reactions.     

Enhancement of chlorophyll a production in Gulf Stream surface seawater by synthetic versus natural rain

Rainwater concentrations of either ammonium or nitrate were sufficient to stimulate chlorophyll a (chl a) production in bioassay experiments using Gulf Stream surface water collected off North Carolina during the summer of 1991. Previous studies primarily examined inshore waters and did not address the impact of rainwater ammonium. An increase in chl a occurred within 1 d of the addition of synthetic rainwater (2 or 5% rainwater, 98 or 95% seawater) containing up to 10 M ammonium; this increase was followed by a decrease in chl a the following day. A similar response to nitrate addition (5% addition of 20 M nitrate rain) was observed. In separate experiments, natural rainwater having nitrate and ammonium concentrations less than those in the experimental synthetic rain yielded a greater chl a response than synthetic rain when added at similar dilutions (0.5 to 5.0% rain). The maximum dissolved inorganic nitrogen concentration in the enriched seawater in these bioassays was 1.8 M; prior to enrichment the maximum was < 0.4 M. Bioassay experiments begun 2 d after a major storm event (sustained NE winds with gusts to 13 m s^-1 and ca. 390 mol m^-2 inorganic nitrogen deposition from rain) showed a chl a increase in response to addition of natural rainwater, but not to synthetic rainwater with similar dissolved inorganic nitrogen concentration. These results suggest that phytoplankton stimulants, in addition to nitrate and ammonium, exist in natural rain but not in the synthetic rain used in these experiments.     

Interactions between light and temperature on the physiological ecology ofGracilaria tikvahiae (Gigartinales: Rhodophyta)

Main effects and interactions of light and temperature on rates of growth (), net photosynthesis (P_s), and dark respiration (R) of the red seaweedGracilaria tikvahiae were investigated in outdoor, nutrient-replete continuous-flow seawater culture chambers. Below 15°C,G. tikvahiae did not grow and between 15° and 30°C, both main effects and interactions of light and temperature on and P_s were significant, which explains the occurrence of this alga as a summer annual in its northern range. Temperature interacted with light (I) through its influence on the vs I and P_s vs I curves. The initial slope of the vs I curve, , the light saturation intensity, I_s, and maximum growth rate, _max, were all significantly lowerat 15°C compared to 20°, 25°, or 30°C. Maximum values of _max, the P_s:R ratio and the net photosynthesis:gross photosynthesis ratio (P_s:P_g) all occurred at 25°C, suggesting that this is the best temperature for growth ofG. tikvahiae. Values for P_max increased up to 30°C, indicating that the temperature for maximum growth and net photosynthesis are not the same forG. tikvahiae. Significant photoinhibition of growth and photosynthesis at full incident sunlight (I₀) occurred at 15°C but not at 20°, 25°, or 30°C. Steele's equation fit the 15°C vs I data best, whereas the hyperbolic tangent function fit the 20°, 25°, and 30°C data best. Main effects and interactionof light intensity and temperature on rates of R were also significant (P<0.001). R was highly intercorrelated with and P_s (0.86r0.94), indicating that R inG. tikvahiae is primarily regulated by growth rate and not temperatureper se. Environmental factors that regulate growth, such as light intensity, exert a great influence on R inG. tikvahiae.