Effects of prey abundance and light intensity on the mixotrophic chrysophyte Poterioochromonas malhamensis from a mesotrophic lake

1. Previous studies of mixotrophy in the flagellate Poterioochromonas malhamensis (Chrysophyceae) were performed on strains that had been in culture for > 30 years. This study aims to compare mixotrophy in a cultured strain with one recently isolated from a mesotrophic lake (Lacawac) in Pennsylvania, U.S.A. 2. P. malhamensis from the lake exhibited a nutritional flexibility similar to that of the culture strain, growing phototrophically but inefficiently in comparison to other nutritional modes (growth rate (μ) = 0.015 h^?1). Supplementing an inorganic salts medium with 1 mM glucose resulted in a doubling of μ to 0.035 h^?1 and 0.033 h^?1 in the light and the dark, respectively. Addition of an algal prey, Nannochloris, to the inorganic salts medium increased growth to rates similar to those observed with glucose. Maximum growth of the lake strain, 0.095 h^?1, was achieved when bacteria was supplied as food. During growth on bacteria, cellular chlorophyll a (Chl a) decreased from 140 fg cell^?1 to 10 fg cell^?1 over 22 h when cultured either in the light or dark. In illuminated cultures, cell-specific Chl a concentration recovered to 185 fg cell^?1 after bacteria became limiting. 3. In contrast to the cultured strain, however, the lake isolate exhibited an inverse relationship between light intensity and ingestion rate. Calculated grazing rates, based upon the ingestion of fluorescently labeled bacteria, were 3.2, 5.2 and 9.4 bacteria flagellate^?1 h^?1, for P. malhamensis incubated in high light, low light and darkness, respectively. Phagotrophy is thus influenced by a light regime in this predominately heterotrophic mixotroph.     

Relationship between phototrophy and phagotrophy in the mixotrophic chrysophytePoterioochromonas malhamensis

The time scales involved in the transition between phototrophic and phagotrophic modes of nutrition were examined in the mixotrophic chrysophytePoterioochromonas malhamensis. Phagotrophy began almost immediately when bacteria were added to phototrophically growing cultures of the alga, and chlorophylla concentration per cell in these cultures decreased over a 24-hour period. Chlorophyll concentrations per cell began to increase when bacteria were grazed to a density of approximately 10⁶ ml^–1, but after more than 24 hours they had not returned to the higher chlorophyll concentrations observed in the phototrophically grown cultures. Bacterivory was the dominant mode of nutrition in all cultures containing heat-killed bacteria. Photosynthesis did not contribute more than 7% of the total carbon budget of the alga when in the presence of abundant heat-killed bacteria. Bacterial density was the primary factor influencing the ability ofP. malhamensis to feed phagotrophically, while light intensity, pH, and the presence of dissolved organic matter had no effect on phagotrophy. We conclude thatP. malhamensis is capable of phagotrophy at all times. In contrast, phototrophy is inducible in the light during starvation and is a long-term survival strategy for this mixotrophic alga (i.e., it operates on time scales greater than a diel cycle).     

Environment-dependent metabolic investments in the mixotrophic chrysophyte Ochromonas

Mixotrophic protists combine photosynthesis and phagotrophy to obtain energy and nutrients. Because mixotrophs can act as either primary producers or consumers, they have a complex role in marine food webs and biogeochemical cycles. Many mixotrophs are also phenotypically plastic and can adjust their metabolic investments in response to resource availability. Thus, a single species's ecological role may vary with environmental conditions. Here, we quantified how light and food availability impacted the growth rates, energy acquisition rates, and metabolic investment strategies of eight strains of the mixotrophic chrysophyte, Ochromonas. All eight Ochromonas strains photoacclimated by decreasing chlorophyll content as light intensity increased. Some strains were obligate phototrophs that required light for growth, while other strains showed stronger metabolic responses to prey availability. When prey availability was high, all eight strains exhibited accelerated growth rates and decreased their investments in both photosynthesis and phagotrophy. Photosynthesis and phagotrophy generally produced additive benefits: In low-prey environments, Ochromonas growth rates increased to maximum, light-saturated rates with increasing light but increased further with the addition of abundant bacterial prey. The additive benefits observed between photosynthesis and phagotrophy in Ochromonas suggest that the two metabolic modes provide nonsubstitutable resources, which may explain why a tradeoff between phagotrophic and phototrophic investments emerged in some but not all strains.     

Effect of light intensity and eye development on prey capture by larval striped bass Morone saxatilis

The efficacy of visual and non-visual feeding among pelagic striped bass Morone saxatilis larvae adapted to a turbid estuary was determined in the laboratory in clear water. Capture of Artemia salina (density 100 l⁻¹) was significantly affected by the interaction between age of larvae (range: 8–25 days post-hatch, dph) and light intensity (range: 0–10·6 μmol s⁻¹ m⁻² at the water surface). Visual feeding by larvae aged 9–11 dph was highest in dim light (0·086–0·79 μmol s⁻¹ m⁻²), with fish capturing up to 5 prey larva⁻¹ h⁻¹. As the larvae grew, prey capture in brighter light improved, associated with an increasing proportion of twin cone photoreceptors and improving ability of the retina to light- and dark-adapt. By age >22 dph, mean prey capture was greatest at highest light intensities (0·79 and 10·6 μmol s⁻¹ m⁻²) exceeding 100 prey larva⁻¹ h⁻¹. Incidence of feeding larvae generally improved as the larvae grew, reaching >80% in all light intensities from 16 dph onwards. The lower threshold for visual feeding, between 0·0084 and 0·03 μmol s⁻¹ m⁻², remained constant as the larvae grew, despite an increasing density of rod photoreceptors. Below this threshold, non-visual feeding was evident at a low rate (<6 prey larva⁻¹ h⁻¹) that was independent of larval age.     

INTENSE GRAZING AND PREY‐DEPENDENT GROWTH OF PFIESTERIA PISCICIDA (DINOPHYCEAE)1

Grazing and growth of Pfiesteria piscicida (Pfiest) were investigated using batch and cyclostat cultures with Rhodomonas sp. (Rhod) as prey. Observed maximum growth rates (1.4 d^?1) and population densities (2 × 10⁵ cells·mL^?1) corresponded to values predicted by Monod functions (1.76 d^?1; 1.4 × 10⁵ cells·mL^?1). In batch cultures under a range of prey‐to‐predator ratios (0.1:1 to 180:1) and prey concentrations (1000–71,000 cells·mL^?1), Rhodomonas sp. was always depleted rapidly and P. piscicida concentrations increased briefly. The rate of Rhodomonas sp. depletion and the magnitude of P. piscicida population maxima depended on the prey‐to‐predator ratio and prey concentration. Starvation resulted in cell cycle arrest at G1 and G2+M and ultimately the demise of both P. piscicida and Rhodomonas sp. populations, demonstrating the dependence of P. piscicida on the supply of appropriate prey. The depletion of Rhodomonas sp. populations could be attributed directly to grazing, because P. piscicida did not exert detectable inhibitory effects on the growth of Rhodomonas sp. but grazed intensely, with maximum grazing rates>10 Rhod·Pfiest^?1·d^?1 and with no apparent threshold prey abundance for grazing. The results suggest that 1) the abundance of appropriate prey may be an important factor regulating P. piscicida abundance in nature, 2) P. piscicida may control prey population, and 3) high growth and grazing potentials of P. piscicida along with cell cycle arrest may confer survival advantages.     

Inducible Mixotrophy in the Dinoflagellate Prorocentrum minimum

Prorocentrum minimum is a neritic dinoflagellate that forms seasonal blooms and red tides in estuarine ecosystems. While known to be mixotrophic, previous attempts to document feeding on algal prey have yielded low grazing rates. In this study, growth and ingestion rates of P. minimum were measured as a function of nitrogen (‐N) and phosphorous (‐P) starvation. A P. minimum isolate from Chesapeake Bay was found to ingest cryptophyte prey when in stationary phase and when starved of N or P. Prorocentrum minimum ingested two strains of Teleaulax amphioxeia at higher rates than six other cryptophyte species. In all cases ‐P treatments resulted in the highest grazing. Ingestion rates of ‐P cells on T. amphioxeia saturated at ~5 prey per predator per day, while ingestion by ‐N cells saturated at 1 prey per predator per day. In the presence of prey, ‐P treated cells reached a maximum mixotrophic growth rate (μ_max) of 0.5 d^?1, while ‐N cells had a μ_max of 0.18 d^?1. Calculations of ingested C, N, and P due to feeding on T. amphioxeia revealed that phagotrophy can be an important source of all three elements. While P. minimum is a proficient phototroph, inducible phagotrophy is an important nutritional source for this dinoflagellate.     

VITAMIN B12-BINDER AND OTHER ALGAL INHIBITORS1

Euglena gracilis Klebs, Poterioochromonas malhamensis (Pringsheim) Peterfi, Monochrysis lutheri Droop, Isochrysis galbana Parke and Phaeodactylum tricornutum Bohlin are known to release into the medium a substance which binds free vitamin B₁₂. The binder, apparently a glycoprotein, makes vitamin B₁₂ unavailable and inhibits growth of vitamin B₁₂-requiring microorganisms. Culture filtrates of selected marine diatoms, chrysophytes, cryptophytes, dinoflagellates and green algae contained the binder, indicating that binder release was not restricted to any algal group. However the three prokaryotic bluegreens tested do not produce B₁₂-binder. Production is also independent of the nutritional requirements of the donor, being produced by autotrophs and auxotrophs. The binders produced by these marine species have similar properties: they are heat labile; inhibition is not species-specific; it is competitive, being reversed by adding B₁₂. Production increases with density of the culture and is not restricted to stationary or scenescent cells. The marine species tested produced much less B₁₂ than the freshwater Euglena and Poterioochromonas; the inhibition is reversed by 20–50 ng · 1^?1 of B₁₂ for marine species which reach moderate densities and by 150–300 ng · 1^?1 B₁₂ for the densely growing species. Consequently the binder may affect the growth of B₁₂-requiring species only in environments like the open ocean poor in B₁₂. By contrast, the marine algae produce other inhibitors which are often heat stable and very inhibitory. These inhibitors, being species-specific and less labile, may be more important in affecting the succession of algal species in natural waters.     

Direct uptake of nitrogen by Pfiesteria piscicida and Pfiesteria shumwayae, and nitrogen nutritional preferences

The rates of uptake of a range of forms of nitrogenous nutrients were measured in cultures of Pfiesteria piscicida and Pfiesteria shumwayae maintained at varying physiological states. The measured rates of dissolved N uptake under some conditions approached the rates of N uptake that are achieved through phagotrophy. Rates of dissolved N uptake by P. piscicida contributed <10% of the cellular N of flagellated cells feeding on algae, but were equal to or greater than phagotrophic N acquisition in cells recently removed from fish cultures. Specific N uptake rates (V, h⁻¹) were higher for cells that were maintained on algal prey for long periods (months) than those that were grown with live fish. However, rates of N uptake on a cellular basis for cells grown on or recently removed from fish were comparable to those maintained on algal prey, likely reflecting differences in the sizes of cells of different physiological condition. Preferences for form of N generally followed a decreasing trend of amino acids > urea > NH₄⁺ > NO₃⁻. Nitrate consistently was not a preferred form of N. Although Pfiesteria spp. are often found in eutrophic environments, the relationship between Pfiesteria spp. and nutrient availability is likely to be primarily indirect, mediated through the production of various prey on which Pfiesteria spp. feed. These findings also confirm, however, that when dissolved N concentrations are elevated, they can contribute to the supplemental nutrition of these cells, and thus may provide a significant source of N to Pfiesteria spp. in nature.     

Grazing impact on the cyanobacterium Microcystis aeruginosa by the heterotrophic flagellate Collodictyon triciliatum in an experimental pond

We estimated the grazing impact of the heterotrophic flagellate Collodictyon triciliatum on the harmful, bloom-forming cyanobacterium Microcystis aeruginosa in an experimental pond during a Microcystis bloom from summer to winter in 2010. For these experiments, we calculated the grazing rates from the digestion rate of C. triciliatum and its food vacuole contents. During the study period, M. aeruginosa exhibited one bloom event with a maximum density of 1.1 × 10⁵ cells ml^?1. The cell density of C. triciliatum fluctuated from below the detection limit to 291 cells ml^?1. The number of M. aeruginosa cells ingested by C. triciliatum food vacuoles ranged between 0.4 and 10.8 cells flagellate^?1, and the digestion rate of C. triciliatum at 25 °C was 0.73 % cell contents min^?1. The grazing rate of C. triciliatum on the M. aeruginosa prey was 0.2–6.9 cells flagellate^?1 h^?1, and its grazing impact was 0.0–25.3 % standing stock day^?1. The functional response of C. triciliatum to the M. aeruginosa prey followed the Michaelis–Menten model of significance (r ² = 0.873, p < 0.001) in our experimental systems, in which the prey concentration varied from 1.0 × 10⁴ to 2.1 × 10⁶ cells ml^?1. The maximum grazing rate was 6.2 prey cells grazer^?1 h^?1, and the half-saturation constant was 1.2 × 10⁵ cells ml^?1. We present evidence that C. triciliatum grazing explained the remarkable decrease in M. aeruginosa cell density in the pond. The present study is the first demonstration of the high potential of protistan grazing on M. aeruginosa to reduce cyanobacterial blooms.     

KINETICS OF GROWTH AND DEATH IN ANABAENA FLOS-AQUAE (CYANOBACTERIA) UNDER LIGHT LIMITATION AND SUPERSATURATION

The kinetics of population growth and death were investigated in Anabaena flos-aquae (Lyngb.) Bréb grown at light intensities ranging from limitation to photoinhibition (5 W·m⁻² to 160 W·m⁻²) in a nutrient-replete turbidostat. Steady-state growth rate (μ, or dilution rate, D) increased with light intensity from 0.44·day⁻¹ at a light intensity of 5 W·m⁻² to 0.99·day⁻¹ at 20 W·m⁻² and started to decrease above about 22 W·m⁻², reaching 0.56·day⁻¹ at 160 W·m⁻². The Haldane function of enzyme inhibition fit the growth data poorly, largely because of the unusually narrow range of saturation intensity. However, it produced a good fit (P < 0.001) for growth under photoinhibition. Anabaena flos-aquae died at different specific death rates (γ) below and above the saturation intensity. When calculated as the slope of a v_x⁻¹ and D⁻¹ plot, where v_x and D are cell viability (or live cell fraction) and dilution rate, respectively; γ was 0.047·day⁻¹ in the range of light limitation and 0.103·day⁻¹ under photoinhibition. Live vegetative cells and heterocysts, either in numbers or as a percentage of the total cells, showed a peak at the saturation intensity and decreased at lower and higher intensities. The ratio of live heterocysts to live vegetative cells increased with intensity when light was limiting but decreased when light was supersaturating. In cells growing at the same growth rate, the ratio was significantly lower under light inhibition than under subsaturation and the cell N:C ratio was also lower under inhibition. The steady-state rate of dissolved organic carbon (DOC) production increased with light intensity. However, its production as a percentage of the total C fixation was lowest at the optimum intensity and increased as the irradiance decreased or increased. The rate and percentage was significantly higher under photoinhibition than limitation in cells growing at the same growth rate. About 22% of the total fixed carbon was released as DOC at the highest light intensity. No correlation was found between the number of dead cells and DOC.     

EFFECTS OF LIGHT ON PHOTOSYNTHESIS,GRAZING, AND POPULATION DYNAMICS OF THE HETEROTROPHIC DINOFLAGELLATE PFIESTERIA PISCICIDA (DINOPHYCEAE)1

In studying how environmental factors control the population dynamics of Pfiesteria piscicida Steidinger et Burkholder, we examined the influence of light regime on kleptoplastidic photosynthesis, growth, and grazing. Prey (Rhodomonas sp.)‐saturated growth rate of P. piscicida increased (0.67 ± 0.03 d^?1 to 0.91 ± 0.11 d^?1) with light intensity varying from 0 to 200 μmol photons·m^?2·s^?1. No significant effect was observed on grazing, excluding the possibility that light enhanced P. piscicida growth through stimulating grazing. Light‐grown P. piscicida exhibited a higher gross growth efficiency (0.78 ± 0.10) than P. piscicida incubated in the dark (0.32 ± 0.16), and photosynthetic inhibitors significantly decreased growth of recently fed populations. These results demonstrate a role of kleptoplastidic photosynthesis in enhancing growth in P. piscicida. However, when the prey alga R. sp. was depleted, light's stimulating effect on P. piscicida growth diminished quickly, coinciding with rapid disappearance of Rhodomonas‐derived pigments and RUBISCO from P. piscicida cells. Furthermore, the effect of light on growth was reversed after extended starvation, and starved light‐grown P. piscicida declined at a rate significantly greater than dark‐incubated cultures. The observed difference in rates of decline appeared to be attributable to light‐dependent cannibalism. Using a 5‐chloromethylfluorescein diacetate staining technique, cannibalistic grazing was observed after 7 days of starvation, at a rate four times greater under illumination than in the dark. The results from this study suggest that kleptoplastidy enhances growth of P. piscicida only in the presence of algal prey. When prey is absent, P. piscicida populations may become vulnerable to light‐stimulated cannibalism.     

EXTREMELY HIGH GROWTH RATE OF THE SMALL DIATOM CHAETOCEROS SALSUGINEUM ISOLATED FROM AN ESTUARY IN THE EASTERN SETO INLAND SEA,JAPAN1

Small single‐celled Chaetoceros sp. are often widely distributed, but frequently overlooked. An estuarine diatom with an extremely high growth potential under optimal conditions was isolated from the Shinkawa‐Kasugagawa estuary in the eastern part of the Seto Inland Sea, western Japan. It was identified as Chaetoceros salsugineum based on morphological observations. This strain had a specific growth rate of 0.54 h^?1 at 30°C under 700 μmol · m^?2 · s^?1 (about 30% of natural maximal summer light) with a 14:10 L:D cycle; there was little growth in the dark. However, under continuous light it grew at only 0.35 h^?1 or a daily specific growth rate of 8.4 d^?1. In addition, cell density, chlorophyll a, and particulate organic carbon concentrations increased by about 1000 times in 24 h at 30°C under 700 μmol · m^?2 · s^?1 with a 14:10 L:D cycle, showing a growth rate of close to 7 d^?1. This very rapid growth rate may be the result of adaptation to this estuarine environment with high light and temperature. Thus, C. salsugineum can be an important primary producer in this estuary in summer and also an important organism for further physiological and genetic research.     

PRODUCTION AND NUTRIENT REMOVAL BY PERIPHYTON GROWN UNDER DIFFERENT LOADING RATES OF ANAEROBICALLY DIGESTED FLUSHED DAIRY MANURE1

Growing algae to scrub nutrients from manure presents an alternative to the current practice of land application and provides utilizable algal biomass as an end product. The objective of this study was to assess algal growth, nutrient removal, and nitrification using higher light intensities and manure loading rates than in the previous experiments. Algal turfs, with periphyton mainly composed of green algal species, were grown under two light regimes (270 and 390 μmol photons·m^?2· s^?1) and anaerobically digested flushed dairy manure wastewater (ADFDMW) loading rates ranging from 0.8 to 3.7 g total N and 0.12 to 0.58 g total P·m^?2·d^?1. Filamentous cyanobacteria (Oscillatoria spp.) and diatoms (Navicula, Nitzschia, and Cyclotella sp.) partially replaced the filamentous green algae at relatively high ADFDMW loading rates and more prominently under low incident light. Mean algal production increased with loading rate and irradiance from 7.6±2.71 to 19.1±2.73 g dry weight· m^?2·d^?1. The N and P content of algal biomass generally increased with loading rate and ranged from 2.9%–7.3% and 0.5%–1.3% (by weight), respectively. Carbon content remained relatively constant at all loading rates (42%–47%). The maximum removal rates of N and P per unit algal biomass were 70 and 13 mg·g^?1 dry weight·m^?2·d^?1, respectively. Recovery of nutrients in harvested algal biomass accounted for about 31%–52% for N and 30%–59% for P. Recovery of P appeared to be uncoupled with N at higher loading rates, suggesting that algal potential for accumulation of P may have already been saturated. It appears that higher irradiance level enhancing algal growth was the overriding factor in controlling nitrification in the algal turf scrubber units.     

Interrelations of photosynthetic behaviour and buoyancy regulation in a natural population of a blue-green alga

The photosynthetic activity of Anabaena cirdnalis and associated changes in buoyancy were determined from prepared suspensions exposed in the natural light field of Crose Mere. The observations are related to variations in subsurface irradiance and temperature. Parallel experiments, aimed at trapping algal colonies undertaking controlled vertical movements within the lake system, are also described. Buoyancy loss and downward migration are clearly associated with specific photosynthetic rates: rates as low as 1.8 mg O₂ (mg chlorophyll a) h⁻¹ are shown to be sufficient to effect buoyancy loss, while movements in the lake tend towards a depth where rates of 5–7 mg O₂ (mg chlorophyll a)⁻¹ h⁻¹ are possible. These rates are significantly less than those possible at light saturation. The effect of increasing temperature is to depress the population in the light-gradient. The significance of this response is discussed in relation to the growth of natural populations of blue-green algae.     

LIGHT AND ELECTRON MICROSCOPY OF GRAZING BY POTERIOOCHROMONAS MALHAMENSIS (CHRYSOPHYCEAE) ON A RANGE OF PHYTOPLANKTON TAXA1

Grazing of fluorescent latex beads, bacteria, and various species of phytoplankton by Poterioochromonas malhamensis (Pringsheim) Peterfi (about 8.0 μm in diameter) was surveyed. The alga ingested fluorescent beads and various live or killed and nomnotile or motile organisms including bacteria, blue-green algae, green algae, diatoms, and chrysomonads. The size range of grazed prey was from 0.1 to 6.0 μm for latex beads and from 1.0 μm (bacteria) to about 21 μm (Carteria inverse) for organisms. As many as 17 latex beads (2.0 μm) or more than 10 Microcystis cells (5–6 μm) were ingested by a single P. malhamensis cell. Following such grazing, the cell increased in volume by up to about 30-fold. The range of cell volume of ingested prey was from 0.52 μm³ (bacteria) to about 3178 μm³(Carteria inversa). This study demonstrates for the first time that P. malhamensis is capable of grazing algae 2–3 times larger in diameter than its own cell and of grazing intact motile algae. Poterioochromonas malhamensis is an omnivorous grazer. Food vacuole formation and digestion processes were examined. The membrane that was derived from the plasma membrane and surrounded the prey disappeared sometime after ingestion. The food vacuole was then formed by successive fusion of numerous homogeneous vesicles accumulated around the prey. The prey was enclosed in a single membrane-bound food vacuole and then digested.     

Combined Effects of Temperature,Irradiance, and pH on Teleaulax amphioxeia (Cryptophyceae) Physiology and Feeding Ratio For Its Predator Mesodinium rubrum (Ciliophora)1

The cryptophyte Teleaulax amphioxeia is a source of plastids for the ciliate Mesodinium rubrum and both organisms are members of the trophic chain of several species of Dinophysis. It is important to better understand the ecology of organisms at the first trophic levels before assessing the impact of principal factors of global change on Dinophysis spp. Therefore, combined effects of temperature, irradiance, and pH on growth rate, photosynthetic activity, and pigment content of a temperate strain of T. amphioxeia were studied using a full factorial design (central composite design 2³*) in 17 individually controlled bioreactors. The derived model predicted an optimal growth rate of T. amphioxeia at a light intensity of 400 μmol photons · m⁻² · s⁻¹, more acidic pH (7.6) than the current average and a temperature of 17.6°C. An interaction between temperature and irradiance on growth was also found, while pH did not have any significant effect. Subsequently, to investigate potential impacts of prey quality and quantity on the physiology of the predator, M. rubrum was fed two separate prey: predator ratios with cultures of T. amphioxeia previously acclimated at two different light intensities (100 and 400 μmol photons · m⁻² s⁻¹). M. rubrum growth appeared to be significantly dependent on prey quantity while effect of prey quality was not observed. This multi-parametric study indicated a high potential for a significant increase of T. amphioxeia in future climate conditions but to what extent this would lead to increased occurrences of Mesodinium spp. and Dinophysis spp. should be further investigated.     

Light-dependent phagotrophy in the freshwater mixotrophic chrysophyte Dinobryon cylindricum

The mixotrophic (bacterivorous), freshwater chrysophyte Dinobryon cylindricum was cultured under a variety of light regimes and in bacterized and axenic cultures to investigate the role of phototrophy and phagotrophy for the growth of this alga. D. cylindricum was found to be an obligate phototroph. The alga was unable to survive in continuous darkness even when cultures were supplemented with high concentrations of bacteria, and bacterivory ceased in cultures placed in the dark for a period longer than one day. Axenic growth of the alga was poor even in an optimal light regime. Live bacteria were required for sustained, vigorous growth of the alga in the light. Carbon (C), nitrogen (N), and phosphorus (P) budgets determined for the alga during growth in bacterized cultures indicated that bacterial biomass ingested by the alga may have contributed up to 25% of the organic carbon budget of the alga. Photosynthesis was the source of most (75%) of the organic carbon of the alga. D. cylindricum populations survived but did not grow when cultured in a continuous low light intensity (30 E m^–2 sec^–1), or in a light intensity of 150 E m^–2 sec^–1 for only two hours each day. Net efficiency of incorporation of bacterial C, N, and P into algal biomass under these two conditions was zero (i.e., no net algal population growth). We conclude that the primary function of bacterivorous behavior in D. cylindricum may be to provide essential growth factor(s) or major nutrients for photosynthetic growth, or to allow for the survival of individuals during periods of very low light intensity or short photoperiod. Offprint requests to: David A. Caron     

DIFFERENT RESPONSE MECHANISMS OF TWO PLANKTONIC DESMID SPECIES (CHLOROPHYCEAE) TO A SINGLE SATURATING ADDITION OF PHOSPHATE

Two planktonic algal species, Staurastrum chaetoceras (Schr.) G. M. Smith and Cosmarium abbreviatum Rac. var. planctonicum W. et G. S. West, from trophically different alkaline lakes, were compared in their response to a single saturating addition of phosphate (P) in a P-limited growth situation. Storage abilities were determined using the luxury coefficient R = Q_max/Q₀. Maximum cellular P quotas differed, depending on whether cells were harvested during exponential growth at μ_max (Q_max, R being 26.7 and 9.1 for C. abbreviatum and S. chaetoceras, respectively) or harvested after a saturating pulse at P-limited growth conditions (Q′_max, R being 53.5 and 20.2 for C. abbreviatum and S. chaetoceras, respectively). At stringent P-limited conditions, maximum initial uptake rates were higher in S. chaetoceras than in C. abbreviatum (0.094 and 0.073 pmol P·cell^?1·h^?1, respectively), but long-term (net) uptake rates (over ～20 min) were higher in C. abbreviatum than in S. chaetoceras (0.048 and 0.019 pmol P·cell^?1·h^?1, respectively). Before growth resumed after the onset of a large P addition (150 μmol·L^?1), a lag phase was observed for both species. This period lasted 2–3 days for S. chaetoceras and 3–4 days for C. abbreviatum, corresponding with the time to reach Q^′_max. Subsequent growth rates (over ～10 days) were 0.010 h^?1 and 0.006 h^?1 for S. chaetoceras and C. abbreviatum, respectively, being only 20%–30% of maximum growth rates. In conclusion, S. chaetoceras, with a relatively high initial P-uptake rate, short lag phase, and high initial growth rate, is well adapted to a P pulse of short duration. Conversely, C. abbreviatum, with a high long-term uptake rate and high storage capacity, appears competitively superior when exposed to an infrequent but lasting pulse. These characteristics provide information about possible strategies of algal species to profit from temporarily high P concentrations.     

Glutamine synthetase activity and free amino acid pools of eelgrass (Zostera marina L.) roots

Activity of the enzyme glutamine synthetase (GS, EC 6.3.1.2) was determined in vitro for roots of the marine angiosperm Zostera marina L. (eelgrass) collected from a population in Great Harbor, Woods Hole, Massachusetts, U.S.A. The GS synthetase activity was lowest in roots of plants collected from the shallow region of the eelgrass bed (12.0 μmol·g⁻¹ (fresh wt)· h⁻¹) and increased in the mid (3.0 m, 40.3 μmol·g⁻¹ (fresh wt)·h⁻¹) and deep (5.0 m, 72.3 μmol·g⁻¹ (fresh wt)·h⁻¹) plant collection depths. GS transferase activity increased with collection depth in a similar manner: shallow, 28.6 μmol·g⁻¹ (fresh wt)·h⁻¹; mid, 52.0 μmol·g⁻¹ (fresh wt)·h⁻¹; deep, 92.8 μmol·g⁻¹ (fresh wt)·h⁻¹. When sediment-embedded plants were held in continuous darkness for 2 days to create extended root anoxia, root GS activities nearly doubled. In contrast, in vivo incorporation of ¹⁴C-glutamate into glutamine and protein residue remained constant or declined under short-term hypoxia and anoxia. During aerobic recovery from anoxia, root labelling of glutamine and protein increased markedly. Free amino acid patterns of eelgrass roots growing in situ were determined over a diurnal cycle. Total free amino acid content was maximal at dawn and decreased 50% by noon. In contrast, the proportion of glutamine was lowest at dawn and maximal at noon for both shallow and deep-growing plants. Despite differences in depth-specific plant sizes, root/rhizome/shoot ratios, and relative growth rates, the daily whole plant nitrogen demand of shallow and deep growing plants were equivalent. When corrected for assay temperature response, the enzyme synthetase activities measured in vitro suggest that all of the plant nitrogen assimilation requirements can be met within daylight hours during the period of peak summer biomass.