pH effect on the susceptibility to parasitoid infection in the marine diatom Coscinodiscus spp. (Bacillariophyceae)

The pH on the frustule of individual cells of the marine centric diatoms Coscinodiscus granii and Coscinodiscus wailesii (Bacillariophyceae) was measured with pH microsensors in culture media with increasing pH values of 8.04, 8.14, and 8.22, respectively. In 85–96% of the C. granii cells the pH on the frustule was up to 0.4 units higher than that of the medium, reaching a maximum pH 8.95. Only in 2–3% the surface pH exceeded that of the medium by up to 0.7 pH units. These results strongly suggest that diatoms in batch cultures differ, at least temporarily, in their individual photosynthetic activities. Infection experiments with the parasitoid nanoflagellate Pirsonia diadema (Stramenopile) showed that flagellates failed to infect when the culture pH was 8.8 and above. pH measurements on freshly infected C. granii showed that the prevalence of infection was higher in tendency on diatoms with low surface pH. Application of these results to parasitoid-diatom interactions in natural waters suggests that within phytoplankton populations a strong photosynthetic activity might prevent diatom cells temporarily from infection by pH-sensitive parasitoids.     

Buoyancy of natural populations of marine phytoplankton

Buoyancy of natural populations of marine phytoplankton was studied in a fjord in western Norway during the diatom bloom and in autumn. The study was carried out under approximate in situ conditions by means of an apparatus described in the paper. During the spring bloom, positive buoyancy was observed only once. Sinking rates of individual fractions ranged from 0 to more than 9 m day^-1, and the mean sinking rates of the total chlorophyll content from 0 to at least 2. 2 m day^-1. The highest rates occurred in the post-bloom period, while sinking appeared negligible from the onset of the bloom up to its culmination. In autumn, the population was dominated by small, flagellated cells. Positive buoyancy, or upward migration, was then observed in two out of three experiments.     

Light-dependency of nitrate uptake by phytoplankton over the spring bloom in Auke Bay,Alaska

The effect of light intensity on nitrate uptake by natural populations of phytoplankton was examined by ¹⁵N traceruptake experiments during the spring (March–May 1987) in Auke Bay, Alaska. The data were fit to a rectangular hyperbolic model which included a term for dark uptake. Three types of curves described nitrate uptake as a function of light intensity. The first (Type I) had a low half-saturation light intensity (K _I), low chlorophyll-specific uptakes rates, no dark uptake and occasional photoinhibition. These were observed during a period of biomass decrease, accompanied by low daily light and strong wind, prior to the major bloom. The second type (Type II) had relatively high K _I, high chlorophyll-specific uptake rates, and no dark uptake. Type II curves were observed during most of the period prior to nitrate depletion in the surface waters. Types I and II both appeared prior to nitrate depletion in the water and reflected variations in the light history of the phytoplankton population. The third type (Type III) occurred in nitrate-deplete conditions, when nitrate uptake was less dependent on light intensity (i.e., high rates of dark uptake and lower K _I). Decreased light-dependency during this period was coupled with physiological nitrogen deficiency in the population. Comparing these parameters to those of photosynthetic carbon fixation, K _Ivalues of nitrate uptake were generally higher than those of photosynthesis prior to nitrate depletion, and lower during nutrient-deplete conditions.     

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.     

Sinking-rate response of natural assemblages of temperate and subtropical phytoplankton to nutrient depletion

Phytoplankton assemblages were collected during spring blooms in 1982 in Washington State and in Hawaii. Sinking rate responses of these assemblages were examined under nitrate, phosphate, and silicate depletion. Ambient nutrient concentrations, chlorophyll concentrations, photosynthetic rates, sinking rates, and floristic compositions were determined. Under nutrient-replete conditions, the temperate assemblage, composed primarily of large centric diatoms, had a sinking rate of 0.96 m d^-1; sinking rates did not change appreciably over 4 d without nitrate. Without phosphate or silicate, the sinking rates remained constant for 3 d and then increased after biomass indices began to decline. These findings illustrate the potential importance of phosphate or silicate depletion to the sedimentation of spring-bloom diatom populations. The subtropical assemblage, composed primarily of diatoms, coccolithophorids, and dinoflagellates, had an initial sinking rate of 0.22 m d^-1 and did not display substantial sinking rate changes in the absence of nitrate, phosphate or silicate. Floristic data consistently showed a proliferation of pennate diatoms, which had lower settling rates than centric diatoms. Growth and sedimentation patterns indicated a competitive advantage for pennate diatom components of subtropical assemblages; this in turn may limit phytoplankton sedimentation losses in such ecosystems.     

Spring bloom sedimentation in a subarctic ecosystem

Results from a 5-yr study (1985 to 1989) in Auke Bay, Alaska show that termination of the spring bloom consistently occurred at limiting nitrate concentrations. Following nutrient exhaustion, phytoplankton sinking rates increased and displayed greater temporal variability. Threshold nitrate concentrations, approximating K_s values of the species present, were found to signal initiation of increased sedimentation. For Thalassiosira aestivalis, the threshold was ～2 μmol l^-1, while for Skeletonema costatum the threshold was ～1 μmol l^-1, suggesting genus-specific differences in sinking-rate sensitivity to nitrate exhaustion. Overall, sinking rates of the three principal genera ranked (high to low) Thalassiosira spp.> S. costatum>Chaetoceros spp., while the nitrate sensitivities of the sinking rates of the genera ranked (high to low) Thalassiosira spp.> Chaetoceros spp.> S. costatum. Thalassiosira spp. showed the most consistent sinking rate increases following nutrient impoverishment. During a bloom dominated by T. aestivalis, a decrease of cell sinking rate with depth coincided with a decrease in short-term nutrient stress as measured by intracellular nitrate pools. In addition, no correlation was found between chain length or aggregate formation and sinking rate for this species. Though we measured only small-scale cell-cell adhesion, not larger-scale marine snow formation, this supports the notion that the sinking rates of Thalassiosira spp. were controlled primarily by cell physiology. For S. costatum, however, shorter chains sank faster. The sinking behavior of the species studied here figures prominently in their pelagic ecology and in the carbon flux of coastal ecosystems, both of which are driven by short-term variability.     

Saturated uptake kinetics: transient response of the marine diatom Thalassiosira pseudonana to ammonium,nitrate, silicate or phosphate starvation

Changes in the saturated uptake kinetics of the limiting nutrient were followed as Thalassiosira pseudonana (Clone 3 H) batch cultures entered ammonium, nitrate, silicate and phosphate starvation. Cultures starved of ammonium or phosphate developed very high specific uptake capacities over a 24 to 48 h starvation period, due to both decreases in cell quota and increases in uptake rates per cell. In particular, the cell phosphorus quota decreased ca. 8-fold during phosphate starvation and specific uptake rates exceeded 100 d^-1. In contrast, cultures entering nitrate or silicate starvation underwent little or no further cell division, and the uptake capacity declined during starvation. After 24 to 48 h starvation, an induction requirement for uptake of nitrate or silicate was apparent. These responses are consistent with adaptation to the pattern of supply of these nutrients in the field.     

Effects of sublethal concentrations of mercuric chloride on ammonium-limitedSkeletonema costatum

The effect of sublethal additions of mercuric chloride on the marine diatomSkeletonema costatum (Grev.) Cleve grown in NH₄-limited chemostats and batch cultures was assessed. In short-term Hg exposure experiments (up to 5 h), the effect of Hg on ammonium uptake rates was studied by simulatneously perturbing the culture with 5 M NH₄ Cl and Hg concentrations ranging from 0.04 to 5.52 nM HgCl₂. The threshold of Hg toxicity occurred between 1.8 and 3.7 nM, based on a decrease in ammonium uptake rates. When the NH₄-limited culture was starved of ammonium for 30 h, the threshold of Hg toxicity decreased about an order of magnitude to 0.2 nM. In long-term Hg exposure experiments (679.5 h), NH₄-limited continuous cultures were semi-continuously exposed to 0.37 and 3.68 nM HgCl₂. After 4 days, the cell density in the Hg-treated chemostats began to drastically decline. After about 16 days these populations recovered, even though Hg additions continued. At the end of the experiment (26 days), cell densities had reached the levels observed at the beginning of the experiment. The reason for the recovery is unknown, but several possibilities are discussed. Ammonium uptake rates determined during the time-course of this long-term Hg exposure, indicated that these NH₄-limited cultures exhibited a significant loss in their ability to take up ammonium at low concentrations (e.g. 1 M). Thus, mercury pollution may seriously decrease the ability of a species to utilize the limiting nutrient during periods of seasonal nutrient limitation.     

Synergistic effects of nitrate and ammonium ion on the growth and uptake kinetics of Monochrysis lutheri in continuous culture

Continuous-culture results for Monochrysis lutheri grown on 12 h light-12 h dark cycles with a spectrum of ratios of nitrate and ammonium serving as limiting nutrients are compared with continuous light, exclusively nitrate, and exclusively ammonium-limited data for this species. The diel effects of the light regime on the maximum specific uptake rate are examined for both nitrate and ammonium. Synergistic effects on uptake by various initial concentrations of these two nutrients are presented. Preconditioning with light-dark cycles did not affect maximum uptake rate, but preconditioning on a combination of nitrate and ammonium gave much lower uptake rates than those observed for populations preconditioned on either nutrient exclusively. The implications of high maximum specific-uptake rates compared to maximum specific-growth rates in terms of the range of nitrate and ammonium ion concentrations associated with nutrient limitation are reviewed.Hawaii Institute of Marine Biology Contribution No. 477.     

Marine diatoms grown in chemostats under silicate or ammonium limitation. I. Cellular chemical composition and steady-state growth kinetics of Skeletonema costatum

Skeletonema costatum was grown in chemostats under ammonium or silicate limitation to examine its growth kinetics and changes in cellular chemical composition at different steady-state growth rates. When the relationship between the effluent limiting substrate concentration and steady-state growth rates was examined, deviations from the simple hyperbolic form of the Monod growth equation were noted at low and high dilution rates. The data from the plot of growth rate and substrate concentration were divided into 4 regions and the relationship of these region to cell quota is discussed. Two physiological states were identified. All populations grown at D<0.05 h^-1, regardless of the size of the cells or the magnitude of Q, exhibited a maximal growth rate of approximately 0.05 h^-1, while populations grown at higher dilution rates (D>0.06 h^-1 to 0.14 h^-1). The maximal value of growth rate is obtained only in cultures grown at very high dilution rates where nutrient shift-up appears to occur, the cell quota approaches a maximum and the heterogeneous cell population becomes more homogeneous.Contribution No. 881 from the Department of Oceanography, University of Washington, Seattle, Washington 98195, USA. This paper represents a portion of two dissertations submitted by P.J.H. and H.L.C. to the Department of Oceanography, University of Washington, Seattle, in partial fulfillment of the requirements for the Ph.D. degree.     

Species-specific sedimentation and sinking velocities of diatoms

Sedimentation rates were determined for various diatom species, and both average and maximum sinking velocities of sedimenting diatoms were calculated during a spring bloom investigation in the central Baltic Sea in 1986. Up to 25 and 50% of theChaetoceros spp. andThalassiosira levanderi populations, respectively, sedimented daily. Daily sedimentation rates of other diatoms, dinoflagellates andMesodinium rubrum, however, were less than 1% of their respective standing stocks. TheT. levanderi population was divided into two subpopulations: while one was sinking, the second was actively dividing (recognizable by paired-cell stages) with a specific growth rate of >0.2 to 0.3 d^–1. These paired cells were never found in sediment trap samples. The average sinking velocity ofChaetoceros spp. was 15 to 30 m d^–1; that ofT. levanderi was higher. The maximum sinking velocity of cells was at least 70 m d^–1. According to these observations, the formation of aggregates (which enhances sinking velocity), and their sedimentation, represent a highly selective process. This indicates that diatom aggregates do not act as roving filters, sweeping the water clear while sinking.     

The effects of ammonium and phosphate enrichments on clorophyll a,pigment ratio and species composition of phytoplankton of Vineyard Sound

Seawater containing natural phytoplankton populations from Vineyard Sound, USA was enriched in the laboratory with three levels each of ammonium and phosphate and with a combination of ammonium and phosphate which provided three different N:P ratios. The addition of ammonium produced more cells and chlorophyll a than the control or the phosphate enrichments. However, enrichment with ammonium and phosphate, regardless of the N:P ratio, yielded the most cells and chlorophyll a. Thus, nitrogen seems to be the primary limiting nutrient, with phosphate showing secondary limiting effects. The ratios of photosynthetic pigments decreased with the increased chlorophyll a production in the enriched cultures. There were no significant changes in the species composition within the cultures, so that the observed changes in pigment ratio and chlorophyll a content were due to physiological responses.     

Life-form population responses of a marine planktonic diatom,Chaetoceros pseudocurvisetus,to oligotrophication in regionally upwelled water

Life-form population responses of a centric planktonic diatom,Chaetoceros pseudocurvisetus Mangin, were investigated in summer 1986 and 1988 in the Izu Islands, Japan, in regionally upwelled water where nutrient concentration changed from favorable to unfavorable conditions for active growth and reproduction (oligotrophication). Two types of life form were observed: vegetative cells of healthy and unhealthy looking conditions and resting spores. The observed life-form responses were experimentally evaluated along with a depletion of limiting nutrients. The algal population ceased vegetative growth and initiated resting spore formation with a disappearance of limiting nitrate, and this life-form response seemed to be triggered by the decrease of cellular nitrogen content below a certain level. Since a large amount of silicon was required for the resting spore formation, a part of vegetative cells were unable to form resting spores and formed unhealthy looking vegetative cells under insufficient concentrations of silicic acid. Percentage shares of the resting spores in the population were linearly related to the amounts of available silicic acid. Vegetative cells which did not form resting spores showed greater mortality than resting spores by attack of bacteria and protozoa; however, vegetative cells could respond quickly to possible nutrient replenishment.     

Marine diatoms grown in chemostats under silicate or ammonium limitation. IV. Transient response of Chaetoceros debilis,Skeletonema costatum,and Thalassiosira gravida to a single addition of the limiting nutrient

The kinetic response of ammonium- or silicate-limited and ammonium- or silicatestarved populations of Chaetoceros debilis, Skeletonema costatum, and Thalassiosira gravida was determined by a single addition of the limiting nutrient to a steady-state culture and subsequent monitoring of the nutrient disappearance of the limiting and non-limiting nutrients at frequent time intervals. The kinetic response of nonlimited (nutrient) populations of these three species was also determined. Three distinct modes of the uptake of the limiting nutrient were observed for ammonium-or silicate-limited populations of these three species, surge uptake (V _s ), internally (cellular) controlled uptake (V _i ), and externally (ambient limiting nutrient concentration) controlled uptake (V _e ). Non-limited populations did not exhibit the three distinct segments of uptake, V _s , V _i and V _e . Estimates of the maximal uptake rate (V _max) and the Michaelis constant (K _s ) were obtained from nutrient-limited populations during the V _e segment of the uptake curve. Pooled values of V _e for the three ammonium-limited populations yielded V _max and K _s estimates of 0.16 h^-1 and 0.5 g-at NH₄–N l^-1. Kinetic data derived from the V _e segment of the uptake curve for silicate-limited populations yielded different values of V _max and K _s for each of the three species. In a number of parameters that were measured, T. gravida was clearly different from C. debilis and S. costatum and its recovery from nutrient starvation was the slowest. Recovery of all species from silicate limitation or starvation was slower than from ammonium limitation or starvation. Ammonium-starved populations maintained a maximal uptake rate at a substrate concentration an order of magnitude lower (0.1 g-at NH₄–N l^-1) than that observed for NH₄-limited populations (1.0 g-at NH₄–N l^-1). Adaptation to the severity of the nutrient limitation occurred as changes in the magnitude of cellular characteristics, such as short-term uptake potential (V _s ) and affinity for the substrate (K _s ). The consequence of these results are discussed in terms of another possible mechanism to explain changes in species composition and succession in nutrient-depleted environments.Contribution No. 944 from the Department of Oceanography, University of Washington, Seattle, Washington 98195, USA.     

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.     

Marine diatoms grown in chemostats under silicate or ammonium limitation. II. Transient response of Skeletonema costatum to a single addition of the limiting nutrient

Skeletonema costatum was grown at different steady-state growth rates in ammonium or silicate-limited chemostats. The culture was perturbed from its steady-state condition by a single addition of the limiting nutrients ammonium or silicate. The transient response was followed by measuring nutrient disappearance of the liliting perturbation experiment indicate that three distinct modes of uptake of the limiting nutrient can be distinguished; surge uptake (V _s ), internally controlled uptake (V _i ), and externally controlled uptake (V _e ). An interpretation of these three modes of uptake is given and their relation to control of uptake of the limiting nutrient is discussed. The uptake rates of the non-limiting nutrients were shown to be depressed during the surge of the uptake of the limiting nutrient. Kinetic uptake parameters, K _s and V _max, were obtained from data acquired during the externally controlled uptake segment, V _e . The same V _max value of 0. 12 h^-1, was obtained under either silicate or ammonium limitation. Estimates of K _s were 0.4 g-at NH₄-N l^-1 and 0.7 g-at Si l^-1. Short-term ¹⁵N uptake-rate measurements conducted on nitrogen-limited cultures appear to be a combination of V _s or V _i , or at lower substrate concentrations V _s and V _e . It is difficult to separate these different uptake modes in batch or tracer experiments, and ensuing problems in interpretation are discussed.Contribution No. 882 from the Department of Oceanography, University of Washington, Seattle, Washington 98195, USA. This work represents portion of three dissertations submitted to the Department of Oceanography, University of Washington, Seattle, in partial fulfillment of the requirements for the Ph.D. degree.     

Membrane voltage of marine phytoplankton,measured in the diatom Coscinodiscus radiatus

In order to investigate the role of the ionic relations in buoyancy of marine phytoplankton, voltage recordings have been made on the planktonic diatom Coscinodiscus radiatus using conventional glass microelectrode techniques. The most negative Nernst equilibrium voltage in C. radiatus is E _K, the potassium equilibrium voltage of around-85 mV. Accordingly, stable voltages of-40 to-80 mV were recorded from C. radiatus which conforms to the general theory of electro-diffusion of ions through membranes (voltage range V _d). In addition, membrane voltages much more negative, e.g. up to-140 mV (voltage range V _p), have been recorded in C. radiatus; these voltages demonstrate the operation of an electrogenic pump. Within the voltage range V _d, light-on and-off (microscope illumination) caused weak hyper- and depolarizations by about 2 mV with a time constant of about 10 s. Also within V _d, spontaneous oscillations could be observed with a frequency of about 0.03 Hz and irregular amplitudes up to 30 mV. These phenomena are simulated by a model for electrocoupling of the major ion transporters in plants, as worked out for guard cells with their subtle osmoregulatory system. Equivalent mechanisms are suggested to operate in planktonic diatoms for adjustment of buoyancy by appropriate uptake and release of ions.     

Spring bloom sedimentation in a subarctic ecosystem

A 5-yr study (1985 to 1989) of spring bloom sedimentation in Auke Bay, Alaska, indicates that the sinking response of diatoms to ambient nutrients influences both species succession during the spring bloom and the subsequent sedimentation of new production. Diatoms from the genera Thalassiosira, Chaetoceros and Skeletonema formed the bulk of the spring bloom each year. Growth of Thalassiosira spp. consistently initiated the primary bloom, while Skeletonema costatum tended to grow later in, or after, the primary bloom. We postulate that this successional pattern is driven by interspecific nutrient competition. Overall, sedimentation flux of the dominant species of bloom diatoms was correlated with surface concentrations of cells integrated over the bloom period. In fact, different linear relationships existed when Thalassiosira and Chaetoceros spp. were considered separately, but not for Skeletonema sp., indicating that marked differences exist between the sedimentation tendencies of these genera. The observed inter-generic differences are explicable by the different overall sinking rates, as well as different nutrient-sensitivities of the sinking rates of each genus. Thalassiosira spp., the fastestsinking and most nutrient-sensitive species, contributed up to 10 x more carbon to the benthos in all years of the study, reaching a maximum of 11.1 gCm^-2 over a single spring bloom event in 1988. This study indicates that the tendency to sink to the benthos during and/or after a bloom is highly dependent on species-specific cell physiology, and supports the idea that it is the fast-sinking, nutrient-sensitive diatoms, such as Thalassiosira species, that constitute the major source of vertical carbon flux in this embayment and other such coastal ecosystems during the spring bloom.     

Phytoplankton sinking rates in oligotrophic waters off Hawaii,USA

The sinking rates in two size fractions of natural phytoplankton were measured over much of the photic zone in a subtropical environment. At 24, 40 and 71 m, sinking rates (± SD) of the 3 to 20 m fractions were 0.72±0.05, 0.83±0.05, and 0.34±0.04 m · d^-1, respectively, and rates for the 20 to 102 m fraction were 1.50±0.21, 1.65±0.14, and 0.95±0.22 m · d^-1, respectively. At all depths sampled, the 20 to 102 m size fraction was observed to sink significantly (P<0.01) faster than the 3 to 20 m fraction. considering vertical variability, both size fractions were observed to have significantly (P<0.01) lower sinking rates at 71 m than at more shallow depths. The finding of lower sinking rates in the region which lies just above the subsurface chlorophyll maximum provides empirical support for the hypothesis that variations in phytoplankton buoyancy may be related to the maintenance of this feature which is typical in oceanic environments. Analysis of sinking rate traces describes the distributions of specific sinking velocities for each size fraction and their variation.Oceanic Institute Contribution No. 162     

Host feeding and nutrient sufficiency for zooxanthellae in the sea anemone Aiptasia pallida

Nutrient sufficiency of zooxanthellae in the sea anemone Aiptasia pallida cultured in low nutrient seawater depends on the availability of particulate food to the host. Zooxanthellae in anemones unfed for 20 to 30 d exhibited the following characteristics of nutrient deficiency: cell division rates decreased; chlorophyll a content gradually decreased from 2 to <1 pg cell^–1; and C:N ratios increased from 7.5 to 16. Over a 3-mo period, algal populations in unfed anemones gradually decreased, indicating that zooxanthellae were lost faster than they were replaced by division. The mitotic index of zooxanthellae in unfed anemones was stimulated either by feeding the host or by the addition of inorganic N and P to the medium. Whether algae are nutrient-limited in hosts under field conditions has not been examined fully; however, C:N ratios in zooxanthellae from field-collected hosts are slightly higher (9.4 vs 7.5) than in hosts fed to repletion in laboratory cultures. This observation might indicate N limitation in the field.