Impact of UV-B radiation on uptake of 15N-ammonia and 15N-nitrate by phytoplankton of the Wadden Sea

Natural marine phytoplankton populations from the German Wadden Sea and unialgal cultures of the haptophycean Phaeocystis pouchetii were tested in 1989 under controlled UV-B stress conditions. Assimilation of ¹⁵N-nitrate in phytoplankton consisting mainly of P. pouchetii, or in pure cultures of this alga, was found to be very sensitive to enhanced UV-B dosage in comparison ¹⁵N-ammonia uptake. In contrast, in phytoplankton samples containing Ceratium spp., Coscinodiscus sp., Noctiluca sp. or others, rate of ¹⁵NO₃ ^- uptake was higher and only slightly affected by UV-B irradiance compared to the P. pouchetii sample. UV-B inhibitory effect on uptake of inorganic nitrogen by P. pouchetii was more pronounced under strong white-light conditions and after a UV-B pre-illumination period of several hours than under low white light. Pools of glutamine and alanine decreased after UV-B exposure. Results are discussed with reference to the damaging effects of white light and UV-B on nitrogen metabolism.     

UV-B protecting compounds in the marine algaPhaeocystis pouchetii from Antarctica

Phaeocystis pouchetii (Hariot) Lagerheim is widely distributed in polar waters, and forms massive near-surface blooms in the marginal ice-edge zone around Antarctica during spring and summer. UV irradiance in the Antarctic marine environment is reportedly as high in October and November as in mid-summer due to stratospheric ozone depletion. Because of the location and timing of theP. pouchetii bloom, this prymnesiophyte will be exposed to high levels of UV-B (280 to 320 nm) radiation. Colourless water-soluble compounds, produced by the colonial stage in the life cycle of this alga, absorb strongly between 250 and 370 nm, with absorbance maxima at 271 and 323 nm. The concentration of these compounds in culturedP. pouchetii depends on the strain, stage in the life cycle, and presence of bacteria. As well as conferring substantial protection to this alga, these substances may also provide UV protection to other organisms present in the water column.     

Influence of phosphate depletion on the growth and colony formation of Phaeocystis pouchetii

The haptophycean alga Phaeocystis pouchetii (Hariot) Lagerheim, a bloom-forming species in the North Sea, formed massive colonies in batch cultures only when the phosphate concentration in the medium was below 1 mol l^-1. In general, colony cells and single cells showed a similar response to phosphate depletion: decreased cellular phosphate (up to a factor of 32), decreased chlorophyll-a concentration (up to a factor of 4.2) and high activities of alkaline phosphatase (APA). However, the growth rate of colony cells was reduced at low phosphate concentrations (<1 mol l^-1) in contrast to that of single cells. Colony cells tended to have a slightly higher phosphate concentration than single cells, while the increase in APA was delayed. The differences between single cells and colonies in phosphate utilization, cellular phosphate content, and growth rate cause, at low phosphate levels, a shift towards the formation of colonies. Similar changes seem to occur in nature during transition of nutrient-sufficient to nutrientlimited conditions.This investigation was supported by the Foundation for Fundamental Biological Research (BION), which is subsidized by the Dutch Organization for the Advancement of Pure Research (ZWO). Publication No. 8 of the project Ecological Research of the North Sea (EON)     

Nutrient starvation effects on the allelochemical potency of Alexandrium tamarense (Dinophyceae)

Batch culture experiments were performed to investigate potential effects of nutrient starvation on the allelochemical potency of the toxic dinoflagellate Alexandrium tamarense. Triplicate cultures with reduced nitrate (−N) or phosphate (−P) seed were compared to nutrient-replete (+N+P) cultures. Total depletion of the dissolved inorganic limiting nutrient, reduced cell quotas, changed mass ratios of C/N/P and reduced cell yield clearly indicate that treatment cultures at stationary phase were starved by either N or P, whereas growth cessation of +N+P cultures was probably due to carbon limitation and/or a direct effect of high pH. Pulsed addition of the limiting nutrient allowed −N and −P cultures to resume growth. Lytic activity of A. tamarense as quantified by a Rhodomonas bioassay was generally high (EC₅₀ around 100 cells mL⁻¹) and was only slightly modulated by growth phase and/or nutrient starvation. Lytic activity per cell increased with time in both +N+P and −P cultures but not −N cultures. P-starved stationary-phase cells were slightly more lytic than +N+P cultures, but this difference may be due to increased cell size and/or accumulation of extracellular compounds. In conclusion, only slight changes but no general and major increase in lytic activity in response to nutrient starvation was observed.     

Crop demand of manganese

The objectives of this study were to evaluate some of the popular rotation crops grown in Hungary for tolerance to low external Mn²⁺ levels and to determine the critical tissue concentration of Mn²⁺ deficiency during early stages of growth. The minimum Mn²⁺ concentration required in soil nutrient contents was 42.5 mg kg⁻¹ for sunflower, 24.3 mg kg⁻¹ for tobacco and 10.2 mg kg⁻¹ for triticale. Sunflower, tobacco and triticale achieved optimum growth at 48.0–65.0 mg Mn²⁺ kg⁻¹, 24.9–32.1 mg Mnⁿ⁺ kg⁻¹ and 28.7 to 29.6 mg Mn²⁺ kg⁻¹, respectively. Critical shoot Mn²⁺ concentration at early stages of growth was 53.6 mg kg⁻¹ in sunflower, 458.0 mg kg⁻¹ in tobacco and 193.8 mg kg⁻¹ in triticale. Our results demonstrate that the tolerance to low external Mn²⁺ (triticale: <30.2 mg kg⁻¹; sunflower: <56.2 mg kg⁻¹; tobacco: <69.3 mg kg⁻¹) and the critical tissue Mn²⁺ levels for deficiency varied significantly between crop species tested.

     

Dissolved organic substances during a Phaeocystis pouchetii bloom in the German Bight (North Sea)

Dissolved organic substances (carbohydrates, organic nitrogen, free amino acids) were measured in the German Bight (North Sea) in June, 1981. During and before this survey, sea foam was observed in the east Frisian coastal water and it accumulated on the nearby beaches to an unusually high extent. In this coastal water area, a large Phaeocystis pouchetii Lagerheim bloom and very high concentrations of dissolved organic matter were found. The above dissolved organic substances were all positively correlated to a highly significant degree with P. pouchetii cell numbers in the bloom area. An influence of salinity (or river water) on this correlation could be excluded. Thus, exudation or decomposition products of P. pouchetii were most likely the cause of the unusually high concentrations of dissolved organic matter in the bloom area off the east Frisian coast, where P. pouchetii blooms have been reported for many years. Ammonia concentrations were very low in the P. pouchetii bloom area; this and the accumulation of dissolved organic substances might lead to speculation that decomposition of dissolved organic matter by bacteria could have been reduced due to antibiotic activity of P. pouchetii.     

Possible cause of inhibition of seed germination in two rice cultivars by heavy metals Pb2+ and Hg2+

Seeds of two rice cultivars (Oryza sativa) cv. PR-116 and Pant Dhan-12 subjected to heavy metal lead (Pb²⁺) and mercury (Hg²⁺) exposure showed an inhibition in germination percentage, shoot and root length, and lower fresh and dry weight after 7 days. Both Pb²⁺ and Hg²⁺ inhibited the solubilization process of starch due to reduction in α-amylase activity, which is also evident from greater starch content and reduced soluble carbohydrate content of endosperms of treated seeds of the two cultivars. Mercury was more tolerated by Pant Dhan-12 when grown under in vitro culture medium containing 2% sucrose. The inhibitory effect of Pb²⁺ on embryo growth was not only abolished but also accelerated by 2% sucrose. The inhibitory effect, however, was not significantly blocked in Hg²⁺-treated embryos grown in vitro in sucrose containing medium. Embryos did not grow normally in a medium devoid of sucrose in either case. Data indicated that Pb²⁺ inhibited germination and seedling growth by impairing the solubilization of endosperm starch without markedly affecting the embryo, while Hg²⁺ inhibited germination and seedling growth by damaging the embryo itself.     

Cytochrome c used as a model system to probe the structural properties of benthic mucilage

Benthic mucilage, whether native or artificially fragmented to microscopic dimensions by ultrasonic treatment, was mixed with cytochrome c, which was used as a 12 000 Da polycationic model compound. Cytochrome c binding profiles proved to depend on the aggregation state of the mucilage. The native mucus matrix binds cytochrome c quasi hyperbolically with an apparent affinity constant K = 1 × 10⁶  M ⁻¹. As shown by chemical modification of both the mucilage and cytochrome c, the binding expression is dependent on the availability of both the positive charges on the cytochrome c surface and the negative charges within the mucus matrix. The extent of binding is sensitive to the ionic strength of the medium. The ultrasonic-stabilized mucus fragments display a peculiar binding profile, with an apparent low affinity, abruptly entering into a high affinity binding region. The results suggest that, depending on the mucus to polycation ratio, a polymeric reticulus builds up. This reticulus can accommodate molecules of at least 12 000 Da molecular weight. The results are also discussed with respect to biological implications. Received: 23 April 1997 / Accepted: 24 November 1997     

Mechanisms for copper tolerance in Amphora coffeaeformis-internal and external binding

Growth of the ship-fouling diatom Amphora coffeaeformis and accumulation of copper in the cells were evaluated for cultures exposed to copper. Comparisons with literature reports for other species revealed that A. coffeaeformis shows no ability to maintain normal growth rates in the presence of high cellular copper levels. This suggests that internal binding is not the principal copper tolerance mechanism for this species. In addition, the copper complexing capacity of A. coffeaeformis exudates was evaluated. Significant complexing by these exudates was demonstrated by DPASV analysis. When added to the culture medium of another species (Thalassiosira profunda), A. coffeaeformis exudates were also able to reduce copper toxicity and accumulation in the cells of that species. However, the copper tolerance of A. coffeaeformis was greater than that acquired by T. profunda grown with A. coffeaeformis exudates; thus exudate production was deemed not to be a primary tolerance mechanism. Comparison of copper accumulations inside and outside cells of A. coeffeaeformis suggests that binding at the cell surface or to mucilage may be an important factor in the tolerance of this species to copper.     

Carbon assimilation and lipid production in phytoplankton in northern Norwegian fjords

Carbon assimilation and lipid production were studied in phytoplankton in Balsfjorden and Ullsfjorden, northern Norway, during the exponential growth phase of the spring bloom in 1983 (6–7 April). In Balsfjorden, phytoplankton biomass was constant with depth and equivalent to 1.5 g chlorophyll a 1^-1. Phytoplankton biomass in Ullsfjorden varied with depth, with a maximum of ca. 7 g chlorophyll a 1^-1 occurring at 5 to 10 m. Particulate carbon-14 assimilation was about 18 mg C per m^-2 h^-1 in Balsfjorden and about 39 mg C per m^-2 h^-1 in Ullsfjorden over the depth range 4 to 8 m. In Balsfjorden, the percentage of total fixed carbon recovered as total lipid was 14.7 and 20.4% at 4 and 8 m depth, respectively. In Ullsfjorden, the corresponding values were 8.8 and 28.1% at 4 and 8 m, respectively. The percentages of total fixed carbon present as fatty acids were 1.1 and 1.6% at 4 and 8 m, respectively, in Balsfjorden, and 0.8 and 6.4% at 4 and 8 m in Ullsfjorden. The majority of the radioactivity in lipid at both locations and at both depths was present as polar lipid, with small percentages present in triacylglycerols and very small percentages present in free fatty acids. On average, about 18% of the total carbon-14 incorporated into phytoplankton over a 6 to 7 h mid-day period was recovered as total lipid and its percentage tended to increase with depth. The relatively low percentage of incorporated carbon-14 present as fatty acids in total lipid implies that most of the radioactivity is present in glyceryl and/or glucosyl moieties and that measurement of total radioactivity in total lipid does not necessarily give an accurate estimation of lipogenesis in phytoplankton. Fatty acid analyses of total phytoplankton in Balsfjorden and Ullsfjorden in 1983 and of a surface slick at the end of a bloom of Phaeocystis pouchetii in Balsfjorden in May 1980 showed an abundance (more than 40% of the total) of (n-3) polyunsaturates in all cases. C-18 polyunsaturates, especially 18:4 and 18:5, were very abundant (about 30% of the total) in the P. pouchetii surface slick in Balsfjorden in 1980. Both P. pouchetii biomass and C-18 polyunsaturates were more abundant in Ullsfjorden than in Balsfjorden (1983). Lipids from the P. pouchetii surface slick were deficient in C-16 polyunsaturates and relatively deficient in C-20 polyunsaturates, but both these classes of fatty acids were abundant in Balsfjorden and Ullsfjorden in 1983. The phytoplankton in both locations in 1983 was dominated by P. pouchetii and diatoms; Chaetoceros socialis was especially abundant in Balsfjorden. The results are discussed in terms of the fatty acids present in herbivorous zooplankton in northern Norwegian fjords.     

Phaeocystis pouchetii blooms in the East Frisian coastal waters (German Bight,North Sea)

In 1982 and 1983, blooms of the planktonic alga Phaeocystis pouchetii were studied in the East Frisian coastal waters off Norderney, FRG. Morphological variability of the colonies, population development, effects on inorganic nutrient and oxygen budget, and the role in the seasonal plankton bloom sequence were investigated. in both years two separate Ph. pouchetii blooms occurred in late spring and early summer that were formed by the globosa-type exclusively. The pouchetii-type was only present during the interval between the first and second blooms in 1982. Growth conditions for Ph. pouchetii seem to be most favourable after a breakdown of a diatom bloom when the silicate reserves have been exhausted, phosphate concentrations are relatively low, but a rich supply of dissolved inorganic nitrogen is available. Contrary to diatom blooms, Ph. pouchetii blooms result in a high production of particulate organic carbon (POC). Another characteristic is the release of great amounts of dissolved organic substances, which cause extensive sea foam formation during the peaks and the subsequent breakdown of the blooms. Due to increased assimilation during mass development and intense disintegration after the breakdown, variations of the oxygen content of the water are significant. Presumably the intensity of blooms has increased in the past decade as indicated by sea foam formation which has actually been excessive in some years.     

Nutritional value of Phaeocystis pouchetii (Prymnesiophyceae) and other phytoplankton for Acartia spp. (Copepoda): ingestion,egg production,and growth of nauplii

Ingestion and egg production by Acartia hudsonica (5°C) and A. tonsa (10°C) from Narragansett Bay, Rhode Island, USA, was measured in the laboratory over a range of concentrations of the prymnesiophyte Phaeocystis pouchetii and the diatom Skeletonema costatum. Both Acartia species reproduced well when fed unialgal diets of S. costatum. Egg production by females fed P. pouchetii, either as gelatioous colonies (primarily >200 m in diameter) or solitary cells (3 to 5 m), was not significantly different than that of starved females. Acartia spp. fed selectively on S. costatum in multialgal treatments. The presence of P. pouchetii did not reduce ingestion of S. costatum; the availability of S. costatum did not increase ingestion of P. pouchetii. Nauplii of A. hudsonica grew equally well on separate diets of P. pouchetii and Isochrysis galbana, two flagellates of similar size. Adult Acartia spp. reproduced poorly when fed these phytoplankton, suggesting that particle size may be more important than food quality in describing the responses. Grazing by Acartia spp. does not directly impact the dynamics of P. pouchetii, but may indirectly contribute to blooms of this prymnesiophyte by removal of competing phytoplankton.     

Blooms of surf-zone diatoms along the coast of the Olympic Peninsula,Washington. I. Physiological investigations of Chaetoceros armatum and Asterionella socialis in laboratory cultures

Two species of diatoms, Chaetoceros armatum T. West and Asterionella socialis Lewin and Norris, which account for virtually all of the plant biomass present in the surf zone along the Olympic Peninsula (USA), have been isolated and grown in culture, and comparisons have been made with respect to their physiological behavior in response to aeration, light, temperature, salinity, and nutrients. A. socialis displayed the greater maximum growth rate under most situations, which would give this species a natural advantage over C. armatum. However, in the natural habitat, C. armalum is by far the more important of the two, since the blooms often consist of almost pure stands of this species. This may be due to its apparent ability to use low light levels more efficiently. The maximum growth rate of C. armatum was reached at light levels of 750 to 1000 lux in cultures, whereas A. socialis required a higher light intensity (4000 lux) for maximum growth. C. armatum did not produce its mucilage envelope under any culture conditions. Such a mucilage envelope, with its accompanying clay particles, is invariably present surrounding the diatom chains when they are collected from the natural habitat. The absence of the mucilage envelope (and clay particles) may explain the peculiarities of iron nutrition which were observed in culture experiments, i.e., a dependence on the presence of iron as NaFe-EDTA or ferric citrate, and failure to utilize ferric choloride as a source of Fe for growth.Contribution No. 652 from the Department of Oceanography, University of Washington.     

Lethal effects of Northwest Atlantic Ocean isolates of the dinoflagellate, Scrippsiella trochoidea, on Eastern oyster (Crassostrea virginica) and Northern quahog (Mercenaria mercenaria) larvae

The thecate dinoflagellate Scrippsiella trochoidea is a cosmopolitan, bloom-forming alga that has been generally considered non-toxic. Here, we report that environmentally relevant cell densities (10⁴ cells mL⁻¹) of Scrippsiella trochoidea strains isolated from the Northwest Atlantic Ocean caused 100% mortality in Eastern oyster (Crassostrea virginica) larvae during 3-day exposures while parallel control larvae exhibited 100% survival. S. trochoidea also exhibited lethal effects on Northern quahog (Mercenaria mercenaria) larvae (70% mortality during 3-day exposure) but were non-toxic to juvenile fish (Cyprinodon variegates). The cultures of S. trochoidea were more lethal to Northern quahog larvae than ten other species of harmful algae, including the highly toxic species Cochlodinium polykrikoides. Scrippsiella trochoidea cultures within later stages of growth were more toxic than exponential growth stages to bivalve larvae, and the toxicity was dose dependent. Furthermore, toxicity was maintained in the cultures that were sonicated, boiled, and frozen as well as in resuspended residues of the culture but was significantly lower in cell-free culture media. Collectively, these results suggest that S. trochoidea causes mortality in bivalve larvae through a physicochemical rather than strictly chemical mechanism, such as clogging of larval feeding apparatuses by materials produced by S. trochoidea (e.g., lipids, extracellular polysaccharides, and/or cell debris) which accumulate as cells in culture or blooms age. This is the first report of the lethal effects of Scrippsiella trochoidea on shellfish larvae.     

Characterization of alkaline phosphatase and organic phosphorous utilization in the oceanic dinoflagellate Pyrocystis noctiluca

Phosphate depleted Pyrocystis noctiluca (Murray) Schuett 1895 has at least one phosphomonoesterase (EC 3:1:3:1) which is triphasic between 0.1 and 222 mol P. The enzyme has a broad temperature range with maximum activity at 50 °C and a Q₁₀ of 1.4 to 1.5. A break in the Arrhenius plot at 35 °C implies the enzyme is membrane-bound. Cytological staining of whole cells and cell fractionation studies (showing 26 times higher specific activity in the particulate compared with the cytoplasmic fraction) suggest the enzyme is plasmalemma-bound. The enzyme has an absolute metal requirement which would be satisfied by Mg⁺⁺ but not Mn⁺⁺, Zn⁺⁺, Fe⁺⁺, or Co⁺⁺ at seawater concentrations. Alkaline phosphatase is a stable enzyme whose activity is not altered by inhibitors of protein synthesis. Orthophosphate inhibition of enzyme activity was largely eliminated in the presence of these inhibitors. Apparently, a protein induced by PO₄ ^3-, rather than PO₄ ^3- itself, inhibits alkaline phosphatase. Cell-free alkaline phosphatase can hydrolyze a variety of phosphate esters and linear polymers of inorganic phosphorus as well as disolved organic phosphorus from tropical oceanic waters. These same hydrolysable organic and inorganic phosphorus compounds support the axenic culture growth of P. noctiluca, suggesting that naturally occurring hydrolysable organic phosphorus compounds may also support the growth of this alga.     

Variable impact of rice (Oryza sativa) on soil metal reduction and availability of pore water Fe2+ and Mn2+ throughout the growth period

Flooding of wetland or agricultural soils can result in substantial alteration of the pore water trace metal profiles and potentially also influence the bioavailability of other trace elements adsorbed to the insoluble oxides. Experimental microcosms were used to quantify the impact of rice (Oryza sativa) plants across an entire growing cycle on the concentrations of Mn²⁺ and Fe²⁺ in two soil types (red sodosol and grey vertosol). Two water management treatments were included: a standard flooded treatment and a saturated treatment (?3?kPa). Soil pore water profiles were established from samples collected at four sampling depths (2.5, 7.5, 15 and 25?cm) on 50 occasions. Fe²⁺ and Mn²⁺ concentrations were higher in flooded soil than in saturated soil and greatest at a depth of 7.5?cm. The presence of rice plants increased Mn²⁺ concentrations in flooded soils, but tended to decrease Mn²⁺ concentrations in saturated soils. The influence of rice plants on Fe²⁺ concentrations was greatest at a depth of 7.5?cm. Changes in soil pore water Fe²⁺ and Mn²⁺ concentrations due to the presence of rice plants were correlated with flowering and reproduction.     

Procedures adopted for the laboratory cultivation of Trichodesmium erythraeum

The present paper describes the procedures adopted for axenic or pure cultivation of the marine blue-green alga Trichodesmium erythraeum (Ehr.)¹ in the laboratory. Strains of T. erythraeum were isolated from the nearshore waters of the Bay of Bengal (11°29 N 79°49 E) during the pre-bloom period in March, 1965, at a depth of 9 to 12m. Modified, enriched, Erdschreiber medium was used for laboratory cultivation of T. erythraeum. A differential concentration of streptomycin 6 mg/100 ml, tetracycline 5 mg/100 ml and sulfadiazine 4 mg/100 ml was used to inhibit the microbial growth in cultures. Difficulties experienced in the initial culture stages of T. erythraeum and the necessary specific instructions employed are described in detail.     

Effects of UV-B irradiation on growth and survival of Antarctic marine diatoms

Growth rate, survival, and stimulation of the production of UV-B (280 to 320 nm) absorbing compounds were investigated in cultures of five commonly occurring Antarctic marine diatoms exposed to a range of UV-B irradiances. Experimental UV-B exposures ranged from 20 to 650% of the measured peak surface irradiance at an Antarctic coastal site (0.533 J m^-2 s^-1). The five diatom species (Nitzschia lecointei, Proboscia alata, P. inermis, Thalassiosira tumida and Stellarima microtrias) appear capable of surviving two to four times this irradiance. In contrast to Phaeocystis cf. pouchetii, another major component of the Antarctic phytoplankton, the concentrations of pigments with discrete UV absorption peaks in diatoms were low and did not change significantly under increasing UV-B irradiance. Absorbance of UV-B by cells from which pigments had been extracted commonly greatly exceeded that of the pigments themselves. Most of this absorbance was due to oxidisable cell contents, with the frustule providing the remainder. Survival of diatoms did not correlate with absorption by either pigments, frustules or oxidisable cell contents, indicating that their survival under elevated UV-B irradiances results from processes other than screening mechanisms.     

Cultures of the pelagic cyanophytes Trichodesmium erythraeum and T. thiebautii in synthetic medium

Trials for determination of culture conditions for the marine cyanophytes of Trichodesmium erythraeum and T. thiebautii were made with use of a synthetic medium. The Aquil medium, either with or without combined nitrogen, brought about stable growth of the two strains, T. erythraeum and T. thiebautii. However, they failed to grow in an ASP₇ medium. The failure was found to be due to the toxic effect of Tris-aminomethane, the pH-buffer in this medium. Two important chemical conditions for the stable growth of Trichodesmium spp. were revealed. (1) Stable growth was supported by Ca²⁺ at high concentrations; in a concentration lower than 0.9 mM, cell-lysis promptly occurred, while the cells could grow without cell-lysis at Ca²⁺ concentrations higher than 7.5 mM even at a salinity as low as 19 S. Ca²⁺ is probably essential for the osmotic regulation in this organism. (2) Phosphate-toxicity at high concentrations was at least partly due to heavy metal(s) contaminating the reagent of inorganic phosphate. After treatment with a Chelex-100 column, phosphate concentration could be increased up to four times the previous concentrations without toxicity.     

Growth and competition of the marine diatoms Phaeodactylum tricornutum and Thalassiosira pseudonana. I. Nutrient effects

Two marine diatoms, Phaeodactylum tricornutum (Bohlin) and Thalassiosira pseudonana (Hasle and Heimdal), were grown both separately and together in batch cultures on a mixture of waste water and seawater enriched with different components of f medium. At 17°C, the maximum division rates of the two species were statistically indistinguishable. The waste water-seawater mixture used proved to have insufficient Si, relative to N and P, for the growth of T. pseudonana, which requires approximately 5x10^-14 g-at Si cell^-1 to divide at a maximum rate. P. tricornutum, on the other hand, although capable of taking up nearly 9x10^-15 g-at Si cell^-1, could sustain maximum rates of division with 4.3x10^-18 g-at Si cell^-1 or less. No allelopathic interaction between the two species could be detected. We conclude that P. tricornutum enjoys a considerable competitive advantage over T. pseudonana in a waste water-seawater-based mariculture system that is not supplemented with Si. Although Si proved necessary for T. pseudonana to complete more successfully with the other diatom, the presence of excess amounts of Si is not necessarily sufficient for the maintenance of T. pseudonana in mixed continuous culture with P. tricornutum: other factors, such as light-related or photoperiod-related growth response, are believed to determine the ultimate outcome of competition between these algae in light-limited continuous culture.Contribution No. 3999, from the Woods Hole Oceanographic Institution.Communicated by M.R. Tripp, Newark