Growth interactions between marine dinoflagellates in multispecies culture experiments

The growth interactions between the marine dinoflagellates Scrippsiella faeroense (Paulsen) Balech et Soares, Prorocentrum micans Ehrenberg and Gymnodinium splendens Lebour were investigated in batch and continuous-culture combinations. Generation times were 1.3±0.2 days for S. faeroense; 1.6±0.1 days for P. micans and 1.8 ± 0.2 days for G. splendens. In mixed batch cultures, growth depended strongly on the ratio between cell numbers of the two species inoculated at the start of the experiments. In various inocula, the more abundant species prevailed and suppressed the competitor even during exponential growth. Total cell production of the suppressed forms was lowered drastically; initial generation time, however, remained more or less unaffected. In equal inocula, S. faeroense dominated over G. splendens, P. micans over S. faeroense and G. spendens over P. micans. In continuous cultures, the species grew independently of each other during the exponential stage; cell number was regulated only by generation times. Competitive inhibition did not occur until maximum cell densities of the dominating forms had been reached. Filtration experiments indicated inhibiting effects mainly in filtered culture liquids of stagnating cultures: reinoculated cells of all three test populations showed a somewhat reduced total cell production. Short initial lag phases were indicated in filtered culture liquids of P. micans and G. spendens; following this period, generation times of the test population remained unaffected, however. The experiments show that growth in multispecies cultures is regulated during the exponential stage mainly by nutrient competition, while at maximum cell densities an additional effect of inhibiting metabolic products is involved. Whether this effect is caused by toxic algal excretions or by bacterial decomposition products of dead cells could not be determined. Sexual stages occurred in aged cultures of S. faeroense and G. splendens both in monocultures and in multispecies combinations and influenced the equilibrium of the competing species. The results are discussed in the light of the findings of other authors.This paper was presented at a poster presentation of the Second International Conference on Toxic Dinoflagellate Blooms, held in Key Biskayne, Florida, USA, 1978.     

Waste-water assay with continuous algal cultures: The effect of mercuric acetate on the growth of some marine dinoflagellates

The effect of mercuric acetate was studied in culture experiments with the dinoflagellates Scrippsiella faeroense (Paulsen) Balech et Soares, Prorocentrum micans Ehrenberg and Gymnodinium splendens Lebour. Impairment of growth rates, in vivo chlorophyll fluorescence, maximum cell densities and morphological changes served as criteria for assessing sublethal influences. Tests were made using the batch- and continuous-culture techniques. Addition of Hg at concentrations of 0.001 mg.1^-1 and higher resulted in reduction of relative growth rates. In a few cases populations recovered from the initial decline and showed new growth. Cell counts corresponded very closely to in vivo chlorophyll fluorescence measurements. Morphological variations were observed in S. faeroense, which responded (even in sublethal concentrations) by bursting it's thecae, releasing naked motile cells and forming vegetative resting stages. The problems of optimal algal-bioassay methods are discussed also, in the light of results obtained by other authors.     

Humic substances. Part II†: Photophysical,photochemical and free radical characteristics

The effects of mercury (HgCl₂) on cell population, chlorophyll a concentration and rates of photosynthesis and excretion were investigated in the phytoplanktonic species Dunaliella minuta in laboratory cultures. Mercury, above 25ppb inhibited both cell population and chlorophyll a concentration approximately to the same extent, whereas the photosynthetic rate was inhibited to a significantly lesser degree. Although, the total photosynthetic rate of the tested organism was reduced, above a threshold concentration, the photosynthetic activity was not reduced under these conditions, but it was in fact significantly greater than that in the control culture. This may suggest that in D. minuta the inhibitory effect of mercury is primarily on cell division rather than cellular photosynthesis, which is enhanced by the fact mercury caused a significant increase of the mean cell volume. Mercury, also, decreased the growth rate and final cell yield. The excretory rate was markedly increased at concentrations ≥ 250 ppb of mercury, but at lower concentrations it tended to depend more on the physiological state of cells than on mercury concentration. In the different cultures, the photosynthetic activity showed variations which occurred without major changes in the chlorophyll a content per cell, which remained almost constant and independent of variations in cell size and growth conditions.     

Regulation of photochemical activity in cultured symbiotic dinoflagellates under nitrate limitation and deprivation

The effects of nitrate limitation and nitrate starvation on the photochemical efficiency of photosystem II (Fv/F m) were examined in batch cultures of two species of symbiotic dinoflagellates, Symbiodinium kawagutii and S. pilosum. F v/F m values were determined along growth curves and show that the F v/F m values are negatively correlated with external nitrogen concentrations in cultures of both species. Changes in growth irradiances in the batch cultures due to increments of the cell densities were estimated S. kawagutii cultures showed a negative correlation between F v/F m and growth irradiance. These results indicate that F v/F m is dependent on the light history of the cultures and on the individual sensitivity of each species, and independent of their nutrient status. Nitrate starvation was analyzed by measuring changes in the quantum yield of fluorescence (Fv/F m), electron transport rate (ETR) and non-photochemical quenching (NPQ) at five time points along the growth curves under three conditions: control (C), without nitrogen (N–), and with ammonia (N+) as a nitrogen source sufficient to meet daily nitrogen requirements. Cells collected during the exponential growth phase and exposed to N– and N+ showed significant reductions in their maximum ETR relative to controls (20% in S. pilosum and 40% in S. kawagutii). The loss of electron transport capacity is consistent with a sink limitation rather than the result of nitrogen starvation. Under nitrate-starvation, the induction of NPQ resulted in effective protection against photosystem II damage in S. pilosum. In contrast, S. kawagutii cells failed to induced NPQ resulting in a concomitant increase in the excitation pressure over photosystem II leading to damage. Collectively the data indicate that F v/F m is not a robust indicator of nitrogen limitation in symbiotic dinoflagellates and that protection against photosystem II damage under sink limitations, is largely dependent on the differential capacities of each species to induce NPQ.Communicated by P.W. Sammarco, Chauvin     

Development of rapid ammonium uptake during starvation of batch and chemostat cultures of the marine diatom Thalassiosira pseudonana

Cell nitrogen quotas and uptake rates following ammonium additions were measured during ammonium-limited growth transients obtained by starving batch and chemostat cultures of Thalassiosira pseudonana (Clone 3 H). During starvation, cell quotas decreased by more than 50% in batch cultures. In chemostat cultures, the drop in cell quota during starvation decreased with dilution rate, from more than 50% at 1.45 d^-1, to less than 10% at 0.22 d^-1. Minimal levels of 3 to 4×10^-2 pg-at. N cell^-1 were reached after 24 h starvation in both batch and chemostat cultures. Uptake rates over the first minute of perturbation experiments were 3 times the long-term (10 to 30 min) rates. In batch cultures, specific uptake rates increased from 4 d^-1 to 20 d^-1 after 24 h starvation. Uptake rates per cell were independent of starvation time and dilution rate in chemostat cultures, but lower in non-starved batch cultures. The implications of these data for models of phytoplankton growth are discussed: the data support models which predict a depression in average growth rates when diatoms encounter microscale patches in oligotrophic environments.     

Dynamics and physiology of saxitoxin production by the dinoflagellatesAlexandrium spp.

Toxin content (fmol cell^–1) and a suite of elemental and macromolecular variables were measured in batch cultures of the dinoflagellatesAlexandrium fundyense, A. tamarense andAlexandrium sp. from the southern New England region, USA. A different perspective was provided by semicontinuous cultures which revealed sustained, steady-state physiological adaptations by cells to N and P limitation. Two types of variability were investigated. In batch culture, changes in nutrient availability with time caused growth stage variability in toxin content, which often peaked in mid-exponential growth. A second type of variability that could be superimposed on growth stage differences is best exemplified by the high toxin content of cells grown at suboptimal temperatures. Calculations of the net rate of toxin production (R _tox ; fmol cell^–1 d^–1) for these different culture treatments and modes made it possible to separate the dynamics of toxin production from cell division. Over a wide range of growth rates, cells produced toxin at rates approximating those needed to replace losses to daughter cells during division. The exception to this direct proportionality was with P limitation, which was associated with a dramatic increase in the rate of toxin production as cells stopped dividing due to nutrient limitation in batch culture. Growth stage variability in batch culture thus reflects small imbalances (generally within a factor of two) between the specific rates of toxin production and cell division. N limitation and CO₂ depletion both affect pathways involved in toxin synthesis before those needed for cell division; P limitation does the opposite. The patterns of toxin accumulation were the same as for major cellular metabolites or elemental pools. The highest rates of toxin production appear to result from an increased availability of arginine (Arg) within the cell, due to either a lack of competition for this amino acid from pathways involved in cell division or to increased de novo synthesis. There were no significant changes in toxin content with either acclimated growth at elevated salinity, or with short term increases or decreases of salinity. These results demonstrate that toxin production is a complex process which, under some conditions, is closely coupled to growth rate; under other conditions, these processes are completely uncoupled. Explanations for the observed variability probably relate to pool sizes of important metabolites and to the differential response of key biochemical reactions to these pool sizes and to environmental conditions.     

Effects of Olisthodiscus luteus on the growth and abundance of Tintinnids

The effects of the red tide flagellate Olisthodiscus luteus Carter on the growth of two tintinnid species, Tintinnopsis lubulosoides Meunier and Favella sp. (Clap. & Lach.) Jorg., were measured in batch culture. T. tubulosoides and Favella sp. grew at rates equivalent to 1.2 (10°C) and 2.0 (20°C) population doublings per day, respectively, when offered nutritionally adequate phytoplankton species. The growth rates of both tintinnid species were reduced in the presence of 10²–10³ O. luteus cells · ml^-1 in multialgal treatments. Growth rate inhibition was proportionately greater at higher O. luteus densities. Lethal effects were observed for both tintinnid species at O. luteus concentrations of 5x10³ cells · ml^-1 in multi-algal treatments. T. tubulosoides mortality occurred at all O. luteus concentrations in unialgal culture. O. luteus-conditioned medium did not substantially inhibit tintinnid growth when combined with acceptable food species, suggesting that toxicity is induced by ingestion or direct contact with O. luteus cells, or by exposure to a short-lived exudate. In agreement with these results, an inverse relationship between O. luteus concentration and tintinnid abundance was observed in Narragansett Bay, Rhode Island, over a two year period. The small lorica diameter of the species apparently inhibited by these O. luteus blooms suggests a detrimental effect independent of cell ingestion. In addition to the absolute concentration of O. luteus cells, the availability of nutritionally adequate algal food may be an important factor determining the impact of O. luteus blooms on tintinnid populations.Contribution no. 5048 from the Woods Hole Oceanographic Institution     

Growth inhibition and formation of morphologically abnormal cells of <Emphasis Type="Italic">Akashiwo sanguinea</Emphasis> (Hirasaka) G. Hansen et Moestrup by cell contact with <Emphasis Type="Italic">Cochlodinium polykrikoides</Emphasis> Margalef

We investigated the growth interaction between Cochlodinium polykrikoides and Akashiwo sanguinea using bi-algal cultures. When cultured together, morphologically abnormal cells of A. sanguinea appeared and the growth of A. sanguinea notably decreased. When C. polykrikoides and A. sanguinea were cultured separately in inner and outer wells separated by a membrane with a pore size of 3 μm, neither species showed suppressed growth and no morphologically abnormal cells of A. sanguinea were observed. Furthermore, filtrates from C. polykrikoides cultures did not affect the cell morphology or the growth of A. sanguinea. When the abnormal cells of A. sanguinea were transferred from bi-algal cultures into fresh medium and cultured as a mono-algal culture, cells recovered their normal morphology and multiplied. Therefore, the growth inhibition and formation of morphologically abnormal cells of A. sanguinea would be induced by constant cell contact with C. polykrikoides.     

Algal plating as a tool for investigating allelopathy among marine microalgae

Chemical interactions among marine microalgae were studied in cultures of pennate diatoms on agar plates. Nine marine and freshwater pennate diatoms were surveyed as potential bioassay organisms; Cylindrotheca fusiformis was most favourable for assays because of its rapid and even growth on agar. Diatom and bacterial bioassays were used to screen cell and filtrate extracts of 14 microalgal cultures. A number of these algal species, which were grown axenically, produced extractable, intracellular and/or extracellular substances that inhibited the growth of C. fusiformis. Our results suggest that the culturing of pennate diatoms on a solid medium can provide a simple bioassay for screening algal extracts which potentially contain growth inhibitors involved in microalgal allelopathy.     

Respiration and physiological state in marine bacteria

The relationship between oxygen consumption (R) and respiratory electron-transport-system (ETS) activity was investigated in batch cultures of 5 species of marine bacteria, Vibrio adaptatus, V. anguillarum, a partially identified Vibrio sp. SA774, Serratia marinorubra, and Pseudomonas perfectomarinus. Although cellular levels of R and ETS varied widely among the species tested, the R:ETS ratios for growing or senescent populations were relatively constant among the species; these ratios were 5.02 in growth and 0.426 in senescence, with coefficients of variation of 29 and 20%, respectively. The lower senescent-phase R:ETS ratio was due to a depression of the respiration rates following growth termination. The regression log (R per cell) = 0.832 log (ETS per cell) + 0.727 for the growing populations was similar to that determined for marine zooplankton. The slight dependency of the R:ETS ratio on organism dry weight found for zooplankton was supported by our data. Planktonic respiration rates estimated from measured ETS-depth profiles in the eastern tropical North Pacific Ocean using the senescent-phase R:ETS ratio were similar to published oxygen consumption rates in the deep sea.Contribution No. M79-61 from the University of Washington's Department of Oceanography, and No. 79032 from the Bigelow Laboratory for Ocean Sciences.     

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.     

Vibrio alginolyticus,a tetrodotoxin-producing bacterium,in the intestines of the fish Fugu vermicularis vermicularis

The influence of nutrient deprivation on cell-cycle progression was examined in two phytoplankton species, the diatom Thalassiosira weissflogii (actin) and the coccolithophorid Hymenomonas carterae (cocco II). The diatom was starved for nitrogen, silicon or both, whereas only nitrogen limitation was examined in H. carterae. In both species, nitrogen-starved cells were arrested in the early part of the cell cycle (G₁ phase). In the diatom, silicon-starvation arrested cells in late G₁ phase and also in the last part of the cell cycle (G₂+M). In all cases, cell-cycle arrest could be reversed by addition of fresh medium, but cell-cycling times during the first generation were increased in comparison to those in nutrient replete, steady-state growth conditions. These results supply evidence for simultaneous dual-nutrient limitation of population growth and provide a mechanistic interpretation for the division patterns observed in cultures where nutrients are supplied periodically.     

Questions on the mechanism of temperature adaptation in marine phytoplankton

The rate of light-saturated photosynthesis in 3 marine algae [Phaeodactylum tricornutum Bohlin, Nitzschia closterium (Ehrenberg) Smith and Dunaliella tertiolecta Butcher] varies during growth in batch culture. The photosynthetic rare declines most rapidly during growth at the higher temperatures. Because of these changes in photosynthesis rate, the previously reported enhanced photosynthetic abilities caused by growth at lower temperatures (generally interpreted as evidence for higher enzyme levels) can only be observed when measurements are made late in the exponential phase or after the onset of the stationary phase of growth. When allowance is made for the earlier peak of photosynthetic ability in cultures growing at higher temperatures, there is no evidence for adaptation to lower temperatures being caused by increased levels of the enzymes required for carbon-dioxide fixation. When the changes due to growth in batch culture are taken into account, certain effects of temperature can be recognized. the dry weight: chlorophyll ratio of all 3 algae increases with decreasing growth temperatures. For P. tricornutum and N. closterium, growth at lower temperatures reduces the cellular content of chlorophyll a, but has little effect on the chlorophyll content of D. tertiolecta. The dry weight: cell-number ratio of D. tertiolecta and P. tricornutum increases with lower growth temperatures, but growth temperature has little effect on the cell mass of N. closterium. Growth of the 3 algae at lower temperatures does not increase their ability to photosynthesize at these lower temperatures. Rather, it reduces their ability to assimilate carbon dioxide at the higher temperatures.     

Aspects of iron and nitrogen nutrition in the red tide dinoflagellateGymnodinium sanguineum

Iron-stress-mediated effects on biochemical constituents of the red tide dinoflagellateGymnodinium sanguineum Hirasaka were examined in 1988 by comparing Fe-replete and Fe-deplete batch cultures. The influence of nitrogen source (NO₃ or NH₄) on characteristics of Fe-deplete cells was also studied [i.e., Fe-deplete/NO₃-grown (= — Fe/NO₃) vs Fe-deplete/NH₄-grown (= — Fe/NH₄)]. Common to both N sources were reductions of chlorophylla (chla) and Fe quotas (per cell volume) by 75% and ca. 1.5 orders of magnitude, respectively, under Fe depletion. The Fe requirement ofG. sanguineum exceeded those of certain neritic diatoms by one to two orders of magnitude. — Fe/NH₄ cells exhibited 30 to 50% greater N quotas and free amino acid:protein ratios than did Fe-deplete cells grown on NO₃. In vivo fluorescence:chla increased with Fe deficiency particularly in — Fe/NO₃ cultures, surpassing — Fe/NH₄ values by ca. two-fold. Effects of Fe depletion were consistent with this element's essential role in the biosynthesis of chla and components of the photosynthetic electron transport (PET) system, and also in NO₃ utilization. Fe:N ratios were larger (1.5-fold) for iron-deficient NO₃-grown than NH₄-grown cells, likely reflecting the Fe content of NO₃ assimilatory enzymes [nitrate (NR) and nitrite (NiR) reductase] and of electron transport components needed to provide reductant, coupled with a diminished capacity of — Fe/NO₃ cells to acquire and assimilate nitrogen. Indicators of PET efficiency suggested that under iron stress, supply of Fe for NR and NiR is partly at the expense of iron-containing PET components. Utilization of nitrate by NO₃-grown cells was inhibited sufficiently by Fe depletion to yield symptoms bordering on N deficiency. In an ecological context, the most important effect mediated by nitrogen source may be the determination of critical Q_Fe (i.e., Fe required to just sustain maximal growth), thereby regulating the degree of growth limitation for a given subsaturating iron concentration.     

Effects of water-soluble fraction of the Mexican crude oil “Isthmus Cactus” on growth,cellular content of chlorophylla,and lipid composition of planktonic microalgae

Phytoplankton species were grown in batch cultures in the presence of the water-soluble fraction (WSF; 50 and 100%) of a Mexican crude oil (Isthmus Cactus). The algae exhibited various responses ranging from retarded growth to stimulation of growth. The cellular content of chlorophylla and the lipid composition of the algae were examined. Four algae, the bacillariophytesNitzschia closterium andAsterionella glacialis, the cryptophytesRhodomonas lens, and the chlorophyteDunaliella tertiolecta, exhibited retarded growth. In most of these algae, cellular chlorophylla, lipid pigments, glycolipids and triglycerides decreased whereas sterols and hydrocarbons accumulated. Phospholipids did not exhibit any specific pattern of change during the experiments. The cyanophyteAgmenellum quadruplicatum and the bacillariophyteSkeletonema costatum were less sensitive to the WSF. The cell yield of the dinophyteProrocentrum minimum was stimulated by the WSF. In these three latter species, lipid pigments were enhanced or remained at control levels. We concluded that the toxic effect of the WSF disrupts the biosynthesis mechanisms required for a functional photosynthetic apparatus (biosynthesis of chlorophylla, glycolipids and lipid pigments) in sensitive algae, a phenomenon coupled to sterol accumulation in these algae.     

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.     

Allelopathic effect of Cylindrospermopsis raciborskii extracts on the germination and growth of several plant species

Cylindrospermopsis raciborskii is an invasive cyanobacterium and a potential producer of the alkaloid toxin cylindrospermopsin (CYN). Extracts of two strains of C. raciborskii were tested for their effects on the germination and growth of Lactuca sativa, Phaseolus vulgaris, Pisum sativum and Solanum lycopersicum. Germination was not significantly inhibited for any of the plant species tested, but growth was affected, depending on the species. Root and stem growth in L. sativa was generally stimulated by both strains. Ph. vulgaris root growth was stimulated by both strains but no effect was visible in stem growth. S. lycopersicum root growth was inhibited by both strains and stem growth was inhibited only by the CYN strain. P. sativum root growth was also inhibited by both strains but stem growth was stimulated. CYN accumulation was also differential with toxin transfer to the stem. Ph. vulgaris accumulated the highest CYN concentration. This study suggests that plants behave differently in their response to this toxin and that roots and stems also show different abilities to react and accumulate the toxin. Knowledge of the impact of CYN- and non-CYN-producing cyanobacteria in different plant species and translocation of the toxin to different plant parts is essential for the avoidance of human as well as environmental health hazards.     

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.     

Sinking rate response to depletion of nitrate,phosphate and silicate in four marine diatoms

The effect of nutrient (N, P, and Si) depletion on sinking rates was studied for two small (Skeletonema costatum (Grev.) Cleve and Chaetoceros gracile Schütt) and two large [Ditylum brightwellii (West) Grun and Coscinodiscus wailesii (Gran et Angst)] centric diatoms obtained from stock cultures. Each diatom was examined under conditions of (1) nutrient repletion (=log growth phase), (2) nutrient depletion (48 h without a given substrate), and (3) recovery (24h after addition of limiting substrate to nutrient-deplete populations). All nutrient-replete cultures displayed low sinking rates despite large differences in cell size. In nutrient-deplete populations, sinking rate was related to the kind of nutrient depleted and varied among species. Silicate depletion elicited by far the greatest increase in sinking rates in all 4 species, indicating that biochemical aspects of silicon metabolism are more important to buoyancy regulation than density-related variations in the amount of silicon per cell. Since N- and P-depletion caused lower sinking rates in 3 of the species, this observation calls for re-evaluation of the axiom that nutrient depletion necessarily causes increased sinking rates. The exception was Coscinodiscus wailesii, which sank faster under all types of nutrient limitation. In most cases, sinking rates typical of log-phase cultures were not regained within 24 h after the addition of limiting nutrient to nutrient-depleted populations. Ultimately, the length of the recovery period may be useful in identifying the metabolic processes responsible for buoyancy regulation in actively growing cells.     

Interactions between nutritional status and long-term responses to ultraviolet-B radiation stress in a marine diatom

Influences of nutritional status on the photoinhibitory effects of ultraviolet-B radiation (UVBR: 290 to 320 nm) on the specific growth rates (_obs) and biomass of Phaeodactylum tricornutum were determined using nutrient-replete batch cultures and nutrient-limited continuous cultures. P. tricornutum cultures were exposed to UVBR doses representative of current mid-latitude and ozone-depletion intensities. Specific growth rates and biomass were inhibited from 2 to 16% by UVBR during nutrient-replete growth. However, no effect of UVBR on _obs or biomass was detectable when nutrient limitation exceeded the potential for limitation by UVBR. Thus, a competitive interaction appears to occur between macronutrient stress and UVBR stress, such that _obs and biomass will be determined by the most limiting factor. Our results suggest that measurable decreases in phytoplankton _obs and biomass from UVBR are most likely in nutrient-rich areas of the ocean, while these parameters may not be appropriate for measuring UVBR stress in regions of nutrient limitation.