Minimum Cell Quota Research Articles

Acclimation by diverse phytoplankton species determines oceanic carbon to nitrogen ratios

AbstractThe carbon to nitrogen (CN) ratio of phytoplankton connects the carbon and nitrogen cycles in the ocean. Any variation in this ratio under climate change will alter the amount of carbon fixed by photosynthesis, and ultimately the amount sequestered in the ocean. However, a consistent mechanistic explanation remains lacking for observed species‐specific variations in phytoplankton CN ratios. We show that acclimation to ambient environmental conditions explains the observed variation of phytoplankton CN ratios by incorporating phytoplankton acclimation theory based on a resource allocation trade‐off between carbon vs. nitrogen acquisition capacity into a three‐dimensional marine ecosystem model. Inter‐specific differences in CN ratio and its sensitivity are caused by inter‐specific differences in Droop's minimum nitrogen cell quota. Our model, constrained by observed phytoplankton parameters, shows that the global mean phytoplankton CN ratio is greater than the canonical Redfield ratio, as suggested previously based on regional in situ observations.

Minimum nitrogen cell quota for maximal growth rate in cycloturbidostat cultures of Picochlorum oklahomense

Replicate cycloturbidostats cultures of Picochlorum oklahomense were manipulated to quantify the minimum nitrogen cell quota supporting maximum growth rate and to study the effects of nitrogen limitation on cycloturbidostat cultures growing at near-μmax under steady-state, balanced growth conditions. Results show that cyclical diurnal neutral lipid accumulation and ultradian growth are not mutually exclusive, and that cycloturbidostat cultures may be maintained near or at the threshold of N-limitation to control cell composition and modulate biomass and lipid productivity. Using this novel approach, we have identified the threshold of N limitation in P. oklahomense and investigated the effects of N limitation on replicate cycloturbidostat cultures. Mildly N-limited cultures grew rapidly (87 % μmax) and maintained near optimal physiology, while also accumulating neutral lipids during the photoperiod. Flow cytometry revealed differences in temporal patterns between treatments, demonstrating the importance of the cell-based approach in manipulating microalgal cultures. This study highlights the utility of turbidostats in experimental phycology and defines a scantly explored concept in the study of N limitation, the threshold of N cell quota necessary to support μmax below which cells are forced to re-allocate resources and slow growth, denoted QN, T.

Competitive growth characteristics between Microcystis aeruginosa and Cyclotella sp. accompanying changes in river water inflow and their simulation model

SummaryWhen the water quality in Lake Tega was the worst among Japanese lakes in the 1970s, cyanobacteria (mainly the genus Microcystis) were observed to be the dominant species in the summer. Since 2000, for water quality improvement, a large amount of the Tone River water was discharged into Lake Tega, resulting in an improvement in the water quality together with transition of algal dominant species from cyanobacteria to diatoms (mainly the genus Cyclotella). Although several factors including nutrient concentration and daily renewal rate (d) could have been related to the succession of algal dominant species, these effects have not been understood very well. This study investigated the effects of the daily renewal rate and nitrate‐nitrogen (hereafter nitrogen) concentration (N) on the competition between the cyanobacterium Microcystis aeruginosa and the diatom Cyclotella sp. through monoculture and monoxenic culture experiments. Based on the experimental results, a simulation model was constructed to predict the competitive growth pattern of each species. Monoxenic culture experiments showed that M . aeruginosa outcompeted Cyclotella sp. under conditions of N = 0.5 (1.0 mg‐N L−1) and d = 5, 15, and 20%. The domination of M . aeruginosa could be attributed to smaller values for the half‐saturation constant and the minimum nitrogen cell quota for this species with respect to Cyclotella sp. However, Cyclotella sp., possessing higher values of nitrogen cell quota and nitrogen uptake rate, dominated by an increasing nitrogen concentration (N = 1.0 mg L−1) and daily renewal rate (d = 30%). The competitive growth patterns of M . aeruginosa and Cyclotella sp. could roughly be predicted by the simulation model. These results suggested that the daily renewal rate as well as the nitrogen concentration, could affect the competition and be influential factors as to which species dominates between M . aeruginosa and Cyclotella sp.

The dynamics of the intracellular contents of carbon, nitrogen, and chlorophyll a under conditions of batch growth of the diatom Phaeodactylum tricornutum (Bohlin, 1897) at different light intensities

It was found that during batch growth of the diatom Phaeodactylum tricornutum (Bohlin, 1897) the specific content of nitrogen in the cells (relative to carbon) decreased with decreasing nitrogen concentration in the medium below ∼10 µM/L. When the nitrogen of the medium was exhausted, the microalga continued their growth for some time using the cell reserve of this element. The C/N ratio in cells increased from 5 to 14 and the density of the culture (relative to carbon) increased by approximately twofold. The correlation between the specific-growth rate of P. tricornutum and the C/N ratio in the cells was described by an equation similar to the Droop (1968) model. The ratio of the intracellular concentrations of carbon and chlorophyll a (C/Chl a), while reducing the specific nitrogen in the cells, increased exponentially; this effect increased with increasing light intensity. The value of the minimum cell quota for nitrogen did not depend on the light condition of culture growth.

Estimation of the nutrient consumption by various cell sizes of the diatom Eucampia zodiacus: A representative organism causing bleaching of aquacultured nori

The diatom Eucampia zodiacus is a harmful species that indirectly causes bleaching to nori (Pyropia) cultivation through competitive utilization of nutrients during its bloom, however cellular storage and changes in physiology by asexual reproduction remains unclear. In the present study, we experimentally investigated the nitrate (N), phosphate (P) and silicic acid (Si) consumption by various cell sizes of E. zodiacus strains, the apical axis length of which ranged from 10.2 to 77.3μm. Nutrient cell quotas of E. zodiacus ranged from 2.7 to 8.4pMcell−1 for N, 0.34–0.76pMcell−1 for P and 1.7–7.3pMcell−1 for Si, and they increased with cell size, in which there is a significant correlation between these two elements. The N and P quotas were estimated to be several times higher than the minimum cell quotas. In contrast, the Si cell quotas were approximately equal to those of the minimum values. Based on the present cell quotas, total nitrate consumption by E. zodiacus population when the blooms reached maximum cell density (=1000cellsml−1) were estimated to be 6.5μM. Monthly mean concentrations of dissolved inorganic nitrogen (DIN) range from 3.5 to 8.2μM during the period of late nori harvest season when E. zodiacus blooms occur, and nori bleaching is reported at the condition of DIN concentration of less than 3μM in Harima-Nada, eastern Seto Inland Sea, Japan. Therefore, the present results suggest that E. zodiacus causes serious damage to nori cultivation due to high levels of nutrient consumption.

Monster potential meets potential monster: pros and cons of deploying genetically modified microalgae for biofuels production

Biofuels production from microalgae attracts much attention but remains an unproven technology. We explore routes to enhance production through modifications to a range of generic microalgal physiological characteristics. Our analysis shows that biofuels production may be enhanced ca fivefold through genetic modification (GM) of factors affecting growth rate, respiration, photoacclimation, photosynthesis efficiency and the minimum cell quotas for nitrogen and phosphorous (N : C and P : C). However, simulations indicate that the ideal GM microalgae for commercial deployment could, on escape to the environment, become a harmful algal bloom species par excellence, with attendant risks to ecosystems and livelihoods. In large measure, this is because an organism able to produce carbohydrate and/or lipid at high rates, providing stock metabolites for biofuels production, will also be able to attain a stoichiometric composition that will be far from optimal as food for the support of zooplankton growth. This composition could suppress or even halt the grazing activity that would otherwise control the microalgal growth in nature. In consequence, we recommend that the genetic manipulation of microalgae, with inherent consequences on a scale comparable to geoengineering, should be considered under strict international regulation.

Growth and uptake kinetics of nitrate and phosphate by benthic microalgae for phytoremediation of eutrophic coastal sediments

In the present study, the effect of monochromatic light (blue, yellow and red) and mixed wavelength on the nutrient uptake and growth kinetics of benthic microalgae Achnanthes sp., Amphora sp., Navicula sp. and Nitzschia sp. were investigated. The maximum uptake rate (ρmax) for nitrate and phosphate obtained by short-term experiments were high in the order of blue, mixed, red, yellow wavelength, and among the 4 benthic microalgae, Nitzschia sp. was the highest ρmax. The half-saturation constant (Ks) was higher than other taxon. The specific maximum growth rate (μmax′) and minimum cell quota (q0) for the nitrogen and phosphorus-limited condition, Nitzschia sp. showed the highest μmax′ and q0 values among the 4 benthic microalgae. These results suggest that the benthic microalgae are adapted to high nutrient concentration. In particular, Nitzschia sp., which have a higher capability of storage and uptake, may be a useful species for phytoremediation.

Growth and uptake kinetics of phosphate by benthic microalga Nitzschia sp. isolated from Hiroshima Bay, Japan

SUMMARYIn the present study, we experimentally investigated the phosphate uptake kinetics of benthic microalga Nitzschia sp. isolated from Hiroshima Bay, Japan. The maximum uptake rate (ρmax) obtained by short‐term experiments was 6.84 pmol cell−1 h−1 for phosphate. The half‐saturation constant for uptake (KS) was 61.2 µmol cell−1 h−1. Both the ρmax and Ks of this species were extremely high, suggesting that Nitzschia sp. is adapted to benthic environments, where nutrient concentrations are much higher than in the water column. The specific maximum growth rate (µ'max) and minimum cell quota (Q0) for the P‐limited condition, obtained by a semi‐continuous growth experiment, were 0.48 day−1 and 0.045 pmol cell−1, respectively. It is concluded that Nitzschia sp. could be a ‘storage strategist’ species, meaning it adapts so as to minimize the influence of fluctuations in phosphate conditions resulting from the change in redox conditions of sediment due to bioturbation.

Potassium — A Non-Limiting Nutrient in Fresh Waters?

Abstract Possible limitation of growth and distribution of freshwater organisms by the availability of potassium, an essential major bio-constituent and plantnutrient, is considered for inland waters. It is interpreted in relation to the range of concentrations normally encountered and experimental work on specific growth rates of algae at low concentrations, minimum cell and biomass quotas of K, biomass yields under graded additions, inhibition at higher concentrations, and response to cation-ratios in the medium. The range of K+ concentrations in inland waters is surveyed. Most concentrations exceed 10 µmol L-1, and are greatly in excess of those (under 1 µmol L-1) found limiting specific growth rates of test species. They are also in excess of the content in most natural populations of phytoplankton when yields have the experimental minimum or limiting cell/biomass quota, of the order of 1% dry weight. Limiting concentrations for specific growth rate in nature are therefore probably rare. They, and yie...

Nitrate and phosphate uptake kinetics of the harmful diatom Coscinodiscus wailesii, a causative organism in the bleaching of aquacultured Porphyra thalli

The large diatom Coscinodiscus wailesii is one of the problematic species which indirectly cause bleaching damage to “Nori” ( Porphyra thalli) cultivation through competitive utilization of nutrients during its bloom. In the present study, we experimentally investigated the nitrate (N) and phosphate (P) uptake kinetics of C. wailesii, Harima-Nada strain. Maximum uptake rates ( ρ max), obtained by short-term experiments, were 58.3 and 95.5 pmol cell −1 h −1 for nitrate and 41.9 and 59.1 pmol cell −1 h −1 for phosphate at 9 and 20 °C, respectively. The half saturation constants for uptake ( K s ) were 2.91 and 5.08 μM N and 5.62 and 6.67 μM P at 9 and 20 °C, respectively. The ρ max values of C. wailesii, much higher than those of other marine phytoplankton species, suggest that C. wailesii is able to take up large amounts of nutrients from the water column. On the other hand, V max/ K s ( V max; V max = ρ max/ Q 0, Q 0; minimum cell quota) values of C. wailesii, which is a better measure to evaluate the competitive ability for nutrient uptake, were low in dominant diatom species. This parameter indicates that C. wailesii is disadvantaged compared to other diatom species in competing for nutrients, and the decreasing nutrient concentrations from winter to spring is an important factor limiting C. wailesii blooming in early spring.

Nitrate and phosphate uptake kinetics of the harmful diatom Eucampia zodiacus Ehrenberg, a causative organism in the bleaching of aquacultured Porphyra thalli

The diatom Eucampia zodiacus Ehrenberg is a harmful diatom which indirectly causes bleaching of aquacultured Nori (Porphyra thalli) through competitive utilization of nutrients during bloom events. In the present study, we experimentally investigated the nitrate (N) and phosphate (P) uptake kinetics of E. zodiacus, Harima-Nada strain. Maximum uptake rates (ρmax), which were obtained by short-term experiments, were 0.777 and 0.916pmolcell−1h−1 for nitrate and 0.244 and 0.550pmolcell−1h−1 for phosphate at 9 and 20°C, respectively. The half-saturation constants for uptake (Ks) were 2.59 and 2.92μM N and 1.83 and 4.85μM P at 9 and 20°C, respectively. Although the maximum specific uptake rate (Vmax; Vmax=ρmax/Q0, Q0; minimum cell quota) and Vmax/Ks for nitrate at 9°C are about 1/2 of those obtained at the optimum temperature (20°C), they are still higher than those obtained for many other phytoplankton at their optimum temperature conditions for uptake. These results suggest that E. zodiacus utilizes nitrogen efficiently at low water temperature, and it is one of the important factors causing the serious damage to Porphyra thalli by bleaching due of this species. For phosphate, the Ks values of E. zodiacus were higher than those reported for other species; the Vmax and Vmax/Ks values were much lower than those of other diatoms such as Skeletonema costatum (Greville) Cleve. These results suggest that E. zodiacus is disadvantaged compared to other diatom species during competitive utilization of phosphate.

Competition patterns among phytoplankton functional groups: How useful are the complex mathematical models?

Simple models have significant contribution to the development of ecological theory. However, these minimalistic modeling approaches usually focus on a small subset of the causes of a phenomenon and neglect important aspects of system dynamics. In this study, we use a complex aquatic biogeochemical model to examine competition patterns and structural shifts in the phytoplankton community under nutrient enrichment conditions. Our model simulates multiple elemental cycles (org. C, N, P, Si, O), multiple functional phytoplankton (diatoms, green algae and cyanobacteria) and zooplankton (copepods and cladocerans) groups. It also takes into account recent advances in stoichiometric nutrient recycling theory, and the zooplankton grazing term is reformulated to include algal food quality effects on zooplankton assimilation efficiency. The model provided a realistic platform to examine the functional properties (e.g., kinetics, growth strategies, intracellular storage capacity) and the abiotic conditions (temperature, nutrient loading) under which the different phytoplankton groups can dominate or can be competitively excluded in oligo, meso and eutrophic environments. Based on the results of our analysis, the intergroup variability in the minimum cell quota and maximum transport rate at the cell surface for phosphorus along with the group-specific metabolic losses can shape the structure of plankton communities. We also use classification tree analysis to elucidate aspects (e.g., relative differences in the functional group properties, critical values of the abiotic conditions, levels of the other plankton community residents) of the complex interplay among physical, chemical and biological factors that drive epilimnetic plankton dynamics. Finally, our study highlights the importance of improving the mathematical representation of phytoplankton adaptive strategies for resources procurement (e.g., regulation of transport kinetics, effects of transport kinetics on the kinetics of assimilation, relationship between assimilation and growth) to effectively link variability at the organismal level with ecosystem-scale patterns.

Low potassium and inorganic carbon concentrations influence a possible phosphorus limitation in Chlamydomonas acidophila (Chlorophyceae)

Chlamydomonas acidophila, a dominant phytoplankton species in the very acidic Lake 111 (pH 2.7) situated in Germany, faces low concentrations of inorganic phosphorus (Pi), inorganic carbon (Ci) and potassium (K+) in its environment, which may lead to a complex colimitation by these nutrients. We performed laboratory and field investigations to test for Pi limitation and its dependence on Ci and K+ concentrations. The minimum cell quota for phosphorus (Q0) and phosphatase enzyme activity were similar to those for neutrophilic algae, despite the low pH and high concentrations of iron and aluminium, indicating no extra metabolic costs or inhibition of enzymes by the extreme environment. The threshold concentration of soluble reactive phosphorus for growth (SRPt), the algal C:P ratio and the alkaline phosphatase enzyme activity all suggested a moderate Pi limitation of C. acidophila in Lake 111. SRPt and Q0 were higher at low CO2 and K+ concentrations in culture, showing a relationship between Ci and Pi acquisition. Furthermore, SRPt and Q0 were higher under K+/Pi-colimiting conditions than under Pi-limiting conditions alone, suggesting that K+ concentrations influence Pi limitation in C. acidophila as well. Our results show that a limitation by one macronutrient requires consideration of the availability of the others as their uptake mechanisms depend on each other. Notwithstanding these interactions, Ci or K+ concentrations had no clear influence on the Pi limitation of C. acidophila in Lake 111.

Nitrogen and phosphorus utilization in the cyanobacterium Microcystis aeruginosa isolated from Laguna de Bay, Philippines

Phytoplankton supports fisheries and aquaculture production. Its vital role as food for aquatic animals, like mollusks, shrimp, and fish cannot be overemphasized. Because of its contribution as a food source for fish, the growth kinetics of Microcystis aeruginosa, a dominant cyanobacterium in the lake, was studied. The regular occurrence of M. aeruginosa is experienced during the months of May to July or from September to November in Laguna de Bay, the largest freshwater lake in the Philippines. M. aeruginosa was collected from Laguna de Bay, isolated, and established in axenic conditions. Data on the growth kinetic parameters for nitrate-nitrogen and phosphate-phosphorus utilization by M. aeruginosa gave the following values: half-saturation constant (K s ), 0.530 mg N. L−1 and 0.024 mg P. L−1 respectively; maximum growth rate (μ max ), 0.671. d−1 and 0.668. d−1 respectively; maximum cell yield, 6.5 and 6.54 log, cells. ml−1 respectively; nutrient level for saturated growth yield, 8.71 mg N. L−1 and 0.22 mg P. L−1 respectively; and minimum cell quota (Q 0 ), 2.82 pg N. cell−1 and 0.064 pg P. cell−1 respectively. The low K s value and high maximum growth rate (μ max ) for phosphorus by M. aeruginosa would suggest a high efficiency of phosphorus utilization. On the other hand, the high K s value for nitrogen indicated a low rate of uptake for this nutrient.

Nutrition and growth kinetics in nitrogen- and phosphorus-limited cultures of the novel red tide flagellate Chattonella ovata (Raphidophyceae)

In the summer of 2004, a harmful bloom caused by Chattonella ovata (Raphidophyceae) occurred over almost the entire area of the Seto Inland Sea and caused fishery damages. This incident was the first record of a bloom and damage to the fisheries caused by this species in Japanese waters. In order to elucidate the mechanism of the bloom outbreak, we examined the nutrition and the growth kinetics in nitrogen (N)- and phosphorus (P)-limited semi-continuous cultures of this species. Inorganic N compounds, such as nitrate, nitrite, and ammonium, were found to be good nitrogen sources for the growth of C. ovata, while organic nitrogen (urea and uric acid) was not utilized. This species was capable of using ATP, ADP and inorganic phosphorus compounds, but did not utilize phosphate monoesters as a sole P source. Under both N-limited and P-limited steady state conditions, the growth rate as a function of cell nitrogen and phosphorus quota, respectively, followed the Droop equation. Kinetic parameters μ m (maximum growth rate) and k q (minimum cell quota) obtained for N- and P-limited cultures were 0.79 day −1 and 5.5 pmol N cell −1 and 0.86 day −1 and 0.48 pmol P cell −1, respectively. The minimum cell quotas were 23–30% lower than those of C. antiqua. The nutrient availability and kinetic parameters of C. ovata are compared with other harmful algae and the ecological implications of these characteristics discussed.

여수해만산 유독 와편모조류 Gymnodinium catenotum (Graham)의 용존태 무기인에 대한 성장 및 흡수

We investigated the growth and phosphate uptake of a toxic dinoflagellate, Gymnodinium catenatum, isolated from Yeosuhae Bay, South Korea. A short-term phosphate uptake experiment revealed that its maximum uptake and the half-saturation constant were 1.39 pmol/cell/hr and <TEX>$2.65{\mu}M$</TEX>, respectively. In a semicontinuous culture, the maximum specific growth rate and minimum phosphorus cell quota of G. catenatum were 0.39/day and 1.27 pmol/cell, respectively. Thus, G. catenatum is a poor competitor in terms of inorganic nutrient use and is unlikely to form blooms in Yeosuhae Bay.

Growth and uptake kinetics for nitrate, ammonium and phosphate by the toxic dinoflagellate Gymnodinium catenatum isolated from Hiroshima Bay, Japan

: Growth and uptake kinetics for nitrate, ammonium and phosphate by Gymnodinium catenatum isolated from Hiroshima Bay were experimentally investigated. Maximum uptake rate (ρmax) and half saturation constant (Ks), which were obtained by short-term uptake experiments, were 6.48 pmol/cell per h and 7.60 µM for nitrate, 3.37 pmol/cell per h and 33.6 µM for ammonium, and 1.42 pmol/cell per h and 3.40 µM for phosphate, respectively. Specific maximum growth rate (µ′m) and minimum cell quota (Q0), which were obtained by semicontinuous growth experiments, were 0.36/day and 31.3 pmol/cell for the N-limited conditions, respectively. The µ′m and QP0 for P-limited conditions were 0.37/day and 1.83 pmol/cell, respectively. These results indicate that G. catenatum is a poor competitor in terms of utilization of inorganic nutrients and is unlikely to form blooms in Hiroshima Bay where phosphate is chronically depleted unless the growth is supported by organic species (i.e. dissolved organic phosphorus) of the nutrients.

Effects of salinity and nutrient conditions on growth and haemolytic activity of Fibrocapsa japonica (Raphidophyceae)

The influence of salinity and a number of nitrogen (N) and phosphorus (P) levels on growth and haemolytic activity of the harmful algal bloom (HAB) species Fibrocapsa japonica were studied simultaneously. The F. japonica strain studied had a euryhaline growth profile with an optimum growth rate at 26 psu. Measurements of the minimum cell quota of N and P revealed an optimum N:P ratio of 24.5 and a relatively high requirement of nutrients (q0N = 7.1 pmol cell -1 ; q0P = 0.29 pmol cell -1 ). These characteristics fit the habitat of this species, among which is the P-controlled eutrophied Dutch coastal waters. Salinity had a significant effect on the growth rate of F. japonica and on its haemolytic activity, even on the basis of equal cell volume. The highest haemolytic activity was found at 16 psu, the lowest salinity condition tested. The EC50 values that were expressed on a per cell basis revealed that F. japonica had a high haemolytic activity when compared with other HAB species. Under N limitation, cells of F. japonica were less haemolytic than cells not limited for N, but this could be caused by a slight decrease in cell volume only. Remarkably, cells not limited for N appeared to form aggregates, which in combination with the elevated levels of haemolytic compounds per cell may result in obstruction of gills, causing fish kills during F. japonica blooms. So, despite the absence of brevetoxin in our strain of F. japonica, this strain may be lethal to fish due to its haemolytic activity.

Nutrition and growth kinetics in nitrogen- or phosphorus-limited cultures of the ‘novel red tide’ dinoflagellate Heterocapsa circularisquama (Dinophyceae)

Heterocapsa circularisquama is a dinoflagellate that has only recently been recorded as producing red tides; it causes severe damage to Japanese shellfish aquaculture by killing bivalves. In order to elucidate the mechanism of red tide outbreaks, the nutrition and growth kinetics of H. circularisquama were examined in nitrogen (N)– or phosphorus (P)–limited semicontinuous culture. Inorganic N compounds, such as nitrate, nitrite, and ammonium, were found to be good nitrogen sources for the growth, whereas organic nitrogen (urea and uric acid) was not utilized. Heterocapsa circuiarisquama could successfully use a wide variety of inorganic and organic phosphorus compounds of different molecular structures as the sole P source. Under N-limited steady-state conditions, the dilution rate (= growth rate) as a function of cell nitrogen quota followed the Droop equation. Under P-limited steady state culture too, the dilution rate as a function of cell P quota followed the Droop equation. Kinetic parameters Dm, (the maximum dilution rate) and Kq (minimum cell quota) were 0.9 d−1 and 1.1 pmol N cell−1 for N-limited cultures and 1.1 d−1 and 89.4 fmol P cell−1 for P-limited cultures. The nutrient availability and kinetic parameters of H. circularisquama are compared with other red-tide organisms and the ecological implications of these characteristics are discussed.

Effects of temperature, and availability of nitrogen and phosphorus on the abundance of Anabaena and Microcystis in Lake Biwa, Japan: an experimental approach

Under optimal nutrient conditions, both Microcystis sp. and Anabaena sp. isolated from Lake Biwa grew optimally at 28–32°C but differed in maximal growth rates, phosphate uptake kinetics, maximal phosphorus quotas, and growth responses to nitrogen and phosphorus limitation. The maximal growth rates of Microcystis and Anabaena were 1.6 and 1.25 divisions day−1, respectively. With phosphate and nitrate in the growth-limiting range, the growth of Microcystis was optimal at an N : P ratio of 100 : 1 (by weight) and declined at lower (nitrogen limitation) and higher (phosphorus limitation) ratios. In contrast, Anabaena growth rates did not change at N : P ratios from 1000 : 1 to 10 : 1. Starting with cells containing the maximal phosphorus quota, Microcystis growth in minus-phosphorus medium ceased in 7–9 days, compared with 12–13 days for Anabaena. The phosphate turnover time in cultures starved to their minimum cell quotas was 7.9 min for Microcystis and 0.6 min for Anabaena. Microcystis had a higher K s (0.12 μg P l−1 10−6 cells) and lower V max (9.63 μg P l−1 h−1 10−6 cells), than Anabaena (K s 0.02 μg P l−1 h−1 10−6 cells; V max 46.25 63 μg P l−1 h−1 10−6 cells), suggesting that Microcystis would not be able to grow well in phosphorus-limited waters. We conclude that in spite of the higher growth rate under ideal conditions, Microcystis does not usually bloom in the North Basin because of low availability of phosphorus and nitrogen. Although Anabaena has an efficient phosphorus-uptake system, its main strategy for growth in low-phosphorus environments may depend on storage of phosphorus during periods of abundant phosphorus supply, which are rare in the North Basin.