PHOSPHORUS UPTAKE KINETICS OF SPIROGYRA FLUVIATILIS (CHAROPHYCEAE) IN FLOWING WATER1,2

The inorganic phosphorus (P_i) uptake kinetics of Spirogyra fluviatilis Hilse were examined as a function of phosphorus cell quota (Q^P) and flow velocity in a laboratory stream apparatus. Short-term uptake and the acclimation of the uptake mechanism to flow were measured by the disappearance of P_i pulses in a recirculating flow cell. Short-term P_i uptake was biphasic. When the alga was P-deficient, Phase 1 and 2 half-saturation constants and maximum uptake rates were 11.0 and 47.2 μg P·L^?1 and 473 and 803 μg P·g dry wt^?1 h^?1, respectively. Flowing water altered short-term uptake when the alga was P-deficient, but not when it was P-replete. When Q^P was less than 0.21%, increases in flow velocity from 3 to 15 cm·s^?1 enhanced uptake with maximum uptake for any P_i pulse at 12 and 15 cm·s^?1. At 22 and 30 cm·s^?1, uptake was reduced by 12% or more relative to the maxima. If, however, the alga was cultivated at 22 and 30 cm·s^?1 and short-term P_i uptake was measured at 12 cm·s^?1, uptake was on average 33% greater than when the alga was cultivated at the latter velocity. Apparently, the alga could adjust short-term uptake to compensate for the suboptimal conditions of the faster velocities. Long-term P_i uptake and net phosphorus efflux were estimated by a non-steady state application of the Droop equation. Long-term uptake of very low P_i concentrations was not reduced by fast flowing water. Instead, uptake increased proportionately with flow velocity. Maximum phosphorus efflux from S. fluviatilis was 3% of cellular P per hour and occurred when Q^P was greater than 0.2%. At lower Q^P, the hourly efflux rate was typically less than 1%. Flowing water did not greatly enhance efflux, although when P_i was undetectable, efflux did tend to increase slightly with velocity. The data show that the effects of flowing water on P_i uptake were varied and not always beneficial. If the effects of flowing water on nutrient acquisition by other lotic algae are similarly varied and complex, flow may be an important determinant of nutrient partitioning among benthic algae in streams.     

TEMPERATURE AND IRRADIANCE EFFECTS ON GROWTH OF PITHOPHORA OEDOGONIA (CHLOROPHYCEAE) AND SPIROGYRA SP. (CHAROPHYCEAE)1

Growth responses of Pithophora oedogonia (Mont.) Wittr. and Spirogyra sp. to nine combinations of temperature (15°, 25°, and 35°C) and photon flux rate (50, 100, and 500 μmol·m^?2·s^?1) were determined using a three-factorial design. Maximum growth rates were measured at 35°C and 500 pmol·m^?2·s^?1 for P. oedogonia (0.247 d^?1) and 25°C and 500 μmol·m^?2·s^?1 for Spirogyra sp. (0.224 d^?1). Growth rates of P. oedogonia were strongly inhibited at 15°C (average decrease= 89%of maximum rate), indicating that this species is warm stenothermal. Growth rates of Spirogyra sp. were only moderately inhibited at 15° and 35°C (average decrease = 36 and 30%, respectively), suggesting that this species is eurythermal over the temperature range employed. Photon flux rate had a greater influence on growth of Spirogyra sp. (31% reduction at 50 pmol·m^?2·s^?1 and 25°C) than it did on growth of P. oedogonia (16% reduction at 50 μmol·m^?2·s^?1 and 35°C). Spirogyra sp. also exhibited much greater adjustments to its content of chlorophyll a (0.22–3.34 μg·mg fwt^?1) than did P. oedogonia (1.35–3.08 μg·mg fwt^?1). The chlorophyll a content of Spirogyra sp. increased in response to both reductions in photon flux rate and high temperatures (35°C). Observed species differences are discussed with respect to in situ patterns of seasonal abundance in Surrey Lake, Indiana, the effect of algal mat anatomy on the internal light environment, and the process of acclimation to changes in temperature and irradiance conditions.     

GROWTH RATE DEPENDENT OPTIMUM RATIOS IN SELENASTRUM MINUTUM (CHLOROPHYTA): IMPLICATIONS FOR COMPETITION,COEXISTENCE AND STABILITY IN PHYTOPLANKTON COMMUNITIES 12

Steady-State growth equations predict that the optimum C:P ratio (R) of Selenastrum minutum (Naeq.) Collins should change by a factor of 20 over the growth range of this organism. Chemostat cultures were established at fixed inflow C:P ratios chosen such that a transition from carbon to phosphorus limitation should occur solely as a result of increasing the steady-state growth rate. Measurements of residual dissolved inorganic carbon (DIC), cellular C:P, the kinetics of photosynthesis with respect to [DIC] and the response of culture biomass lo DIC or K₂HPO₄ additions were obtained. These results show that optimum ratios are growth rate dependent and that this dependency can be predicted based on steady-stale algal growth equations. A theoretical analysis was undertaken evaluating the range of growth rate dependent changes in the optimum ratio which could be expected for different nutrient pairs. Further analysis showed that, under certain conditions, the growth rate dependence of the optimum ratio may alter the breadth of zones of stable coexistence between species and allow for either the formation or complete elimination of such zones.     

EFFECTS OF TEMPERATURE AND IRRADIANCE ON THE SEASONAL VARIATION OF A SPIROGYRA (CHLOROPHYTA) POPULATION IN A MIDWESTERN LAKE (U.S.A.)

Although Spirogyra Link (1820) is a common mat‐forming filamentous alga in fresh waters, little is known of its ecology. A 2‐year field study in Surrey Lake, Indiana, showed that it grew primarily in the spring of each year. The population consisted of four morphologically distinct filamentous forms, each exhibiting its own seasonal distribution. A 45‐μm‐wide filament was present from February to late April or early May, a 70‐μm‐wide form was present from late April to mid‐June, a 100‐μm‐wide form was present from February to mid‐June, and a 130‐μm‐wide form appeared only in February of 1 of 2 study years. The 70‐ and 100‐μm‐wide forms contributed to the peak amount of biomass observed in late May and early June. Multiple regression analysis indicated that the presence of the 45‐, 70‐, and 100‐μm‐wide forms was negatively correlated with temperature. Presence of the 130‐μm‐wide form was negatively correlated with irradiance. Isolates of these filament forms were exposed to temperature (15, 25, and 35° C)/irradiance (0, 60, 200, 400, 900, and 1500 μmol·m^?2·s^?1) combinations in the laboratory. Growth rates of the 45‐μm‐wide form were negative at all irradiances at 35° C, suggesting that this form is susceptible to high water temperatures. However, growth rates of the other forms did not vary at the different temperatures or at irradiances of 60 μmol·m^?2·s^?1 or above. Net photosynthesis was negative at 35° C and 1500 μmol·m^?2·s^?1 for the 100‐ and 130‐μm‐wide forms but positive for the 70‐μm‐wide form. All forms lost mat cohesiveness in the dark, and the 100‐ and 130‐μm‐wide forms lost mat cohesiveness under high irradiances and temperature. Thus, the morphological forms differed in their responses to irradiance and temperature. We hypothesize that the rapid disappearance of Spirogyra populations in the field is due to loss of mat cohesiveness under conditions of reduced net photosynthesis, for example, at no to low light for all forms or at high light and high temperatures for the 100‐ and 130‐μm‐wide forms. Low light conditions can occur in the interior of mats as they grow and thicken or under shade produced by other algae.     

SHORT‐TERM EFFECTS OF ACETATE AND MICROAEROBIC CONDITIONS ON PHOTOSYNTHESIS AND RESPIRATION IN CHLORELLA SOROKINIANA GXNN 01 (CHLOROPHYTA)1

The culture of microalgae using organic carbon sources decreases the cost of operation in closed systems. The effect of carbon sources on microalgae is thus an interesting problem in not only theoretical research but also practical production. The short‐term effects of acetate and microaerobic conditions on the growth, photosynthesis, and respiration of the green microalga Chlorella sorokiniana I. Shihira & R.W. Krauss GXNN 01 were described after acetate addition to autotrophic cultures. As the acetate concentration increased, cells needed a longer lag phase to grow, and 243.8 mM acetate completely inhibited growth. Acetate addition induced an immediate response in photosynthesis and respiration. The activity of PS II and PS I were impaired and declined with different rates, and then recovered compared with autotrophic cells. Carbonic anhydrase and Rubisco activities were also inhibited at the beginning, and respiration was increased. We propose that ATP consumption for acetate assimilation results in surplus NADPH, and then accumulated reducing power over‐reduces inter‐photosystem components and raises the transthylakoid proton gradient, which redistributes energy between PS I and PS II, and leads to a decrease in the PS II/PS I ratio and O₂ evolution. An apparent cyclic electron flow was also observed, which may be mainly mediated by NAD(P)H dehydrogenase‐dependent pathway since NADPH was in excess. These observations pointed to an acclimation process after acetate addition, and suggested the interaction between photosynthesis and respiration involving ATP and reducing power.     

氮磷水平对龙须菜生长和光合特性的影响

研究不同营养盐条件对龙须菜(Gracilaria lemaneiformis)的生理效应, 对深入了解龙须菜与近海环境的相互作用具有重要意义。在低氮低磷(LNLP)、低氮高磷(LNHP)、高氮低磷(HNLP)和高氮高磷(HNHP) 4种营养盐条件下培养龙须菜15 d, 以探讨不同氮、磷水平对龙须菜生长和光合特性的影响。结果表明: 1) LNHP、HNLP和HNHP处理促进了龙须菜的生长, 其中HNHP处理下龙须菜具有最大的相对生长速率和生物量; 2) LNHP、HNLP和HNHP处理提高了龙须菜的光合无机碳利用能力, 其中HNHP处理下龙须菜具有最大的无机碳饱和光合速率和表观半饱和常数, 比LNLP处理分别提高了118%倍和48.71%; 3) LNHP、HNLP和HNHP处理显著影响龙须菜的光化学效率, 与LNLP处理相比, LNHP处理提高了龙须菜的光化学效率, 而HNLP和HNHP处理降低了龙须菜的光化学效率。研究结果表明, HNHP处理条件下, 龙须菜的生长和光合无机碳利用能力最高, 光化学效率最低。     

A COMPARISON OF AIR AND WATER AS ENVIRONMENTS FOR PHOTOSYNTHESIS BY THE INTERTIDAL ALGA FUCUS SPIRALIS (PHAEOPHYTA)1

The response of photosynthesis and respiration of the intertidal brown alga Fucus spiralis L. to light and temperature at ambient and elevated concentrations of inorganic carbon was investigated. The light-saturated rate of photosynthesis was greater in air at 15° C and 20° C, but greater in water at 10° C. Light compensation point and I_k was about 50% lower under submerged relative to emerged conditions, whereas the initial slope of photosynthesis versus irradiance was higher, except at 20° C. Under both submerged and emerged conditions light-saturated photosynthesis was limited to a similar degree (78%, and 65%, respectively) by the availability of inorganic carbon at naturally occuring concentrations. In air, slight desiccation at tissue water contents of about 96% to 92% caused a stimulation in the rate of net photosynthesis to 110–148% of fully hydrated fronds. At lower water contents the rate of net photosynthesis declined linearly with decreasing water content and became zero at a water content of about 15%. Dark respiration declined linearly with tissue water content and remained positive to a water content of 8%. Upon reimmersion the fronds showed a complete recovery within 35 min following desiccation to a water content of 20–30%. Thus F. spiralis seems to be very tolerant to desiccation. Since F. spiralis photosynthesizes effectively in air, even at a higher rate than in water as long as it has not lost a large proportion of its water in desiccation, the alternating exposure to air may be beneficial by increasing the daily carbon gain compared to a fully submerged situation.     

EFFECTS OF INCREASED TEMPERATURE AND CO2 ON PHOTOSYNTHESIS,GROWTH, AND ELEMENTAL RATIOS IN MARINE SYNECHOCOCCUS AND PROCHLOROCOCCUS (CYANOBACTERIA)1

Little is known about the combined impacts of future CO₂ and temperature increases on the growth and physiology of marine picocyanobacteria. We incubated Synechococcus and Prochlorococcus under present‐day (380 ppm) or predicted year‐2100 CO₂ levels (750 ppm), and under normal versus elevated temperatures (+4°C) in semicontinuous cultures. Increased temperature stimulated the cell division rates of Synechococcus but not Prochlorococcus. Doubled CO₂ combined with elevated temperature increased maximum chl a–normalized photosynthetic rates of Synechococcus four times relative to controls. Temperature also altered other photosynthetic parameters (α, Φ_max, E_k, and ) in Synechococcus, but these changes were not observed for Prochlorococcus. Both increased CO₂ and temperature raised the phycobilin and chl a content of Synechococcus, while only elevated temperature increased divinyl chl a in Prochlorococcus. Cellular carbon (C) and nitrogen (N) quotas, but not phosphorus (P) quotas, increased with elevated CO₂ in Synechococcus, leading to ～20% higher C:P and N:P ratios. In contrast, Prochlorococcus elemental composition remained unaffected by CO₂, but cell volume and elemental quotas doubled with increasing temperature while maintaining constant stoichiometry. Synechococcus showed a much greater response to CO₂ and temperature increases for most parameters measured, compared with Prochlorococcus. Our results suggest that global change could influence the dominance of Synechococcus and Prochlorococcus ecotypes, with likely effects on oligotrophic food‐web structure. However, individual picocyanobacteria strains may respond quite differently to future CO₂ and temperature increases, and caution is needed when generalizing their responses to global change in the ocean.     

STEADY-STATE LUXURY CONSUMPTION AND THE CONCEPT OF OPTIMUM NUTRIENT RATIOS: A STUDY WITH PHOSPHATE AND NITRATE LIMITED SELENASTRUM MINUTUM (CHLOROPHYTA)1

Selenastrum minutum (Naeg.) Collins was grown over a wide range of growth rates under phosphate or nitrate limitation with non-limiting nutrients added to great excess. This resulted in saturated luxury consumption. The relationships between growth rate and cell quota for the limiting nutrients were well described by the Droop relationship. The observed variability in N cell quota under N limitation as reflected in k_Q·Q_max^?1*, was similar in magnitude to previously reported values but k_Q·Q_max^?1* for P under P limitation was greater than previously reported for other species. These results were evaluated in light of the optimum ratio hypothesis. Our findings support previous work suggesting that the use of a single optimum ratio (k_Qi·K_Qj^?1) is inappropriate for dealing with a species growing under steady-state nutrient limitation. Under these conditions the optimum ratio should be viewed as a growth rate dependent variable. Two approaches for testing the growth rate dependency of optimum ratios are proposed. The capacity for luxury consumption differed between nutrients and was growth rate dependent. At low growth rates, the coefficient of luxury consumption (R_sat) for P was ca. four times that for N. The set of all possible relationships between N and P cell quota under these conditions was reported and these values were then used to establish the cellular N:P niche boundaries for S. minutum. Cell quotas of non-limiting nutrients were not described by the Droop equation. Analysis showed that as the cellular N:P ratio deviates from the optimum ratio, the ability of the Droop equation to describe the relationship between growth rate and non-limiting cell quotas decreases. When non-limiting nutrient cell quotas are saturated, the Droop equation appears to be invalid. Previously reported patterns of non-limiting nutrient utilization are summarized in support of this conclusion. The physiological and ecological consequences of luxury consumption and growth rate dependent optimum ratios are considered.     

SEASONAL VARIATIONS OF PHOTOSYNTHETIC PIGMENTS, TOTAL C, N, AND P CONTENT, AND PHOTOSYNTHESIS IN PHYLLARIOPSIS PURPURASCENS: (PHAEOPHYTA) FROM THE STRAIT OF GIBRALTAR

Photosynthetic pigments, C, N, and P tissue composition, and photosynthetic rate were measured from April to October in the brown alga Phyllariopsis purpurascens (C. Agardh) Henry et South (Laminariales, Phaeophyta) growing at a 30-m depth in the Strait of Gibraltar. Ir-radiance reaching the population ranged from 13.5 to 27.5 mol.m^-2.mo^-1. The available light for this species, expressed as a percentage of the irradiance above the water, was 1.8%. Dissolved inorganic nitrogen forms, NO3-and NH₄+, were constant from April to October, whereas phosphate was depleted in August. Chlorophyll a decreased from 520.0 ± 165.0 to 199.6 ± 159.9 μg.g^-1 dry weight; in contrast, chlorophyll c and carotenoids did not change until September but increased threefold in October. C:N and N:P ratios changed in the same way and in the same range. They were constant until July but increased from 15–17 up to 42 (C:N) and from 14 to 40 (N:P) in October, suggesting a severe P limitation of growth of this species. The dark respiration rate and the light compensation point were constant from April to October (0.5 ± 0.1 μmol O₂. m^-2.s^-1 and 6.5 ± 0.2 μmol.m^-2. s^-1, respectively), whereas the maximum rate of apparent photosynthesis, light onset saturation parameter, and half saturation constant for light were maximum in April to May (3.7 μmol O₂. m^-2.s^-1and 40 and 41.5 μmol.m^-2. s^-1, respectively) and October (3.6 μmol O₂. m^-2.s^-1 and 50 and 53.7 μmol.m^-2. s^-1, respectively). They were minimum in August (1.2 μmol O₂.m^-2.s^-1 and 11.3 and 12 μmol.m^-2.s^-1, respectively). These minimum figures yielded a negative carbon budget in August and 0 in September, whereas it was positive the rest of the year. Photosynthetic efficiency, estimated by the ratio between maximum apparent photosynthesis and light half saturation constant, showed a strong agreement with productivity measured by means of an independent method. These results indicate that lamina expansion in this species is controlled by photosynthetic efficiency.     

SHORT‐ AND LONG‐TERM EFFECTS OF ELEVATED CO2 ON PHOTOSYNTHESIS AND RESPIRATION IN THE MARINE MACROALGA HIZIKIA FUSIFORMIS (SARGASSACEAE,PHAEOPHYTA) GROWN AT LOW AND HIGH N SUPPLIES1

The short‐term and long‐term effects of elevated CO₂ on photosynthesis and respiration were examined in cultures of the marine brown macroalga Hizikia fusiformis (Harv.) Okamura grown under ambient (375 μL · L^?1) and elevated (700 μL · L^?1) CO₂ concentrations and at low and high N availability. Short‐term exposure to CO₂ enrichment stimulated photosynthesis, and this stimulation was maintained with prolonged growth at elevated CO₂, regardless of the N levels in culture, indicating no down‐regulation of photosynthesis with prolonged growth at elevated CO₂. However, the photosynthetic rate of low‐N‐grown H. fusiformis was more responsive to CO₂ enrichment than that of high‐N‐grown algae. Elevation of CO₂ concentration increased the value of K_1/2(Ci) (the half‐saturation constant) for photosynthesis, whereas high N supply lowered it. Neither short‐term nor long‐term CO₂ enrichment had inhibitory effects on respiration rate, irrespective of the N supply, under which the algae were grown. Under high‐N growth, the Q₁₀ value of respiration was higher in the elevated‐CO₂‐grown algae than the ambient‐CO₂‐grown algae. Either short‐ or long‐term exposure to CO₂ enrichment decreased respiration as a proportion of gross photosynthesis (Pg) in low‐N‐grown H. fusiformis. It was proposed that in a future world of higher atmospheric CO₂ concentration and simultaneous coastal eutrophication, the respiratory carbon flux would be more sensitive to changing temperature.     

EFFECTS OF NITRATE CONCENTRATION ON THE GROWTH AND PHYSIOLOGY OF LAMINARIA SACCHARINA (PHAEOPHYTA) IN CULTURE1,2

Laminaria saccharina Lamour. sporophytes were grown in enriched and synthetic media through a range of nitrate concentrations, There was an approximately linear relationship between growth and nutrient concentration up to 10 μ substrate concentration. The half-saturation constant (K_{2) was ca. 1.4 μ NO3-. The internal levels of NO3- increased at substrate concentrations above 10 μM b>3- and reached levels several thousand times higher than the surrounding medium. Thus there is evidence for luxury consumption of NOsb>3}^-. The chlorophyll content and photosynthetic capacities of plants also increased with increasing external NO₃^- The ecological implications of this work are considered.     

EFFECTS OF Si:P CONCENTRATION RATIOS AND NUTRIENT LIMITATION ON THE CELLULAR COMPOSITION AND MORPHOLOGY OF ASTERIONELLA FORMOSA (BACILLARIOPHYCEAE)1

The freshwater diatom Asterionella formosa Haas. was grown in semicontinuous culture at 20°C under continuous cool-white fluorescent light of ca. 20 μEin · m^?2· s ·^?1 in a medium containing Si: P in various concentration ratios. The cell quotas of P and Si changed in relation to the available concentrations of P and Si at constant μ= 0.11 and 0.16 d^?1. Under Si-limitation, the P cell quota increased by over an order of magnitude as the influent [Si:P] decreased. The Si cell quota increased with increase in [Si] in the influent medium, and it increased as [P] increased at a specific [Si]. Under P-limitation, the P cell quotas were fairly constant and low; the Si cell quotas were relatively high and decreased slightly as influent [P] and [Si] increased. Asterionella stored up to 28 times more P and 2 times more Si than needed. The number of Asterionella cells per colony varied as a function of the influent [Si:P] and nutrient limitation being usually less than or equal to 6 when P-limited, and greater than 10 when Si-limited.     

THE RELATIVE IMPORTANCE OF WATER MOTION ON NITROGEN UPTAKE BY THE SUBTIDAL MACROALGA ADAMSIELLA CHAUVINII (RHODOPHYTA) IN WINTER AND SUMMER1

The influence of seawater velocity (1.5–12 cm · s^?1) on inorganic nitrogen (N) uptake by the soft‐sediment perennial macroalga Adamsiella chauvinii (Harv.) L. E. Phillips et W. A. Nelson (Rhodophyta) was determined seasonally by measuring uptake rate in a laboratory flume. Regardless of N tissue content, water velocity had no influence on NO₃^? uptake in either winter or summer, indicating that NO₃^?‐uptake rate was biologically limited. However, when thalli were N limited, increasing water velocity increased NH₄⁺ uptake, suggesting that mass‐transfer limitation of NH₄⁺ is likely during summer for natural populations. Uptake kinetics (V_max, K_s) were similar among three populations of A. chauvinii at sites with different mean flow speeds; however, uptake rates of NO₃^? and NH₄⁺ were lower in summer (when N status was generally low) than in winter. Our results highlight how N uptake can be affected by seasonal changes in the physiology of a macroalga and that further investigation of N uptake of different macroalgae (red, brown, and green) during different seasons is important in determining the relative influence of water velocity on nutrient uptake.     

PHOSPHORUS AND NITROGEN NUTRITION IN CHONDRUS CRISPUS (RHODOPHYTA): EFFECTS ON TOTAL PHOSPHORUS AND NITROGEN CONTENT,CARRAGEENAN PRODUCTION,AND PHOTOSYNTHETIC PIGMENTS AND METABOLISM1

The existence of a phenomenon in phosphorus (P) nutrition comparable to the “Neish effect” in nitrogen (N) nutrition (an inverse relation between seawater N enrichment and carrageenan content) was investigated in the temperate red alga Chondrus crispus Stackhouse. Plants were preconditioned for 17 d and then cultured under varying enrichments of P (0, 3, 6, 10, 15 μM P·wk^?1) and a constant N enrichment (53.5 μM N·wk^?1) for 5 wk. Tissue total P, tissue total N, and carrageenan contents were then determined. Identical experiments were performed using C. crispus collected during the fall, winter, spring, and summer seasons. The procedure was repeated using material collected during the following fall season and cultured under constant P (6 μM P·wk^?1) and varying N enrichments (0, 3, 6, 10, 25 μM N·wk^?1). In the fall (P) experiment, carrageenan content was the highest [53.1 ± 0.3% DW (dry weight)], and tissue total P content was the lowest (1.71 ± 0.27 mg P·g DW^?1) in plants that received no P enrichment. Carrageenan content was stable (46.1 ± 1.8% DW) for plants given enrichments of 3 μM P·wk^?1 and greater. Thus, a decrease in carrageenan content, concomitant with an increase in tissue total P content, was observed, but only at tissue total P levels below 2 mg P·g DW^?1. As these levels were always higher than 2 mg P·g DW^?1 in the winter, spring, and summer experiments, carrageenan content remained constant within each season at 46.2 ± 1.3, 43.1 m 0.7, and 44.5 ± 0.6% DW, respectively. Nitrogen enrichment of plants collected in the fall did not affect carrageenan content, which was stable at 49.3 ± 0.9% DW. When these plants were compared with those of the previous fall experiment (6 μM P·wk^?1 and 53.5 μM N·wk^?1), a slight increase in carrageenan content was noted. Thus, at sufficiently high concentration, N also decreased carrageenan content in C. crispus. Phosphorus nutrition had no significant effect on photosynthesis versus irradiance parameters (P_max, α, R_d, I_c, and I_k), the contents of the photosynthetic pigments chlorophyll-a, phycoerythrin (PE), phycocyanin (PC), and allophycocyanin (APC), and the ratios PE:APC and PC:APC. In contrast, N nutrition affected both P_maxand the photosynthetic pigment contents. The data indicate that N limitation reduces the number of phycobilisomes but not their size. The greater reduction in phycobiliprotein than chlorophyll-acontent corroborates the natural bleaching phenomenon regularly observed in C. crispus populations during summer when N levels are generally low in seawater. These results suggest that C. crispus in the temperate waters of the Bay of Fundy may experience N limitation, but P limitation is unlikely.