DIEL VARIATIONS OF CARBOHYDRATES AND NEUTRAL LIPIDS IN NITROGEN‐SUFFICIENT AND NITROGEN‐STARVED CYCLOSTAT CULTURES OF ISOCHRYSIS SP.1

The goal of this study was to investigate the time response of two major carbon (C) reserves, respectively neutral lipids (NL) and total carbohydrate (TC), in the Haptophyte Isochrysis sp. growing in nitrogen (N)‐sufficient or N‐starved conditions and under light:dark (L:D) cycles. Experiments were carried out in a cyclostat culture system that allowed the following of the dynamics of the main cell compounds at both hourly and daily time scales. Under N‐sufficient conditions, the L:D cycles cause the population to be synchronized, with most of the cells dividing at the beginning of the dark period. The C‐specific growth rate was maximal around midday and negative during the dark period due to respiration processes. NL and TC both accumulated during the day and consumed during the night. We showed that NL and TC are highly dynamic compounds, as more than three quarters of NL and TC accumulated during the light period were consumed during the dark period. In contrast to NL, phospholipid and glycolipid to C ratios remained quite stable during the light/dark cycles. The major effect of N starvation on the NL and TC dynamics was to uncouple their diel variations from the L:D cycle, in two different ways depending on their respective role during short‐term acclimation. Whereas the TC per cell ratio increased rapidly to reach a stable value in response to N starvation, NL per cell continued to oscillate, but with a pattern out of phase with the L:D cycle.     

ELEMENTAL AND BIOCHEMICAL COMPOSITION OF RHINOMONAS RETICULATA (CRYPTOPHYTA) IN RELATION TO LIGHT AND NITRATE‐TO‐PHOSPHATE SUPPLY RATIOS1

The biochemical basis for variations in the critical nitrogen‐to‐phosphorus (N:P) ratio, which defines the transition between N‐ and P‐limitation of growth rate, is currently not well understood. To assess this issue, we cultured the cryptophyte Rhinomonas reticulata NOVARINO in chemostats with inflow nitrate‐to‐phosphate ratios ranging from 5 to 60 mol N·(mol P)^?1 at two light intensities. The nitrate‐to‐phosphate ratio marking the transition between N‐ and P‐limitation was independent of light intensity and was between 30 and 45 mol N/mol P. In N‐limited cells, the particulate N:P ratio was stable at around 23 mol N/mol P over a range of inflow nitrate‐to‐phosphate from 5 to 30, whereas in P‐limited cells this ratio was around 90 mol N/mol P at inflow nitrate‐to‐phosphate ratios of 45 and 60. Cell phosphorus decreased with increasing nitrate‐to‐phosphate ratio up to the critical nitrate‐to‐phosphate ratio for each light intensity, above which they remained stable. The C:P of R. reticulata cells increased with increasing inflow nitrate‐to‐phosphate from around the Redfield value (106 mol C/mol P) to around 700. There was a significant effect of light on C:P in the N‐ limited cells, with higher C:P under high light conditions that was not observed in the P‐limited chemostats. Cellular RNA was not influenced by light but was greatly influenced by the type of nutrient limitation. In contrast, chl a, C, N, and protein were not influenced by the nitrate‐to‐phosphate in the inflow medium. Total protein per RNA was independent of light intensity but exhibited a maximum at inflow nitrate‐to‐phosphate of 30. Our results suggest a strong “two‐level” homeostatic mechanism of cellular N and P content in R. reticulata with two distinct states that are determined by the type of nutrient limitation and not by light.     

UNCOUPLING OF SILICON COMPARED WITH CARBON AND NITROGEN METABOLISMS AND THE ROLE OF THE CELL CYCLE IN CONTINUOUS CULTURES OF THALASSIOSIRA PSEUDONANA (BACILLARIOPHYCEAE) UNDER LIGHT,NITROGEN, AND PHOSPHORUS CONTROL1

The elemental composition and the cell cycle stages of the marine diatom Thalassiosira pseudonana Hasle and Heimdal were studied in continuous cultures over a range of different light‐ (E), nitrogen‐ (N), and phosphorus‐ (P) limited growth rates. In all growth conditions investigated, the decrease in the growth rate was linked with a higher relative contribution of the G2+M phase. The other phases of the cell cycle, G1 and S, showed different patterns, depending on the type of limitation. All experiments showed a highly significant increase in the amount of biogenic silica per cell and per cell surface with decreasing growth rates. At low growth rates, the G2+M elongation allowed an increase of the silicification of the cells. This pattern could be explained by the major uptake of silicon during the G2+M phase and by the independence of this process on the requirements of the other elements. This was illustrated by the elemental ratios Si/C and Si/N that increased from 2‐ to 6‐fold, depending of the type of limitation, whereas the C/N ratio decreased by 10% (E limitation) or increased by 50% (P limitation). The variations of the ratios clearly demonstrate the uncoupling of the Si metabolism compared with the C and N metabolisms. This uncoupling enabled us to explain that in any of the growth condition investigated, the silicification of the cells increased at low growth rates, whereas carbon and nitrogen cellular content are differently regulated, depending of the growth conditions.     

Does the Redfield ratio infer nutrient limitation in the macroalga Spirogyra fluviatilis?

1. The cellular nutrient contents of microalgae, when growing at or approaching maximum rates, approximate the Redfield C : N : P (molar) ratio of 106 : 16 : 1. Deviations from this optimal ratio can be used to infer nutrient limitation of microalgal growth. However, this ratio may not be applicable to macroalgae, which are distinguished from microalgae by forming a thallus that is a discrete structure visible to the naked eye. The utility of the Redfield ratio to infer nutrient limitation of the growth of macroalgae was tested for Spirogyra fluviatilis in a field experiment conducted in tropical Australia. 2. The optimal cellular C : N : P ratio for S. fluvialitis was estimated by means of in situ nutrient addition. This was compared with S. fluvialitis cellular ratios determined from eight sites with a wide range of soluble N concentrations (<1–90 μg L^?1), a smaller range of soluble P concentrations (5–12 μg L^?1), and soluble molar N : P ratios of 0.11– 27. 3. Spirogyra fluviatilis had an optimal molar C : N : P ratio of 1800 : 87 : 1 which differs substantially from the Redfield ratio, and suggests that the latter ratio is not applicable to this macroalga. Concentrations of N and P in the river deviated from the optimal N : P ratio of 87 : 1, inferring nutrient limitation of growth. 4. C : P and C : N ratios of S. fluviatilis varied in accordance with general stoichiometric relationships for autotrophs under nutrient limitation of growth. Ratios of C : P and C : N increased, respectively, with increased severity of P‐ and N‐limitation. Additionally, C : P ratios increased with increased N : P ratios, whilst the C : N ratio increased with decreased N : P ratios. The C : N molar ratio however was an insensitive indicator of nutrient depletion compared with the C : P ratio. Under N‐limitation of growth, luxury amounts of P were stored by S. fluviatilis. 5. In aquatic environments where macroalgae are sufficiently abundant to be sampled, their cellular carbon, nitrogen and phosphorus stoichiometry can be used to infer nutrient limitation of growth when their optimal C : N : P ratio is known.     

Growth characteristics of flagellated cells of Phaeocystis pouchetii revealed by diel changes in cellular DNA content

The present study reports on effects of different light:dark periods, light intensities, N:P ratios and temperature on the specific growth rate of flagellated cells of Phaeocystis pouchetii in culture. The specific growth rate was estimated by diel changes in cellular DNA content. The cellular DNA content and cell cycle of flagellated cells of P. pouchetii are shown, and the importance of light:dark period in cell division is demonstrated. Diel patterns of the cellular DNA content showed that cell division was confined to the dark period. The cells dealt with more than one division per day by rapid divisions shortly after each other.The specific growth rates (μ_DNA) based on the DNA cell cycle model were in close agreement with specific growth rates (μ_Cell) determined from cell counts. The temperature affected the specific growth rates (multiple regression, p < 0.01) and were higher at 5 °C (μ ≤ 2.2 d⁻¹) than at 10 °C (μ ≤1.6 d⁻¹). Increasing the light:dark period from 12:12 h to 20:4 h affected the specific growth rate of P. pouchetii at the lower temperature tested (5 °C) (multiple regression, p < 0.01), resulting in higher specific growth rates than at 10 °C. At 10 °C, the effect of light:dark period was severely reduced. Neither light nor nutrients could compensate the reduction in specific growth rates caused by elevated temperature. The specific growth rates was not affected by the N:P ratios tested (multiple regression, p = 0.21). The experiments strongly suggest that the flagellated cells have a great growth potential and could play a dominating role in northern areas at increased day length.     

Light-, nitrogen-, and phosphorus-limited growth of Phaeodactylum tricornutum Bohlin strain TFX-1: Chemical composition,carbon partitioning,and the diel periodicity of physiological processes

Phaeodactylum tricomutum Bohlin (strain TFX-1) was grown under light-, nitrogen-, and phosphorus-limited conditions in continuous or semicontinuous cultures under a 12L-12D light regime. The C, N, and P contents were determined at each steady state, as was the partitioning of cellular organic carbon into protein, lipids, polysaccharides, and metabolic intermediates. All determinations were made at the beginning and again at the end of the light period. The rates of nutrient assimilation and of synthesis of biochemical constituents during the light and dark periods were calculated from the above data, and the periodicities of these processes characterized. The elemental composition of the cells was different under each limitation. In particular, phosphorus limitation severely restricted the ability of the cell to store nitrogen in non-protein forms. Biochemical composition and the diel periodicity of cellular processes also differed between limitations. Nutrient uptake was most strongly periodic under light limitation. Protein synthesis showed increased periodicity under nitrogen limitation, relative to the other limitations, while the periodicity of lipid synthesis was reduced under phosphate limitation. Polysaccharide was synthesized at high rates during the light period and consumed in the dark under all limitations.     

DIEL RHYTHM OF ALGAL PHOSPHATE UPTAKE RATES IN P‐LIMITED CYCLOSTATS AND SIMULATION OF ITS EFFECT ON GROWTH AND COMPETITION1

Oscillations in the phosphate (P_i) uptake rates for three species of green algae were examined in a P‐limited cyclostat. For Ankistrodesmus convolutus Corda and Chlorella vulgaris Beyerinck, the P_i uptake rates increased during the daytime and decreased at night. In contrast, Chlamydomonas sp. exhibited the opposite uptake pattern. Cell densities also oscillated under a light:dark cycle, dividing at a species‐specific timing rather than continuously. In general, the cell densities exhibited an inverse relationship with the P_i uptake rates. A competition experiment between A. convolutus and C. vulgaris in a P‐limited cyclostat resulted in the dominance of C. vulgaris, regardless of the relative initial cell concentrations. Chlorella vulgaris also dominated in a mixed culture with Chlamydomonas sp., irrespective of the initial seeding ratio and dilution rate. However, Chlamydomonas sp. and A. convolutus coexisted in the competition experiment with gradual decrease of Chlamydomonas sp. when equally inoculated. Mathematical expressions of the oscillations in the P_i uptake rate and species‐specific cell division gate were used to develop a simulation model based on the Droop equation. The simulation results for each of the species conformed reasonably well to the experimental data. The results of the competition experiments also matched the competition simulation predictions quite well, although the experimental competition was generally more delayed than the simulations. In conclusion, the model simulation that incorporated the effect of diel rhythms in nutrient uptake clearly demonstrated that species diversity could be enhanced by different oscillation patterns in resource uptake, even under the condition of limitation by the same resource.     

PHOSPHATE‐LIMITED GROWTH OF PAVLOVA LUTHERI (PRYMNESIOPHYCEAE) IN CONTINUOUS CULTURE: DETERMINATION OF GROWTH‐RATE‐LIMITING SUBSTRATE CONCENTRATIONS WITH A SENSITIVE BIOASSAY PROCEDURE1

The relationship between steady‐state growth rate and phosphate concentration was studied for the marine prymnesiophyte Pavlova lutheri (Droop) J. C. Green grown in a chemostat at 22°C under continuous irradiance. A bioassay procedure involving short‐term uptake of 10 picomolar spikes of ³³P‐labeled phosphate was used to estimate the concentration of phosphate in the growth chamber. The relationship between growth rate and phosphate was well described by a simple rectangular hyperbola with a half‐saturation constant of 2.6 nM. The cells were able to take up micromolar spikes of phosphate at rates two to three orders of magnitude higher than steady‐state uptake rates. The kinetics of short‐term uptake displayed Holling type III behavior, suggesting that P. lutheri may have multiple uptake systems with different half‐saturation constants. Chl a:C ratios were linearly related to growth rate and similar to values previously reported for P. lutheri under nitrate‐limited conditions. C:N ratios, also linearly related to growth rate, were consistently lower than values reported for P. lutheri under nitrate‐limited conditions, a result presumably reflecting luxury assimilation of nitrogen under phosphate‐limited conditions. C:P ratios were linearly related to growth rate in a manner consistent with the Droop equation for growth rate versus cellular P:C ratio.     

PHOSPHORUS LIMITATION AND CARBON METABOLISM IN A UNICELLULAR ALGA: INTERACTION BETWEEN GROWTH RATE AND THE MEASUREMENT OF NET AND GROSS PHOTOSYNTHESIS1

Chlamydomonas reinhardii Dangeard was grown in continuous culture under P limitation at a range of dilution rates. Carbon uptake measurements were performed using double isotope (¹²C/¹⁴C) techniques and the fluxes of carbon in the light and dark were analysed over the range of growth rates. ¹⁴C uptake was shown to be equal to gross photosynthesis only at maximum relative growth rates; at low relative growth rates ¹⁴C uptake approximated net photosynthesis. The altered pattern of C uptake was found to be due to the suppression of dark respiration in the light and the release of ¹⁴C0₂ from respiratory pathways at low relative growth rates. Metabolic channelling of ¹⁴C from photosynthetic pathways to respiratory pathways occurred at low growth rates as the specific activity of the respired CO₂ reached 45% of the input gas mixture. These data are discussed in the light of the controversy concerning the measurement of gross and net photosynthesis in natural populations and in the light of models of ¹⁴C uptake in single celled algae. Existing models are shown to be adequate for high relative growth rates but not for low relative growth rates under P limitation.     

Compositional profiles of Rhodosporidium toruloides cells under nutrient limitation

Lipid production by the red yeast Rhodosporidium toruloides was explored under nutrient limitation. To determine the compositional profiles of R. toruloides cells, samples were prepared using a continuous cultivation process under nutrient limitation and analyzed via several methods, including Fourier transform infrared spectroscopy and elemental analysis. Under nitrogen limitation, as the dilution rate increased, the cellular lipid content decreased but the carbohydrate and protein contents increased. Under carbon limitation, the cellular lipid, protein, and carbohydrate contents remained relatively constant at the different dilution rates. Moreover, the cellular elemental composition was essentially identical under nitrogen and carbon limitation at a high dilution rate of 0.20 h⁻¹. We also analyzed the consumed carbon to nitrogen (C/N) under different nutrition conditions. The results indicated that the consumed C/N had a major influence on cell metabolism and product formation, which contributed to our understanding of the physiological characteristics of R. toruloides.

     

CHANGES IN NUTRIENT CONTENT OF PALMARIA PALMATA IN RESPONSE TO VARIABLE LIGHT AND UPWELLING IN NORTHERN SPAIN1

Light has been identified as one of the main factors affecting seaweed ecophysiology. We investigated the dependence of nutrient metabolism on sun and shade light conditions and whether episodes of upwelling of nutrient‐rich subsuperficial water could reduce the summer nutrient limitation driving physiological changes in Palmaria palmata (L.) Kuntze. We measured the major nutrient pools, photosynthetic pigments, and light curves, under sun and shade conditions during a summer period when one upwelling was recorded. The redundancy analysis (RDA) produced two clear groups: sun‐ and shade‐acclimated algae. Light was the major predictive factor. Sun‐acclimated algae exhibited higher carbon (C) and lower nitrogen (N) and phosphorus (P) content in association with the storage of floridoside (main C reserve) to benefit from higher irradiance (under nutrient limitation). Among N pools, N reserves (phycoerythrin, nitrate) were a lower proportion of the total N in sun‐acclimated algae, suggesting their degradation to fulfill the N demands of the cell. The orthophosphate content was also lower in sun‐acclimated algae, indicating its utilization as a nutrient reserve. In contrast, N within cell walls and membranes and chl a contributed to a similar proportion of the total N in sun‐ and shade‐acclimated algae, suggesting a response to sustain cell integrity. Transient high nutrient concentration due to the upwelling was unrelated to the nutrient content of the thallus. The storage of C as floridoside from high light exposure was shown to be the driving force for the metabolic adjustment of P. palmata at the end of summer before the onset of dormancy.     

Temporal plasticity in thermal‐habitat selection of burbot Lota lota a diel‐migrating winter‐specialist

In this study, animal‐borne telemetry with temperature sensors was coupled with extensive habitat temperature monitoring in a dimictic reservoir, to test the following hypotheses: behavioural thermoregulation occurs throughout the year and temperature selection varies on a diel and seasonal basis, in a winter‐specialist diel‐migrating fish. Burbot Lota lota demonstrated nightly behavioural thermoregulation throughout the year, with a large seasonal shift between selection for very cold temperatures (<2° C) optimal for reproduction during the spawning period and selection for warmer temperatures (12–14° C) optimal for hunting and feeding during non‐reproductive periods. During daylight hours, while L. lota avoided habitats warmer than optimal for reproduction and feeding during the spawning and non‐reproductive periods, respectively, active selection was limited to selection for 4–6° C habitat during the prespawning period. Although behavioural thermoregulation explained the night‐time migration, behavioural thermoregulation only partially explained daytime behaviour, indicating that diel migration is best explained by a combination of factors. Thus, thermal‐habitat selection was a good predictor of night‐time habitat occupancy in a diel‐migrating species. Together, these results show that thermal‐habitat selection by fishes may be important throughout the year and a more seasonally plastic behaviour than previously recognized.     

OPTIMUM N:P RATIOS AND COEXISTENCE OF PLANKTONIC ALGAE1

The optimum atomic ratio of N to P, the ratio at which one nutrient limitation changes over to the other, was determined in seven species of freshwater planktonic algae. The ratio varied over a wide range among species; the average for these species was 17. If the cellular nutrient ratios in marine species are comparable with those in freshwater organisms, Redfield's ratio of 15 is remarkably close to the average. Cellular N:P ratios varied over a 24-h period under a light:dark cycle. The variation of the optimum ratio between species and diel change in cellular N:P ratios within a species could play an important role in population dynamics by enhancing the probability of coexistence of species.     

Diel activity pattern and predation rate of the generalist predator Dicyphus hesperus

Abstract This study examined the diel activity pattern and the effect of diel activity pattern on predation rate and prey finding of Dicyphus hesperus Knight (Heteroptera: Miridae). To determine the diel activity pattern of D. hesperus, starved females were placed on tomato leaflets Lycopersicon esculentum Mill. (Solanaceae) under zero, low, or high light intensities at 02:00, 08:00, and 14:00 h, respectively, and the amount of time spent walking or resting during a 30‐min interval was recorded. Predation rates of D. hesperus females on Ephestia kuehniella Zeller (Lepidoptera: Pyralidae) egg patches were determined under either a L16:D8 (long day) or L8:D16 (short day) diel period. Egg patches were removed from D. hesperus females after either 8 or 16 h of dark or 16 or 8 h of light, and the number of eggs consumed was counted. Dicyphus hesperus females spent more time searching for prey at night than during the day. Females ate eggs at a higher rate during the night than during the day. Overall, D. hesperus females had higher predation rates when reared under a long day diel cycle compared with females reared under a short day diel cycle. More females reared at the L16:D8 diel cycle found the egg patch during the night than during the day. There was no difference in egg patch finding between night and day for females reared at L8:D16. Overall, L16:D8 reared females found more egg patches than females reared at L8:D16. Therefore, D. hesperus females are more active and find and consume prey at a higher rate at night than day.     

ELEVATED ATMOSPHERIC CARBON DIOXIDE INCREASES ORGANIC CARBON FIXATION BY EMILIANIA HUXLEYI (HAPTOPHYTA), UNDER NUTRIENT‐LIMITED HIGH‐LIGHT CONDITIONS1

Phytoplankton play a key role in determining the partitioning of CO₂ between the atmosphere and the ocean on seasonal, interannual, and millennial time scales. The magnitude of biological draw‐down of atmospheric CO₂ and C storage in the oceans is affected by concurrent changes in other environmental factors, like nutrient supply. Furthermore, variations in carbon‐to‐nitrogen (C:N) and carbon‐to‐phosphorus (C:P) assimilation ratios modify the oceanic CO₂ storage capacity. Here we show that increased atmospheric CO₂ concentration enhances CO₂ fixation into organic matter by a noncalcifying strain of Emiliania huxleyi (Lohmann) Hay & Mohler only under certain conditions, namely high light and nutrient limitation. Enhanced organic matter production was accompanied by marked deviations of the C:N:P ratio from the canonical stoichiometry of marine particulate matter of 106:16:1 (C:N:P) known as the Redfield ratio. Increased cell organic carbon content, C:N, and C:P were observed at high light when growth was either nitrogen or phosphorus limited. Elevated CO₂ led to further increases in the particulate C:N and C:P ratios. Enhanced CO₂ uptake by phytoplankton such as E. huxleyi, in response to elevated atmospheric CO₂, could increase carbon storage in the nitrogen‐limited regions of the oceans and thus act as a negative feedback on rising atmospheric CO₂ levels.     

Belowground carbon flux links biogeochemical cycles and resource‐use efficiency at the global scale

Nutrient limitation is pervasive in the terrestrial biosphere, although the relationship between global carbon (C) nitrogen (N) and phosphorus (P) cycles remains uncertain. Using meta‐analysis we show that gross primary production (GPP) partitioning belowground is inversely related to soil‐available N : P, increasing with latitude from tropical to boreal forests. N‐use efficiency is highest in boreal forests, and P‐use efficiency in tropical forests. High C partitioning belowground in boreal forests reflects a 13‐fold greater C cost of N acquisition compared to the tropics. By contrast, the C cost of P acquisition varies only 2‐fold among biomes. This analysis suggests a new hypothesis that the primary limitation on productivity in forested ecosystems transitions from belowground resources at high latitudes to aboveground resources at low latitudes as C‐intensive root‐ and mycorrhizal‐mediated nutrient capture is progressively replaced by rapidly cycling, enzyme‐derived nutrient fluxes when temperatures approach the thermal optimum for biogeochemical transformations.     

DIEL CHANGES IN THE COMPOSITION OF PHOTOSYNTHETIC PIGMENTS AND CELLULAR CARBON AND NITROGEN IN PYRAMIMONAS PARKEAE (PRASINOPHYCEAE)1

Diel changes in mean cell volume, cellular carbon (carbon content per cell), cellular Chl a, C/N ratio, Chl a/carbon ratio and pigment composition were determined for an axenic clonal culture of Pyramimonas parkeae Norris et Pearson through three 12:12 h LD cycles in a laboratory culture tank of 1 m³. Mean cell volume and cellular C, N and most pigments increased during the light period as a result of photosynthesis and decreased with an increase in cell density by phased cell division during the dark period. Chi a and Chi b increased in a parallel manner during the light period. Increases in the diel synthesis pattern of carotenoids varied. Violaxanthin and lutein content increased for a few hours at the beginning of the light period and preceeded that of neoxanthin. The diel synthesis pattern of neoxanthin was similar to that of Chi a. Increases of loroxanthin and its ester form were slower than that of Chi a at the beginning of the light period. A net increase of α-carotene was observed during the dark period. Mass spectroscopy of carotenoid structure showed a new xanthophyll, loroxanthin dodecenoate, in this species.     

Effect of temperature and soluble reactive phosphorus on abundance of Aphanizomenon flos-aquae (Cyanophyceae)

Filament density of Aphanizomenon flos‐aquae (Lemmerm.) Ralfs, water temperature and soluble reactive phosphorus (SRP) were measured from April to August in 1993–1996 in Lake Barato, Hokkaido, Japan. In addition, growth characteristics and internal phosphorus (P) utilization of Aph. flos‐aquae were evaluated under P limitation at three temperatures (15, 20 and 25?C) to clarify the role of internal accumulated P for its growth in the incubation experiment. The filament density was highest in early July 1994, when SRP concentration had not yet decreased and the water temperature was high. These are important factors favoring an increase in abundance of this species in L. Barato. During batch culture, the time course of the stationary phase was shortest at 25?C and longest at 15?C; the cellular C:P molar ratio was 111 under P sufficiency and increased eight‐ to 12‐fold under P limitation. As the C:P ratio was significantly higher in the decreasing phase at 15?C, Aph. flos‐aquae may be more adaptable to Plimitation at 15?C than at 20?C and 25?C. However, the low temperatures did not favor the abundance of Aph. flos‐aquae in 1996. This indicates that the filament density of Aph. flos‐aquae decreases before it reaches the maximum value for some reason under P limitation in L. Barato.     

Photosynthetic and growth responses of three freshwater algae to phosphorus limitation and daylength

1. Three common species of freshwater phytoplankton, the diatom Nitzschia sp., green alga Sphaerocystis schroeteri and cyanobacterium Phormidium luridum, were grown under contrasting daylengths [18 : 6 h light : dark cycles (LD) versus 6 : 18 h LD] and phosphorus (P) regimes (P‐sufficient versus 1 μm P). The rates of growth and photosynthesis, as well as growth efficiencies and pigment concentrations, were compared among treatments. 2. The growth and photosynthetic parameters of the three species depended on both P status and daylength in a species‐specific way. The responses to P limitation depended on daylength and, conversely, the responses to daylength depended on P status. 3. Growth rates and the maximum rates of photosynthesis (P_max) of all species decreased under P limitation under both light regimes. However, the decrease of P_max because of P limitation was greater under long daylength. The P_max of the green alga S. schroeteri decreased the most (ca. sixfold) under P limitation compared with the other two species. The photosynthesis saturation parameter I_k also decreased under P limitation; the decline was significant in Nitzschia and Sphaerocystis. P‐limitation significantly increased photoinhibition (β) in Nitzschia and Sphaerocystis, but not in Phormidium. The excess photochemical capacity (the ratio of the maximum photosynthesis rate to the photosynthesis rate at the growth irradiance), characterising the ability to utilise fluctuating light, was significantly lower under P limitation. 4. The growth efficiency (growth rate normalised to daylength) declined with increasing daylength in all species. Under short daylength the cyanobacterium Phormidium had the lowest growth efficiency of the three species. 5. The cellular chlorophyll a concentration in both Nitzschia and Sphaerocystis was significantly higher under short daylength, but only under P‐sufficient conditions. In Nitzschia, under short daylength, P‐limitation significantly decreased cellular chlorophyll concentration. In contrast, P‐limitation increased cellular chlorophyll concentration in Sphaerocystis, but under long daylength only. The ratio of chlorophyll a to b in the green alga also declined under short daylength and under P‐limited conditions.