Minimum Cell Quota Research Articles

Cellular microcystin content in N-limited Microcystis aeruginosa can be predicted from growth rate.

Cell quotas of microcystin (Q(MCYST); femtomoles of MCYST per cell), protein, and chlorophyll a (Chl a), cell dry weight, and cell volume were measured over a range of growth rates in N-limited chemostat cultures of the toxic cyanobacterium Microcystis aeruginosa MASH 01-A19. There was a positive linear relationship between Q(MCYST) and specific growth rate (mu), from which we propose a generalized model that enables Q(MCYST) at any nutrient-limited growth rate to be predicted based on a single batch culture experiment. The model predicts Q(MCYST) from mu, mu(max) (maximum specific growth rate), Q(MCYSTmax) (maximum cell quota), and Q(MCYSTmin) (minimum cell quota). Under the conditions examined in this study, we predict a Q(MCYSTmax) of 0.129 fmol cell(-1) at mu(max) and a Q(MCYSTmin) of 0.050 fmol cell(-1) at mu = 0. Net MCYST production rate (R(MCYST)) asymptotes to zero at mu = 0 and reaches a maximum of 0.155 fmol cell(-1) day(-1) at mu(max). MCYST/dry weight ratio (milligrams per gram [dry weight]) increased linearly with mu, whereas the MCYST/protein ratio reached a maximum at intermediate mu. In contrast, the MCYST/Chl a ratio remained constant. Cell volume correlated negatively with mu, leading to an increase in intracellular MCYST concentration at high mu. Taken together, our results show that fast-growing cells of N-limited M. aeruginosa are smaller, are of lower mass, and have a higher intracellular MCYST quota and concentration than slow-growing cells. The data also highlight the importance of determining cell MCYST quotas, as potentially confusing interpretations can arise from determining MCYST content as a ratio to other cell components.

Modelling Si-N-limited growth of diatoms

A mechanistic model for silicon (Si) physiology is developed, interfaced with a model of nitrogen IN) physiology, which is capable of simulating the major documented facets of Si-N physiology in diatoms. The model contains a cell cycle component that is involved in regulating the timing ol the synthesis of valves, girdles and setae. In addition to reproducing the timing of diatom cell division within a light- dark cycle, the model simulates the following features seen in real diatoms. Synthesis of valves only occurs during G 2 interphase and M, while the girdles and (if appropriate) setae are synthesized during G 1 Si stress alone results in a loss of setae, followed by a thinning of the valves in successive generations until a minimum Si cell quota is attained. After this point, the duration of G 2 increases and growth is Si limited. Concurrently, the carbon (C) cell quota increases, offering the capability to simulate the documented increase in sinking rates with Si stress. N stress alone results in an increase in the duration of G 1 and G 2 interphases, and high Si cell quotas, From this complex model which must be run for arrays of subpopulations to simulate non-synchronous growth, a simpler model is developed. This is capable of reproducing similar growth dynamics, although with no reference to component parts of the frustule. When allied to a photoacclimative submodel, a prediction of the model is that diatoms starved of Si will release increased amounts of dissolved organic C because cell growth is halted more rapidly than the photosystems can be degraded.

Effect of nitrogen and phosphorus on the growth of a red tide dinoflagellateScrippsiella trochoidea (Stein) Loeblich III

Scrippsiella trochoidea (Stein) Loeblich III was grown in a nitrogen-or phosphorus-limited batch culture system in laboratory. Growth rates and cellular Chl-a were measured as functions of nitrate and phosphate concentrations. Growth rates were hyperbolic with both nitrate and phosphate concentration and fit the Monod equation. The minimum cell quota of nitrogen and phosphorus and then the optimum N∶P ratio ofS. trochoidea were estimated in this study. Measurement of phosphate concentration in Jiaozhou Bay suggest that phosphorus is the limiting factor ofS. trochoidea growth.

Growth and phosphate uptake kinetics of the toxic dinoflagellate Alexandrium tamarense from Hiroshima Bay in the Seto Inland Sea, Japan

SUMMARY Shellfish poisoning by the toxic dinoflagellate Alexandrium tamarense (Lebour) Balech occurred for the first time in Hiroshima Bay, Japan, in 1992. Oyster culture in the bay produces as much as 60% of the total production in Japan, and it suffered severe damage. In the present study, we experimentally investigated the growth rate and phosphate uptake kinetics of A. tamarense, Hiroshima Bay strain. A short-term phosphate uptake experiment revealed that the maximum uptake rate was 1.4 pmol P cell-1 per h and the half-saturation constant was 2.6 umol L-1. In semicontin-uous culture, the maximum specific growth rate and the minimum phosphorus cell quota were 0.54 day-1 and 0.56 pmol P cell-1, respectively. These uptake rates suggest that A. tamarense is a poor phosphorus competitor compared with other species. However, the large phosphorus storage capacity (Qpmax/qo= 36), the surge phosphorus uptake ability (Vs/Vi= 4.1) and the low growth rate would be advantageous for surviving brief periods of phosphorus limitation which frequently occur in Hiroshima Bay.

Ｎ：Ｐ比をパラメータとした連続混合培養系における藍藻類の種間競争の数理モデルによる解析

Competition of blue-green algae Microcystis aeruginosa and Phormidium tenue in chemostat was simulated to determine what was the cause of species shift among them. The model consists of 8 equations including Droop's equation, growth of algae depends on internal nutrient content. Simulation of growth of M. aeruginosa and P. tenue at N:P supply ratio 5 to 40 almost agreed with experimental data. It was suggested that outcome of competition in chemostat could be predicted by the simulation model. Differences in kinetic constants such as Qmin (minimum cell quota) and ρmax(maximal nutrient uptake rate) were assumed to be causes of change in weight ratio of each algae with increase in N:P supply ratio in chemostat, suggesting species shift among blue-green algae in lakes occurs due to differences in kinetic constants for growth and nutrient uptake. At N:P supply ratio <60 (P was fixed), total dry weight of M. aeruginosa and P. tenue in competition calculated with the model increased with increase in N:P supply ratio.

The effects of nitrogen, phosphorus, vitamins and trace metals on the growth of the red tide organismProrocentrum Micans

Laboratory culture experiments showed that 100μmol/L of ammonium, or mixture of glycine and glutamate was harmful to growth, and that orthophosphate wasP. micans’ main phosphorous source in the ocean. Presence of 80μ mol/L EDTA, 0.5 to 1 μmol/L Fe3+, 1.0 to 20.0 μ mol/L Mn2+ 0.1 to 0.4 μmol/L Co2+ in the culture medium could improve the growth ofP. micans. Vitamin B1 promoted growth, but vitamin B12 and biotin did not. The estimated minimum cell quotas (q o) for nitrogen and phosphorus being 0.74 pmole/cell and 0.045 pmole/cell show that phosphorus (more than nitrogen) limits the growth ofP. micans in the study area.

Growth Responses of &lt;i&gt;Spirulina Platensis&lt;/i&gt; to Some Physico-chemical Factors and the Kinetics of Phosphorus Utilization

phorus sources. Values on the different growth parameters for orthophosphate and other organic phos phorus sources were as followings: half-saturation constant of 0.02-0.07 mg-P/1; maximum growth rate of 0.8-1.0/d; minimum cell quota of 0.08-0.32 pg-P/cell, and level for saturated growth yield of 0.3-1.0 mg-P/1. The result that this species utilized effectively a rather wide range of both inorganic and organic phosphorus and showed a high growth rate suggests that mass production of this species is pos sible by recycling organic waste.

A comparative study of microalgae isolated from flooded rice paddies: light-limited growth, C fixation, growth efficiency and relative N and P requirement

Seven unialgal isolates from rice paddies,Ankistrodesmus convolutus, Chlorella sp.,Scenedesmus quadricauda, Frustulia vulgaris, Anabaena sp.,Microcystis aeruginosa andPhormidium mucicola, were investigated for their light-limited growth and photosynthetic characteristics, growth efficiency, and relative requirement for N and P as well as their storage potentials. The maximum growth rate (μmax) ranged from 1.27 to 2.72 d−1 among species. The slope of light-limited growth (αg) showed only small interspecies differences (0.126–0.204 d−1 W−1 m2) except inS. quadricauda (0.092 d−1 W−1 m2). Similarly, the ratio of μmax to αg, Ik(g), varied within a narrow range (8.7–13.9 W m−2) with the exception ofS. quadricauda (19.5 W m−2). The slope of the photosynthetic curve based on chlorophylla (chla), αp(a), was significantly higher in cyanobacteria (0.118–0.189 mg C mg chla −1h−1W−1m2) than in other species (0.070–0.094 mg C mg chla −1h−1W−1m2). The maximum photosynthetic rate based on chla (Pmax(a)) was similar (2.95–3.83 mg C mg chla −1 h−1) with the exception of a high value (6.17 mg C mg chla −1 h−1) inM. aeruginosa. A significant correlation (P<0.001) existed between αp(a) and Pmax(a). The C-specific maximum photosynthetic rate (Pmax(c)) was inversely correlated to the C/chla ratio. Growth efficiency (μeff), the efficiency to retain photosynthetically-fixed C for growth, showed an interspecies variation ranging from 73–93% and was correlated to the μmax:Pmax(c) ratio. Cyanobacteria exhibited higher efficiency than others. The ratio of Pmax:αp, Ik(p), was higher than Ik(g) by 2.0 to 3.8 times. The optimum N:P ratio, determined as the ratio of minimum cell quotas of N to P (qoN:qoP), showed a wide interspecies variability. It was highest inP. mucicola (54) and lowest inF. vulgaris (10). The maximum storage capacity for excess P and N also varied among species; the maximum capacity for P ranged from 2 to 14 times of its immediate need and the capacity for N varied from 1.2 to 4 times.

Effects of physico-chemical factors and nutrients on the growth of Spirulina platensis isolated from Lake Kojima, Japan.

The effects of physico-chemical factors and nutrients on the growth responses of Spirulina platensis isolated from Lake Kojima (K-2 strain) were determined. The optimum conditions for growth were the following: light intensity of 2, 500-10, 000 lx, pH 6.5-9.0, 30 to 35°C temperature and 0-7.6% of chlorinity. S. platensis (K-2 strain) requires no vitamins for growth essentially. However, growth enhancement was observed with vitamin B12 among the B-group vitamins tested. Likewise, hormones such as kinetin and giberellin exhibited stimulatory effect on growth. The alga could utilize inorganic and organic phosphorus sources. Values on the different growth parameters for orthophosphate, β-glycerophosphate, adenosine diphosphate, and phosphocreatine were comparatively similar; half-saturation constant, maximum growth rate, minimum cell quota and level for saturated growth yield were about 0.01-0.02mg P/l, 0.8/day, 0.1-0.2pg P/cell, and 0.4-0.7mg P/l, respectively.

淡水赤潮プランクトンＰｅｒｉｄｉｎｉｕｍ ｂｉｐｅｓ ｆｏ．ｏｃｃｕｌｔａｔｕｍの増殖に及ぼす照度，温度，ｐＨおよび窒素栄養塩の影響

The alga causing a fresh water red tide in the Ananai-gawa Dam-controlled Lake was identified to be Peridinium bipes fo. occulutatum. The effects of light intensity, temperature, pH, and nitrogen on the growth of this alga was examined. The growth yield was greatest at a light intensity of 10, 000lx, a temperature of 20°C and a pH of 7.3-8.0. The growth rate was highest at a light intensity of 10, 000 lx and was not affected so much at temperatures from 10 to 25°C or a pH range of 6.0-8.7 as tested. The alga can use NO2--, NO3--, NH4+-, and urea-N under their low concentrations as a sole source of nitrogen and cannot use three amino acids (glycine, alanine, and glutamic acid) effectively. The half-saturation constant for nitrogen was 50-65μgN/l and specific growth rate was saturated at 0.5-1.3mgN/l, but a higher concentration of nitrogen (5mgN/l) was needed for reaching maximum cell yield. The minimum cell quota of nitrogen was calculated to be 0.32ng N/cell. Under the saturated condition of nitrogen, the maximum specific growth rate was 0.17-0.21 d-1. Among freshwater phytoplankton this alga was found to have a rather higher Ks value of nitrogen for growth and to have a smaller growth rate.

TEMPERATURE EFFECTS ON SILICON‐ AND PHOSPHORUS‐LIMITED GROWTH AND COMPETITIVE INTERACTIONS AMONG THREE DIATOMS1

ABSTRACTThree diatom species, Stephanodiscus hantzschii (Ehr.) Grun., Asterionella formosa Hass. and Fragilaria crotonensis Kitt. Hass. were isolated from Lake Maarsseveen where they are dominant and show a successional sequence. The physiological responses of each species to temperature and limitation by silicon and phosphorus were determined over the temperature range of 5° to 20° C using short‐term batch culture methods. Stephanodiscus hantzschii had higher maximum growth rates than the other two species at all temperatures, and the maximum growth rates of all species increased with increasing temperature. Temperature affected not only maximum growth rates but also half‐saturation constants (Ks) and the minimum cell quotas. S. hantzschii had low silicon requirements for growth under Si‐limiting conditions, and A. formosa and F. crotonensis had higher and nearly identical silicon requirements. The Ks values for silicon for S. hantzschii were essentially constant from 5° to 20° C but varied greatly for the other two species. A. formosa had the lowest requirements for growth under phosphorus limitation, F. crotonensis was intermediate and S. hantzschii had the highest growth requirements for phosphorus. The K1 values for phosphorus were constant over the temperature range for both A. formosa and F. crotonensis and were much higher and variable for S. hantzschii.Nutrient competition experiments were performed in continuous cultures at four temperatures and various Si:P ratios. The results generally, but not always, confirmed the predictions based on the Monod relationships for each species. Results not in agreement with predictions were usually because of similar physiological properties of A. formosa and F. crotonensis or because of decreased loss rates for F. crotonensis due to wall growth. In cultures with all three species phosphorus‐limited (Si:P &gt; 75), A. formosa often dominated as predicted, although F. crotonensis was sometimes the most abundant species. As predicted, S. hantzschii never dominated at high Si:P ratios. At intermediate Si:P ratios when A. formosa and F. crotonensis were both Si‐limited and S. hantzschii P‐limited, all three species coexisted because A. formosa and F. crotonensis have almost identical silicon requirements, although sometimes F. crotonensis was more abundant than predicted. At 10°C the results agreed best with the predictions; A. formosa dominated at high Si:P ratios and S. hantzschii dominated as predicted at low Si:P ratios when all three species were Si‐limited.

中筋川下流域に発生する淡水赤潮の原因種Ｐｌａｇｉｏｓｅｌｍｉｓ ｓｐ．（８７株）の増殖生理特性

Freshwater red tides by Plagioselmis sp., a Cryptophyceae, frequently occur in the lower part of the Nakasuji River during the cold seasons. The present paper describes the growth characteristics of Plagioselmis sp. (strain 87) isolated from the river water in 1987. The optimum tempreature and light intensity for the growth of Plagioselmis sp. (strain 87) were found to be 10-25°C and 1, 000-14, 000lx. This alga preferred the neutral and lower alkaline pH (7-9).5 and lower salinity (0-3.4‰). The amounts of nitrate, phosphate, and vitamin B12 required to maximize both growth rate and cell yields were 5mg N/l, 0.15mg P/l, and 50ng/ml, respectively. The falf-saturation constants of nitrate, phosphate, and vitamin B12 based on the growth rate were estimated to be 0.15mg N/l, 0.0078mg P/l, and 2.3ng/l, respectively. The minimum cell quotas of nitrogen, phosphorus, and vitamin B12 were 1.5×10-2ng N/cell, 5.5×10-4ng P/cell, and 2.5×10-4pg/cell, respectively. Phosphate concentration, but not nitrogen and vitamin B12 concentration, in the Nakasuji River water was found to reach almost the level which can maximize the growth rate of this alga. Growth potential in the water was estimated to be more than 6×103cells/ml from the quotients of those nutrient concentrations divided by each minimum cell quota of the alga. These results suggest that the water of the Nakasuji River has the potential of growing enough algae for the abundance of red tide during the cold seasons.

Microbial Recycling of Phytopiankton Phosphorus

The remineralization of phytoplankton-bound phosphorus subsequent to nonpredatory phytopiankton mortality represents a significant source of algal-available phosphorus in many lakes. A unique experimental apparatus (A Dual Culture Diffusion Apparatus) was used to measure the rate and extent of this process and to elucidate some of the governing factors. It was demonstrated that this process is strongly influenced by heterotrophic decomposer activity, because phosphorus regeneration rates were less than 0.01/d for cultures not inoculated with a decomposer community, while they were two to five times higher for decomposer-inoculated cultures. In addition to the character and activity of the microbial decomposer community, the phytoplankton cell phosphorus content was shown to be a significant factor in the rate of phosphorus regeneration for a given cell decay rate. Cell phosphorus above the minimum cell quota appeared to be released in an available form quite rapidly upon algal death and lysis.

Studies on B group vitamin requirements of red tide phytoplankton. III. Physiological ecology of Chattonella antiqua (HADA) ONO on B group vitamin requirements.

Physiological characteristics of the growth of two strains of Chattonella antiqua (HADA) ONO (Raphidophyceae), haarmful red tide phytoplankters, were examined in axenic clonal cultures with special reference to B group vitamin requirements. C. antiqua Harima-77 and TSU-8011 preferred a relatively high concentration of nutrients, and required essentially vitamin B12 among the B group vitamins for their growth. It was found that their growth rates and final cell yields were dependent on the vitamin B12 concentrations in the media. The half-saturation constant of vitamin. B12 for each strain was small:0.05ng/l for Harima-77 and 0.35ng/l for TSU-8011. The amount of vitamin B12 required to maximize final cell yields was around 10ng/l for Harima-77 and 20ng/l for TSU-8011. The maximum specific growth rate of each strain was 0.69 and 0.48 day-1, respectively. The minimum cell quota of vitamin B12 was estimated at 4.7×10-4pg/cell or 11.7 molecules/μm3 of cell for Harima-77 and 1.6×10-3pg/cell or 27.5 molecules/μm3 of cell for TSU-8011. These results suggest that vitamin B12 in seawater may be one of the principal factors con-trolling the growth of C. antiqua, and that the amount of the vitamin required to maximize both growth rates and growth yields is around 10 or 20ng/l.

赤潮プランクトンのＢ群ビタミン要求に関する研究 Ｉ Ｈｅｔｅｒｏｓｉｇｍａ ａｋａｓｈｉｗｏ ＨＡＤＡのＢ群ビタミン要求に関する増殖生理

Physiological characteristics on the growth of Heterosigma akashiwo HADA (Raphidophyceae), a red tide phytoplankton, were examined with special reference to B group vitamin requirements in the axenic clonal culture. The alga was wide-range euryhaline, and preferred relatively high concentration of nutrients. It was found that only vitamin B12 was essential for the growth, and that growth rates and final cell yields were dependent on the vitamin B12 concentrations in the media. Vitamin B12 halfsaturation constants were very small: 0.37-0.60ng/l. However, it needed the higher concentration of 20ng/l to maintain maximum growth yields. Maximum specific growth rates were 0.62-0.82 day-1. The minimum cell quota of vitamin B12 was 8.9×10-5pg/cell or 24.2 molecules/μm3 of the cell volume. Cells grown in the vitamin B12-rich media were able to accumulate it up to 50-fold as much as the minimum quota. These results suggest that vitamin B12 in seawater may be one of the most principal factors controlling the growth of H. akashiwo, and consequently the development of its population to a red tide.

TEMPERATURE EFFECTS ON SILICON LIMITED GROWTH OF THE LAKE MICHIGAN DIATOM STEPHANODISCUS MINUTUS (BACILLARIOPHYCEAE)1

ABSTRACTThe effect of temperature on the silicon limited growth and nutrient kinetics of Stephanodiscus minutus Grun. was examined using batch and semicontinuous culture methods. Short‐term batch culture methods gave maximum growth rates which were essentially constant over the temperature range of 10° to 20°C (μ3= 0.71–0.80 d−1). The half‐saturation constant for growth (Ks) was significantly lowest at 10°C (Ks= 0.31 μM Si; 0.22–0.41), and higher at both 15°C (Ks= 1.03 μM Si; 0.68–1.47) and 20°C (Ks= 0.88 μM Si; 0.60–1.22). Two methods were used to evaluate the semicontinuous experiments. The Droop relationship showed that the minimum cell quota was about 1.50 × 10−7 nmol Si cell−1, but there was much overlap in the results at all three temperatures. The Monod growth relationship for the semicontinuous experiments gave estimates of Ks which were lowest at 15°C (Ks= 0.12 μM Si), and higher at 10°C (Ks= 0.68 μM Si) and 20°C (Ks= 1.24 μM Si), although 95% confidence intervals overlapped. The maximum growth rate estimates for the semicontinuous experiments were similar at 10° and 15°, and higher at 20°C, but the number of points used in making the calculations makes the results less reliable than those from batch cultures. Generally, there were no consistent significant differences in the silicon limited growth of S. minutus over the temperature range studied. Our values of Ks for S. minutus are the lowest recorded for a freshwater diatom, and are consistent with the distribution of this species in nature. Generally, this species becomes abundant in areas with high phosphorus loading and very low silicon levels (low Si:P loading rates). Stephanodiscus species are also fossil indicators of eutrophication in north temperate lakes.

Biomass and Distribution of Cladophora Glomerata in Relation to Some Physical-Chemical Variables at Two Sites in Lake Erie

Cladophora was collected from 0.25-m 2 quadrats on alternate weeks at two sites located in Lake Erie near Hamburg, New York, and Walnut Creek near Erie, Pennsylvania, between June and November 1979. Cladophora biomass and internal nutrients (total Kjeldahl nitrogen and total phosphorus) were determined. Water samples were analyzed for total phosphorus, soluble reactive phosphorus, nitrate + nitrite nitrogen, total Kjeldahl nitrogen, and ammonia nitrogen. Temperature and Secchi disk transparency were also recorded. The peak standing crop for Cladophora was 211 gDW/m 2 (July) at the Hamburg site and 57 gDW/m 2 (July) at Walnut Creek. The depth distribution of Cladophora at the sample sites was influenced by wave activity and drifting sand. As a result of sand scouring at the shallower depths, Cladophora biomass generally was greatest at the 3-m depth. Reductions in Cladophora internal nutrients corresponded to decreased nutrients in the water. Cladophora detached from its substrate during periods when low internal nutrient levels were observed. Internal total phosphorus concentrations were low and approached the minimum cell quota (0.05–0.06% –P) most of the time. Soluble reactive phosphorus values were also low (1–4 μgP/L). Levels of soluble reactive phosphorus and internal total phosphorus were low and almost always limiting, and may be responsible for the low biomass values observed. In addition, internal total phosphorus levels showed a significant positive correlation with Cladophora biomass. Internal total nitrogen values were generally less than 1%-N at Walnut Creek and ammonia nitrogen measurements were near detection limits. Cladophora was nitrogen-limited at the Walnut Creek site during part of the study period. Nitrate + nitrite nitrogen showed a significant positive correlation with Cladophora biomass at that site.

The effect of environmental factors on phytoplankton growth: Temperature and the interactions of temperature with nutrient limitation1

The combined stress of nutrient limitation and suboptimal temperature on growth was studied with turbidostat and chemostat cultures of Scenedesmus sp. and Asterionella formosa. The combined effects were greater than the sum of individual effects and were not multiplicative. In N‐ and P‐limited Scenedesmus sp. and A. formosa the cell (q) of both limiting and nonlimiting nutrients increased with decreasing temperature. At a given temperature cell quotas of limiting nutrients also increased with the growth rate (µ) and followed a saturation function. Higher values of the minimum cell quota (qo) at lower temperatures show that cells require more nutrient with decreasing temperature. The change of qo with temperature varies with the type of limiting nutrient. This change for N and P in Scenedesmus sp. suggests that their optimum ratio, the ratio at which one limitation changes over to the other, is higher at suboptimal temperatures.Cell quotas of nutrient‐sufficient cultures (qm) for C, N, and P and cellular chlorophyll a concentration increased with decreasing temperature. The quota of each nonlimiting nutrient in nutrient‐limited cultures had the same value as qm. The rate of protein synthesis per unit RNA decreased with temperature.The highest apparent maximum N uptake was observed at 15°C for N‐limited Scenedesmus sp. growing at 0.5·d‒1. The optimal growth temperature range, however, was 20°–25°C. The highest apparent maximum P uptake in A. formosa was found at 19°–20°C, when µ = 0.4·d‒1. These temperatures were also optimal for growth.

Steady-State Growth and Chemical Composition of the Marine Chlorophyte Dunaliella tertiolecta in Nitrogen-Limited Continuous Cultures

The marine chlorophyte Dunaliella tertiolecta was grown in continuous cultures under NH(4)-N, NO(2)-N, NO(3)-N, and urea-N limitations. The effect of the nitrogen cell quota (Q(n)) on the steady-state growth rate (mu) was the same regardless of the N source. The relationship between mu and Q(n) was well described by the Droop equation, but only up to the true maximum growth rate ;mu (= cell washout rate). The ratio between the minimum cell quota (k(Q)) and the maximum cell quota (Q(m)) was 0.19. Hence, there is no substitute for determining ;mu experimentally. That there was no difference in growth response to different N sources suggests that no internal pooling of inorganic nitrogen occurred. Both the carbon (Q(c)) and phosphorus (Q(p)) cell quotas under N limitation increased with increasing mu in a threshold fashion: virtually no change in either cell quota up to approximately 0.8 ;mu, followed by a rapid and large increase up to ;mu. In addition, in the region of low mu, there was an increase in Q(p) with a decreasing medium N/P ratio of between 15 and 5 (by atoms). The results generally indicate the physiological limits in cellular constituency under N limitation. The usefulness of this information, however, in describing the response of natural populations of marine phytoplankton to transient nutrient exposures on the temporal and spatial microscales that most likely exist is of limited value.

Temperature effects on steady-state growth, phosphorus uptake, and the chemical composition of a marine phytoplankter

The marine chrysophyteMonochrysis lutheri was grown in phosphorus-limited continuous cultures at temperatures of 15°, 18.8° and 23°C. The effect of temperature on the maximum growth rate was well-defined by the Arrhenius equation, but the Q10 for this alga (1.7) was somewhat lower than has been determined previously for many other phytoplankton species (2.0-2.2). The minimum phosphorus cell quota was relatively unaffected by temperature at 18.8°C and 23°C, but doubled in magnitude at 15°C. As a result, the internal nutrient equation of Droop described the relationship between specific growth rate and phosphorus cell quota well at 18.8° and 23°C, but was less successful at 15°C. The major limitation in using the Droop equation is that the ratio between the minimum and maximum cell quotas must be known, thus necessitating the need to establish the true maximum growth rate by the cell washout technique. In addition, the phosphorus uptake rate on a cell basis at a given steady state growth rate (=specific uptake rate) increased dramatically at 15°C, whereas the turnover rate of total available phosphorus was unaffected by temperature. Both the nitrogen and carbon cell quotas were relatively unaffected by growth rate at a given temperature, but the average values increased slightly with decreasing temperature. The overall conclusion is that phytoplankton growth and limiting-nutrient uptake rates are only synchronous at or near the optimum temperature. Because these types of responses are species specific, much additional data on temperature effects will be required before the importance of including such effects in phytoplankton-nutrient models can be determined.