Temperature dependence of inorganic nitrogen uptake and assimilation in Antarctic sea-ice microalgae

Summary The influence of temperature on NO ₃ ^- and NH ₄ ⁺ uptake, and the activity of the assimilatory enzyme NO ₃ ^- reductase (NR) was compared to inorganic C uptake (photosynthesis) in natural assemblages of Antarctic sea-ice microalgae. NO ₃ ^- and NH ₄ ⁺ uptake reached a maximum between 0.5°–2.0°C and 2.0°–3.0°C, respectively, which was close to that for photosynthesis (2.5°–3.0°C). NR showed a distinctly higher temperature maximum (10.0°–12.0°C) and a lower Q₁₀ value than inorganic N and C transport. Our data imply that, owing to differential temperature characteristics between N transport and N assimilation at in situ temperature (-1.9°C), the incorporation of extracellular NO ₃ ^- into cellular macromolecules, may be limited by transport of NO ₃ ^- into the cell rather than the intracellular reduction of NO ₃ ^- to NH ₄ ⁺ . Despite differences in temperature maxima between N transport and N assimilation, the overall low temperature maxima of inorganic N metabolism characterizes Antarctic sea-ice microalgae as psychrophilic. Our study is the first to examine the temperature dependence of inorganic N uptake and assimilation in sea-ice microbial communities.     

BIOCHEMICAL COMPOSITION AND PHOTOSYNTHETIC CARBON METABOLISM OF NUTRIENT LIMITED CULTURES OF MERISMOPEDIA TENUISSIMA (CYANOPHYCEAE)1

Continuous cultures of Merismopedia tenuissima Lemmerman, limited by phosphorus, nitrogen, sulfur, or carbon, were compared to non limited batch cultures by two methods. The cellular content of photosynthetic pigments (chlorophyll and phycocyanin) was found to decrease in all nutrient limited cultures, except for the carbon limited culture. The ratio of carbohydrate to protein was 4- to 7-fold higher in P, N or S limited cultures than in non-limited or C limited cultures. The macromolecular products of photosynthesis were determined in samples to which NaH¹⁴CO₃ was added. Relative incorporation into protein decreased in P or N limited cultures, increased accumulation of low molecular weight compounds was found in S and P limited cultures, and little change was noted in C limited cultures as compared to non-limited cultures. Although relative incorporation into protein was significantly greater at 20μEin·m^?2·s^?1 light intensity than at 180 μEin·m^?2.s^?1 in non-limited cultures, this effect was abolished in all nutrient limited cultures. These results suggest that measurement of the cellular carbohydrate to protein ratio and the products of photosynthesis would be useful in the analysis of algal population dynamics in nature.     

Growth of Bacteria Degrading Naphthalene and Salicylate at Low Temperatures

A total of 58 bacterial strains degrading naphthalene and salicylate were isolated from soil samples polluted with oil products, collected in different regions of Russia during winter and summer. The isolates were assessed for their ability to grow at low temperatures (4, 8, and 15°C); bacteria growing at 4°C in the presence of naphthalene or salicylate accounted for 65 and 53%, respectively, of the strains isolated. The strains differed in the temperature dependence of their growth rates. It was demonstrated that the type of expression of the Nah⁺ phenotype at low temperatures depended on the combination of host bacterium and plasmid.     

Phytoplankton–bacterioplankton interactions in a neotropical floodplain lake (Laguna Bufeos,Bolivia)

Laguna Bufeos is a floodplain lake of the river Ichilo, a tributary of the Amazon basin situated in Bolivia. Nutrient addition assays involving whole water (<200 μm) as well as fractionated water (<0.8 μm) treatments were carried out in incubation tubes to test whether bacterial growth is limited by the availability of inorganic nutrients and to test whether bacteria are able to utilize inorganic nutrients directly or are stimulated by inorganic nutrients through increased production of phytoplankton. The assays were carried out during two extreme hydrological conditions, the high-water and the low-water period. During the high-water period experiment, neither N or P limited bacterial growth rates. During the low-water period, bacterial growth was P limited. Bacterial growth was stimulated in the fractionated as well as in the whole water treatments, indicating that bacterial growth was directly stimulated by P. Bacterial growth corrected for grazing losses (determined by means of dilution experiments) was significantly higher in the fractionated water containing only bacteria when compared to the whole water containing also grazers and phytoplankton. This suggests that bacterial growth was suppressed by competition with phytoplankton rather than stimulated through the production of dissolved organic matter by phytoplankton.     

Bacterial metabolism in the River Danube: parameters influencing bacterial production

1. Microbial parameters were determined at five sampling sites in the River Danube up-and downstream of Vienna, Austria, twice monthly over an annual cycle. Bacterial production (BP) was estimated from thymidine and leucine incorporations; additionally, the effect of turbulence on BP and the conversion factors for converting incorporation rates into bacterial cell production were determined using the cumulative approach. 2. BP under turbulent conditions was not significantly different from that under stagnant conditions. For thymidine, a mean annual conversion factor of 3.2 ± 10¹⁸ cells mol^?1 thymidine incorporated was calculated. For leucine, the corresponding factor was 0.07 ± 10¹⁸ cells mol^?1 leucine. Average annual BP calculated by thymidine incorporation was significantly higher than BP calculated from leucine incorporation and ranged from 47.2 to 77.5 μg C 1-^?1 day^?1 depending on the tracer and the conversion factor used. 3. Bacterial growth rates ranged from 0.1 day^?1 during winter to 1.7 day^?1 in the summer. A strong correlation was found between temperature as well as chlorophyll a and bacterial growth when temperature was greater than 5 °C; a major spring phytoplankton bloom at a temperature below 5 °C did not increase BP. 4. Dissolved organic carbon (DOC) concentrations varied between 2 and 7.2 mg C 1-^?1 and comprised between 50 and 92% of the total organic carbon pool in the River Danube, Based on the DOC concentration and an assumed bacterial growth yield of 20% we calculated mean DOC turnover times of around 60 days in the winter and less than 8 days during the summer.     

Seasonal and tidal variability of environmental carbon related physico-chemical variables and inorganic C acquisition in Gracilariopsis longissima and Enteromorpha intestinalis from Los Toruños salt marsh (Cádiz Bay, Spain)

The seasonal and tidal variability of inorganic C acquisition mechanisms, photosynthesis, internal composition and growth were studied in two co-occurring macroalgae in Los Toruños salt marsh (Cádiz Bay), Gracilariopsis longissima and Enteromorpha intestinalis. This variability was monitored together with physico-chemical variables affecting carbon availability, photosynthesis, and growth. The environmental variables, such as light, temperature, pH, salinity, oxygen, alkalinity, dissolved inorganic carbon (DIC) and CO₂, displayed not only an expected seasonal cycle but also a daily (tidal) variability, with abrupt and rapid changes influenced by biological activities, physical variables, tidal state and tidal timing. In contrast to environmental variables, photosynthesis, pigments and C:N composition were affected by seasonal changes but not by tidal regimes, as organisms integrated these short-term fluctuations in physico-chemical variables. Photosynthesis, pigments and internal N composition were maximal in autumn and minimal in summer for both species. Growth showed a seasonal trend, displaying a summer drop with negative values. This response can be the result of extreme values of environmental variables (temperature, light, pH, nutrients, and the shortage of DIC) in summer, in comparison with higher growth rates in September onwards. The use of inhibitors of carbon acquisition in situ at normal DIC concentrations (2.2. mM) revealed species-dependent differences. While the external carbonic anhydrase (CA) activity showed a constitutive character in G. longissima, it showed little effect in E. intestinalis, which relies on internal CA activity. The 4, 4′-diisothiocyanatostilbene-2,2′-disulfonate (DIDS)-sensitive bicarbonate transport in G. longissima was effective in winter. In contrast, DIDS stimulated photosynthesis in summer, and relieved AZ inhibition. This response could suggest a stimulation of a H⁺ extrusion mediated-CO₂ transport in periods of low CO₂ availability.     

Nutrient Constraints on Metabolism Affect the Temperature Regulation of Aquatic Bacterial Growth Efficiency

Inorganic nutrient availability and temperature are recognized as major regulators of organic carbon processing by aquatic bacteria, but little is known about how these two factors interact to control bacterial metabolic processes. We manipulated the temperature of boreal humic stream water samples within 0–25°C and measured bacterial production (BP) and respiration (BR) with and without inorganic nitrogen?+?phosphorus addition. Both BP and BR increased exponentially with temperature in all experiments, with Q ₁₀ values varying between 1.2 and 2.4. The bacterial growth efficiency (BGE) showed strong negative relationships with temperature in nutrient-enriched samples and in natural stream water where community-level BP and BR were not limited by nutrients. However, there were no relationships between BGE and temperature in samples where BP and BR were significantly constrained by the inorganic nutrient availability. The results suggest that metabolic responses of aquatic bacterial communities to temperature variations can be strongly dependent on whether the bacterial metabolism is limited by inorganic nutrients or not. Such responses can have consequences for both the carbon flux through aquatic food webs and for the flux of CO₂ from aquatic systems to the atmosphere.     

Nutrient control of bacterioplankton and phytoplankton dynamics

To determine whether positive correlations between phytoplankton and bacterioplankton growth in nutrient addition experiments are due to growth coupling or growth stimulation by the same nutrients, we examined phyto- and bacterioplankton growth in a series of eleven nutrient addition (N × P) and light/dark experiments. In mesotrophic Castle Lake, the phyto- and bacterioplankton growth responses to phosphorus (P) addition were strongly correlated (r²=0.59), while only a weak correlation (r²=0.10) was observed for the nitrogen addition treatments. After normalizing the N + P treatments for the growth stimulation observed in the respective P treatments, we found a substantial stimulation of the phytoplankton (e.g., costimulation by N + P) and no stimulation of the bacterioplankton. Bacteria growth rates were similar in both light and dark incubated P treatments. In these experiments, we found clear evidence suggesting the dynamics of bacteria and phytoplankton were correlated because they are often limited by the same resource (mainly inorganic phosphorus). We found only limited evidence that bacterioplankton growth coupling to algal dynamics was occurring in these experiments. However, we did not consider several factors such as dissolved organic nutrient availability, bacterivory, availability of physical substrates, and temperature which are also thought to influence the nature of bacterial/phytoplankton interactions. Based on the results of our experiments, we conclude the biomass of the bacterio- and phytoplankton covaried because they were stimulated by the same nutrients. This revised version was published online in August 2006 with corrections to the Cover Date.     

Nutrient and Temperature Limitation of Bacterioplankton Growth in Temperate Lakes

Limitation of bacterioplankton production by nutrients and temperature was investigated in eight temperate lakes in summer. Six of the lakes were resampled in autumn. The lakes differ in nutrient content, water color, and concentration of dissolved organic carbon. Nutrients (phosphorus, nitrogen, and organic carbon) were added alone and in all possible combinations to filtered lake water inoculated with bacteria from the lake. After incubation for 36–40 h at in situ temperatures (ranging from 7 to 20°C), the response in bacterioplankton production was determined. The effect of increased temperature on bacterioplankton growth was also tested. Bacterioplankton production was often limited by phosphorus alone, organic carbon alone, or the two in combination. Phosphorus limitation of bacterioplankton production was more common in the summer, whereas limitation by organic carbon was more frequently observed in the autumn. There was a close balance between limitation by phosphorus and organic carbon in the epilimnion in the summer. In the hypolimnion in the summer, bacterioplankton growth was primarily phosphorus-limited. The effect of phosphorus additions decreased with increasing phosphorus concentrations in the lakes. However, there were no correlations between the effect of added organic carbon and water color, dissolved organic carbon concentration, or phosphorus concentration. When temperature was low (in the hypolimnion in the summer, and throughout the water column in the autumn) temperature also limited bacterioplankton production. Thus, temperature and inorganic nutrients or organic compounds can limit bacterioplankton growth both alone and simultaneously. However, at low temperatures, temperature is the most important factor influencing bacterioplankton growth.     

Growth responses ofPterocladiella capillacea(Rhodophyta) in laboratory and outdoor cultivation

The growth and photosynthetic responses ofPterocladiella capillaceato NH₄, PO₄, CO₂-enrichment, pH, irradiance and temperature were evaluated for winter or summer plants cultivated under laboratory and outdoor settings. In the laboratory, using a gradient table, optimal growth temperature and irradiance for winter plants occurred at 10–20 °C and 100 mol photon m^–2s^–1, averaging 24.3% per week. The optimal growth conditions found for summer plants were 10–20 °C and 20 mol photon m^–2s^–1, averaging 29.0% per week. In a pH-stat cultivation system photosynthetic rates and growth rates were largely unaffected by pH in the range 6.5–8.5, however, they both decreased significantly above 8.5. In outdoor settings, using 40 L tanks,P. capillaceawas more responsive to the frequency the algae were fed with NH₄and PO₄rather than the relative concentrations of these nutrients. The average growth rates during winter were 28.3% and 12.5% per week when NH₄and PO₄were included once and twice a week for 24-h periods, respectively, while summer plants grew 15.0% and 25.3% per week at these nutrient regimes. Algae grown in seawater (containing 13.8 ± 1.8 M CO₂) or CO₂-enriched seawater (averaging 33.7 ± 13.2 M CO₂) had similar growth rates or even reduced productivity under CO₂-enrichment during winter. Concentrations of chlorophyllawere in average significantly higher in winter as compared to summer especially when nutrients were included twice a week. Phycoerythrin levels were also higher for plants fed with nutrients twice a week particularly during summer time. Although agar yields were limited and not seasonally dependent, this study shows high growth capacity forP. capillaceaas compared to previous investigations. Future mariculture prospective using current tank cultivation techniques for this species will likely depend on market demands for high quality agar.     

Utilization of Dissolved Nitrogen by Heterotrophic Bacterioplankton: Effect of Substrate C/N Ratio

The significance of dissolved combined amino acids (DCAA), dissolved free amino acids (DFAA), and dissolved DNA (D-DNA) as sources of C and N for marine bacteria in batch cultures with variable substrate C/N ratios was studied. Glucose, ammonium, alanine, and phosphate were added to the cultures to produce C/N ratios of 5, 10, and 15 and to ensure that phosphorus was not limiting. Maximum bacterial particulate organic carbon production (after 25 h of incubation) was inversely correlated with the C/N ratio: with the addition of identical amounts of carbon, the levels of production were 9.0-, 10.0-, and 11.1-fold higher at C/N ratios of 15, 10, and 5, respectively, relative to an unamended control. The bacterial growth efficiency increased from 22% (control cultures) to 44 to 53% in the cultures with manipulated C/N ratios (C/N-manipulated cultures). Net carbon incorporation from DCAA, DFAA, and D-DNA supported on average 19, 4, and 3% (control cultures and cultures to which only phosphate was added [+P cultures]) and 5, 4, and 0.3% of the particulate organic carbon production (C/N-manipulated cultures), respectively. In the C/N-manipulated cultures, a 2.6- to 3.4-fold-higher level of incorporation of DCAA, relative to that in the control cultures, occurred. Incorporation of D-DNA increased with the substrate C/N ratio, suggesting that D-DNA mainly was a source of N to the bacteria. Organic N (DCAA, DFAA, and D-DNA) sustained 14 to 49% of the net bacterial N production. NH₄⁺ was the dominant N source and constituted 55 to 99% of the total N uptake. NO₃^- contributed up to 23% to the total N uptake but was released in two cultures. The studied N compounds sustained nearly all of the bacterial N demand. Our results show that the C/N ratio of dissolved organic matter available to bacteria has a significant influence on the incorporation of individual compounds like DCAA and D-DNA.     

Effect of planting date,ventilation and soil temperature on growth and nutrition of tomato in high tunnels

Growth rates and tissue nutrient concentrations were measured in tomato (Lycopersicon esculentum Mill) grown in unheated high tunnels in the spring in the northeast USA. Two weeks after transplant on 3 April, seedlings had low concentrations of Nitrogen, Magnesium and other nutrients, while later plantings on 17 April and 1 May had adequate nutrition. The low yield and small fruit of the 3 April planting, compared to the later plantings, was likely related to this nutrient stress soon after transplant. Air and soil temperatures were less than 10°C at planting on 3 April. Air and soil were warmed during the day to different extents in tunnels vented at different temperatures. Over all plantings and ventilation regimes, relative growth rates over the two weeks after transplant were correlated to average air temperature. However, there was little uptake of P, N and Mg, when soil was cooler than 12°C. Nutrient concentrations in the shoot became very low because shoot growth continued when soil temperature limited nutrient uptake.     

Stoichiometry of Root and Leaf Nitrogen and Phosphorus in a Dry Alpine Steppe on the Northern Tibetan Plateau

Leaf nitrogen (N) and phosphorus (P) have been used widely in the ecological stoichiometry to understand nutrient limitation in plant. However,few studies have focused on the relationship between root nutrients and environmental factors. The main objective of this study was to clarify the pattern of root and leaf N and P concentrations and the relationships between plant nitrogen (N) and phosphorus (P) concentrations with climatic factors under low temperature conditions in the northern Tibetan Plateau of China. We conducted a systematic census of N and P concentrations, and the N∶P ratio in leaf and root for 139 plant samples, from 14 species and 7 families in a dry Stipa purpurea alpine steppe on the northern Tibetan Plateau of China. The results showed that the mean root N and P concentrations and the N∶P ratios across all species were 13.05 mg g⁻¹, 0.60 mg g⁻¹ and 23.40, respectively. The mean leaf N and P concentrations and the N∶P ratio were 23.20 mg g⁻¹, 1.38 mg g⁻¹, and 17.87, respectively. Compared to global plant nutrients concentrations, plants distributing in high altitude area have higher N concentrations and N∶P, but lower P concentrations, which could be used to explain normally-observed low growth rate of plant in the cold region. Plant N concentrations were unrelated to the mean annual temperature (MAT). The root and leaf P concentrations were negatively correlated with the MAT, but the N∶P ratios were positively correlated with the MAT. It is highly possible this region is not N limited, it is P limited, thus the temperature-biogeochemical hypothesis (TBH) can not be used to explain the relationship between plant N concentrations and MAT in alpine steppe. The results were valuable to understand the bio-geographic patterns of root and leaf nutrients traits and modeling ecosystem nutrient cycling in cold and dry environments.     

SEASONAL VARIATION OF P CONTENT AND MAJOR N POOLS IN PALMARIA PALMATA (RHODOPHYTA)1

The annual variation of major nitrogen pools, phosphorus, carbon, ash, and thallus water content in relation to seasonal environmental changes was studied in two northern Spanish populations of the edible seaweed Palmaria palmata (Linnaeus) Kuntze. Observed patterns were investigated using Spearman rank order correlation coefficients. There were significant relationships between thallus nutrient content and nitrate and orthophosphate seawater concentration, irradiance, temperature, and wave force. The highest levels of total N and P and nitrogenous compounds were observed during autumn and winter because the thallus stored N‐ and P‐rich compounds in response to high nutrient seawater concentration when growth was limited by low light and temperature. Phycoerythrin and other proteins were the main N reserves. Thallus P content was higher in algae from the eutrophic site. During spring, reduced N and P thallus content and increased ash, water, and C content were observed in the growing fronds. N and P seawater concentrations were undetectable during summer when nutrient reserves were low and growth was reduced and eventually suppressed, suggesting nutrient limiting conditions. Palmaria palmata clearly could take advantage of elevated N and P concentrations to create storage reserves in winter to support early summer growth. This storage response reduced the dependence of algal nutrition on the external nutrient supply and supports the use of pulse fertilization to diminish summer nutrient limitation of cultured algae.     

Seasonal changes in bacterial and phytoplankton biomass in a subantarctic coastal area (Kerguelen Islands)

The seasonal variations of bacterial and phytoplanktonic biomass were studied during several pluri-annual surveys in the subantarctic Morbihan Bay (Kerguelen Islands, 49 ° 20 S; 70 ° 10 E). Large interannual variation was observed. Phytoplanktonic biomass showed moderate values during winter and autumn. They increased sharply in spring, reaching a maximum value of about 1 mg C l^–1 corresponding to an important depletion of nutrients. A second phytoplanktonic bloom of similar amplitude occurred in late summer. During algal blooms which were roughly associated with optimal values of solar irradiation for the first one and with the highest temperatures for the second one, phytoplanktonic material represented near 100% of particulate and living carbon. Bacteria showed maximal abundance (0.2 to 0.7 mg C l^–1) during summer or autumn. Their relative abundance, which represented less than 1% of the living biomass in spring and summer, can reach more than 95% in autumn and winter.     

A metabolic theory of ecology applied to temperature and mass dependence of N and P excretion by common carp

Our study used a metabolic theory of ecology (MTE) to explore scaling of metabolic rates by body size and temperature, and to predict nutrient excretion by common carp (Cyprinus carpio). At high biomasses, common carp have negative impacts on water quality, and one mechanism is excretion of the nutrients N and P. We measured whole-body and mass-specific excretion rates during summer and winter for fish of different sizes (wet mass range 28–1,196 g) to produce an allometric scaling model capable of predicting excretion at different temperatures. We found positive relationships between both dissolved and total nutrient concentrations and fish wet mass in summer and winter, with greater excretion rates in summer (mean water temperature 24.2°C) than in winter (mean water temperature 9.2°C). Mass-specific excretion rates decreased with increasing fish size, consistent with the MTE, and the temperature-adjusted model explained more variation for N excretion than for P. The proportion of dissolved nutrients (NH₄ and PO₄) to total nutrients increased with increasing fish size. The significance of these models is that they can be used to predict population-based nutrient excretion by common carp when thermal history, fish density and size distribution in a water body are known.     

Temporal and Vertical Difference in Factors Limiting Growth Rate of Heterotrophic Bacteria in Lake Biwa

Abstract Dilution bioassays were performed to examine the seasonal and vertical difference in the relative importance of factors limiting growth of heterotrophic bacteria in Lake Biwa. The lake water diluted by 0.2 μm lake filtrate (1:6.6) was enriched either with glucose (C), inorganic phosphorus (P), ammonium nitrogen (N), amino acids (AA), or a combination of these, and incubated for 2 days at the depths where lake water was collected (2.5, 20 and 30 m depths). Experiments showed that at 2.5 m, P was the most deficient resource for bacterial growth, but the magnitude of P limitation depended on water temperature. Among others, amino acids showed a slight but significant stimulation of bacterial growth rates during the fall. At 20 and 30 m, however, growth stimulation by resource addition was rarely detected. Vertically reciprocal translocation experiments revealed that the growth rate was limited by low temperature rather than resource supply at the greater depths. The results support a simple view that bacterial growth rate is basically regulated by water temperature, but high growth rate is not realized in summer because of resource depletion. The present study suggests that both temperature and P supply play a crucial role in biogeochemical cycling of organic matter in Lake Biwa through the bacterial growth rate. Received: 10 March 1999; Accepted: 14 May 1999     

Temperature- and Light-Dependent Performance of the Cyanobacterium <Emphasis Type="Italic">Leptolyngbya Foveolarum</Emphasis> and the Diatom <Emphasis Type="Italic">Nitzschia Perminuta</Emphasis> in Mixed Biofilms

This paper investigates the role of species interactions as a mechanism determining the changing seasonal abundance of microphytobenthic species. Different kinds of interactions can occur between microphytobenthic species, e.g., interference competition (a species directly hindering the growth of another) or resource competition. If such interactions are strong, the capacity of species to exploit parts of the seasonal spectrum of temperature and light conditions could be greatly affected. A model system of two freshwater benthic phototrophs, the cyanobacterium Leptolyngbya foveolarum (Rabenhorst ex Gomont) Anagnostidis et Komárek and the diatom Nitzschia perminuta (Grunow) M. Pergallo, was used to study the capacity of each species to grow in ranges of temperature (7, 15 and 25 °C) and light (5, 40 and 200 μmol m⁻² s⁻¹) conditions in single-species and two-species cultures. Growth was followed for 14–17 days by measuring chlorophyll a and maximum photosynthetic capacity using spectrophotometry and pulse amplitude modulated (PAM) fluorimetry. A PHYTO-PAM fluorimeter facilitated simultaneous observations in mixed cultures on the two species. In mixed cultures, the diatom appeared to be a ‘cool season species’ (low temperature and low light intensity) and the cyanobacterium a ‘summer or autumn species’ (higher temperature and light intensities). This is different than predicted by monoculture experiments, where a wide range of optimal growth conditions was found. Two-species biofilm tests indicated inhibitory effects of the cyanobacterium on the diatom species, especially under conditions favorable to the cyanobacterium. High or low light intensities, increase of local pH caused by depletion of inorganic carbon, and limitation of other inorganic nutrients (resource competition) were examined as factors contributing to diatom inhibition, but none provided an acceptable explanation for observed growth patterns. Our results pointed towards interference competition.     

Nitrogen limitation and particulate elemental ratios of seston in hypersaline Mono Lake, California, U.S.A.

Particulate elemental ratios (C:N, N:P and C:Chl a) of seston in hypersaline (70–90 g kg^–1) Mono Lake, California, were examined over an 11-year period (1990–2000) which included the onset and persistence of a 5-year period of persistent chemical stratification. Following the onset of meromixis in mid-1995, phytoplankton and dissolved inorganic nitrogen were substantially reduced with the absence of a winter period of holomixis. C:N, N:P and C:Chl a ratios ranged from 5 to 18 mol mol^–1, 2 to 19 mol mol^–1 and 25 to 150 g g^–1, respectively, and had regular seasonal patterns. Deviations from those expected of nutrient-replete phytoplankton indicated strong nutrient limitation in the summer and roughly balanced growth during the winter prior to the onset of meromixis. Following the onset of meromixis, winter ratios were also indicative of modest nutrient limitation. A 3-year trend in C:N and N:P ratios toward more balanced growth beginning in 1998 suggest the impacts of meromixis weakened due to increased upward fluxes of ammonium associated with weakening stratification and entrainment of ammonium-rich monimolimnetic water. A series of nutrient enrichment experiments with natural assemblages of Mono Lake phytoplankton conducted during the onset of a previous episode of meromixis (1982–1986) confirm the nitrogen will limit phytoplankton before phosphorus or other micronutrients. Particulate ratios of a summer natural assemblage of phytoplankton collected under nitrogen-depleted conditions measured initially, following enrichment, and then after return to a nitrogen-depleted condition followed those expected based on Redfield ratios and laboratory studies.     

Rapid Method of Determining Factors Limiting Bacterial Growth in Soil

A technique to determine which nutrients limit bacterial growth in soil was developed. The method was based on measuring the thymidine incorporation rate of bacteria after the addition of C, N, and P in different combinations to soil samples. First, the thymidine incorporation method was tested in two different soils: an agricultural soil and a forest humus soil. Carbon (as glucose) was found to be the limiting substance for bacterial growth in both of these soils. The effect of adding different amounts of nutrients was studied, and tests were performed to determine whether the additions affected the soil pH and subsequent bacterial activity. The incubation time required to detect bacterial growth after adding substrate to the soil was also evaluated. Second, the method was used in experiments in which three different size fractions of straw (1 to 2, 0.25 to 1, and <0.25 mm) were mixed into the agricultural soil in order to induce N limitation for bacterial growth. When the straw fraction was small enough (<0.25 mm), N became the limiting nutrient for bacterial growth after about 3 weeks. After the addition of the larger straw fractions (1 to 2 and 0.25 to 1 mm), the soil bacteria were C limited throughout the incubation period (10 weeks), although an increase in the thymidine incorporation rate after the addition of C and N together compared with adding them separately was seen in the sample containing the size fraction from 0.25 to 1 mm. Third, soils from high-pH, limestone-rich areas were examined. P limitation was observed in one of these soils, while tendencies toward P limitation were seen in some of the other soils.