Interactions between P-limitation and different C conditions on the fatty acid composition of an extremophile microalga

Phosphorus limitation combined with aluminum triggers synergistic responses on the freshwater microalgae Raphidocelis subcapitata (Chlorophyceae)

In the environment, algae are exposed to several stressors such as limitation of essential nutrients and excess of toxic substances. It is well known the importance of phosphorus (P) supply for healthy metabolism of algae and impacts at this level can affect the whole aquatic trophic chain. Aluminum (Al) is the most abundant metal on Earth and it is toxic to different trophic levels. Processes related to P and Al assimilation still need to be clarified and little is known about the responses of microalgae exposed to the two stressors simultaneously. We evaluated the effects of environmental concentrations of Al and P limitation, isolated and in combination, on growth, pigment production and photosynthesis of the freshwater microalga Raphidocelis subcapitata. Both stressors affected cell density, chlorophyll a, carotenoids, and maximum quantum yield. Al did not affect any other evaluated parameter, while P limitation affected parameters related to the dissipation of heat by algae and the maximum electron transport rate, decreasing the saturation irradiance. In the combination of both stressors, all parameters evaluated were affected in a synergistic way, i.e., the results were more harmful than expected considering the responses to isolated stressors. Our results indicate that photoprotection mechanisms of algae were efficient in the presence of both stressors, avoiding damages to the photosynthetic apparatus. In addition, our data highlight the higher susceptibility of R. subcapitata to Al in P-limited conditions.

Does cobalt antagonize P limitation effects on photosynthetic parameters on the freshwater microalgae Raphidocelis subcapitata (Chlorophyceae), or does P limitation acclimation antagonize cobalt effects? More questions than answers

Phosphorus (P; macronutrient) and cobalt (Co; micronutrient) are essential for algal healthy metabolism. While P provides energy, Co is a co-factor of several enzymes and component of B12 vitamin. However, in concentrations higher or lower than required, P and Co alter algal metabolism, impacting physiological processes (e.g., growth and photosynthesis), usually in a harmful way. In the environment, algae are exposed to multiple stressors simultaneously and studies evaluating the algal response to the combination of macronutrient limitation and micronutrient excess are still scarce. We assessed the effects of P limitation and Co excess, isolated and combined, in Raphidocelis subcapitata (Chlorophyceae), in terms of growth, pigments production, and photosynthetic parameters. Except for the photochemical quenching (qP) and the efficiency in light capture (α) under P limitation, all parameters were affected by both stressors, isolated and combined. Under P limitation, chlorophyll a was the most sensitive parameter; while excess of Co affected most the photoprotective mechanisms of algae, altering the non-photochemical quenchings qN and NPQ, influencing the light use and dissipation of heat by algae. The combination of two stressors resulted in a significant decrease in algal growth, with synergistic responses in growth and pigments production, and antagonism in the photosynthetic parameters. We suggest that algal metabolism was altered during P limitation acclimation and the excess of Co was used in a beneficial way by P-limited algae in photosynthesis, resulting in the well-functioning of the photosynthetic apparatus in the combination of both stressors. However, more studies are needed to understand which mechanisms are involved in this adaptation which resulted in antagonism in photosynthetic processes and synergism in growth and pigments production.

Ecotoxicological response of Spirulina platensis to coexisted copper and zinc in anaerobic digestion effluent

Copper ion (Cu²⁺) and zinc ion (Zn²⁺) are widely co-existent in anaerobic digestion effluent as typical contaminants. This work aims to explore how Cu²⁺-Zn²⁺ association affects physiological properties of S. platensis using Schlösser medium (SM) and sterilized anaerobic digestion effluent (SADE). Microalgae cells viability, biochemical properties, uptake of Cu²⁺ and Zn²⁺, and risk assessment associated with the biomass reuse as additives to pigs were comprehensively assessed. Biomass production ranged from 0.03 to 0.28 g/L in SM and 0.63 to 0.79 g/L in SADE due to the presence of Cu²⁺ and Zn²⁺. Peak value of chlorophyll-a and carotenoid content during the experiment decreased by 70-100% and 40-100% in SM, and by 70-77% and 30-55% in SADE. Crude protein level reduced by 4-41% in SM and by 65-75% in SADE. The reduction ratio of these compounds was positively related to the Cu²⁺ and Zn²⁺ concentrations. Maximum value of saturated and unsaturated fatty acids was both obtained at 0.3 Cu + 2.0 Zn (50.8% and 22.8%, respectively) and 25% SADE reactors (33.8% and 27.7%, respectively). Uptake of Cu in biomass was facilitated by Zn²⁺ concentration (> 4.0 mg/L). Risk of S. platensis biomass associated with Cu²⁺ was higher than Zn²⁺. S. platensis from SM (Cu²⁺ ≤ 0.3 mg/L and Zn²⁺ ≤ 4.0 mg/L) and diluted SADE (25% and 50% SADE) reactors could be used as feed additives without any risk (hazard index <1), which provides sufficient protein and fatty acids for pig consumption. These results revealed the promising application of using S. platensis for bioremediation of Cu²⁺ and Zn²⁺ in anaerobic digestion effluent and harvesting biomass for animal feed additives.

Dark stress for improved lipid quantity and quality in bioprospected acid-tolerant green microalgae

The cost of microalgae cultivation is one of the largest limitations to achieving sustainable, large-scale microalgae production of commercially desirable lipids. Utilizing CO2 as a 'free' carbon source from waste industrial flue gas emissions can offer wide-ranging cost savings. However, these gas streams typically create acidic environments, in which most microalgae cannot survive due to the concentration of CO2 and the presence of other acidic gasses such as NO2 and SO2. To address this situation, we investigated growth of a mixed acid-tolerant green microalgal culture (91% dominated by a single Coccomyxa sp. taxon) bioprospected at pH 2.8 from an acid mine drainage impacted water body. The culture was grown at pH 2.5 and fed with a simulated flue gas containing 6% CO2 and 94% N2. On reaching the end of the exponential growth phase, the culture was exposed to either continued light-dark cycle conditions or continual dark conditions. After three days in the dark, the biomass consisted of 28% of lipids, which was 42% higher than at the end of the exponential phase and 55% higher than the maximum lipid content achieved under light/dark conditions. The stress caused by being continually in the dark also favoured the production of omega-3 and omega-6 polyunsaturated fatty acids (PUFAs; 19.47% and 21.04%, respectively, after 7 days) compared to 7-days of light-dark treatment (1.94% and 9.53%, respectively) and showed an increase in nitrogen content (C:N ratio of 6.4) compared to light-dark treatment (C:N ratio of 11.9). The results of the research indicate that use of acid tolerant microalgae overcomes issues using flue gasses that will create an acidic environment and that applying dark stress is a low-cost stressor stimulates production of desirable dietary lipids.

Novel approach for the management of acid mine drainage (AMD) for the recovery of heavy metals along with lipid production by Chlorella vulgaris

The treatment of acid mine drainage (AMD) is of paramount importance for environmental sustainability. A two-stage process involving AMD remediation and simultaneous lipid production represents a highly efficient approach with zero-waste generation. Alkaline (NaOH) treatment of AMD at pH 8.0, 10.0, and 12.0 had significantly reduced metal loads (copper (Cu), cobalt (Co), chromium (Cr), cadmium (Cd), nickel (Ni), and zinc (Zn)) compared to the acidic pH range (4.0 and 6.0). The concentration levels of sulfates (SO₄ = 4520 mg/L), iron (Fe = 788 mg/L), aluminum (Al = 310 mg/L), and manganese (Mn = 19.4) were reduced to 2971 mg/L, 10.3 mg/L, 16.4 mg/L, and 1.3 mg/L, respectively at pH value 8.0. AMD with a pH value of 8.0 was later chosen as an ideal medium to favor the lipid accumulation by Chlorella vulgaris. Algal biomass was increasing to 5.5 g/L from 0.6 g/L of AMD-based medium within 15 days of cultivation. The FTIR and SEM-EDS studies revealed significant morphological changes in the microbial cell wall. The metals might positively impact lipid production in microalgae, where lipid yield achieved 0.18 g/g of glucose with lipid content of 0.35 g/g of biomass. The fatty acid profile presented 53.4% of saturated fatty acid content with a cetane value of 60.7. Thus, the efficiency of C. vulgaris was demonstrated with AMD treatment proving it to be a good candidate for bioenergy production.

Phenotypic Diversity and Plasticity of Photoresponse Across an Environmentally Contrasting Family of Phytoflagellates

Organisms often employ ecophysiological strategies to exploit environmental conditions and ensure bio-energetic success. However, the many complexities involved in the differential expression and flexibility of these strategies are rarely fully understood. Therefore, for the first time, using a three-part cross-disciplinary laboratory experimental analysis, we investigated the diversity and plasticity of photoresponsive traits employed by one family of environmentally contrasting, ecologically important phytoflagellates. The results demonstrated an extensive inter-species phenotypic diversity of behavioural, physiological, and compositional photoresponse across the Chlamydomonadaceae, and a multifaceted intra-species phenotypic plasticity, involving a broad range of beneficial photoacclimation strategies, often attributable to environmental predisposition and phylogenetic differentiation. Deceptively diverse and sophisticated strong (population and individual cell) behavioural photoresponses were observed, with divergence from a general preference for low light (and flexibility) dictated by intra-familial differences in typical habitat (salinity and trophy) and phylogeny. Notably, contrasting lower, narrow, and flexible compared with higher, broad, and stable preferences were observed in freshwater vs. brackish and marine species. Complex diversity and plasticity in physiological and compositional photoresponses were also discovered. Metabolic characteristics (such as growth rates, respiratory costs and photosynthetic capacity, efficiency, compensation and saturation points) varied elaborately with species, typical habitat (often varying more in eutrophic species, such as Chlamydomonas reinhardtii), and culture irradiance (adjusting to optimise energy acquisition and suggesting some propensity for low light). Considerable variations in intracellular pigment and biochemical composition were also recorded. Photosynthetic and accessory pigments (such as chlorophyll a, xanthophyll-cycle components, chlorophyll a:b and chlorophyll a:carotenoid ratios, fatty acid content and saturation ratios) varied with phylogeny and typical habitat (to attune photosystem ratios in different trophic conditions and to optimise shade adaptation, photoprotection, and thylakoid architecture, particularly in freshwater environments), and changed with irradiance (as reaction and harvesting centres adjusted to modulate absorption and quantum yield). The complex, concomitant nature of the results also advocated an integrative approach in future investigations. Overall, these nuanced, diverse, and flexible photoresponsive traits will greatly contribute to the functional ecology of these organisms, addressing environmental heterogeneity and potentially shaping individual fitness, spatial and temporal distribution, prevalence, and ecosystem dynamics.

Herbivore consumers face different challenges along opposite sides of the stoichiometric knife-edge

Anthropogenic activities have reshaped the relative supply rates of essential elements to organisms. Recent studies suggested that consumer performance is strongly reduced by food that is either very high or very low in relative phosphorus content. However, the generality of such 'stoichiometric knife-edge' and its underlying mechanisms are poorly understood. We studied the response of a planktonic rotifer to a 10-fold food carbon : phosphorus (C : P) gradient and confirmed the existence of the stoichiometric knife-edge. Interestingly, we observed a complete homeostatic breakdown associated with strong growth reductions at high food C : P. In contrast, at low food C : P, animals maintained homeostasis despite pronounced performance reductions. Our results suggest that the mechanisms underlying adverse effects of stoichiometric imbalance are determined by both the identity of elements that are limiting and those that are present in excess. Negative effects of excess P reveal an additional way of how eutrophication may affect consumers.

Direct and Indirect Effects of Resource P-Limitation Differentially Impact Population Growth, Life History and Body Elemental Composition of a Zooplankton Consumer

One of the central tenets of ecological stoichiometry is that consumer growth rate is strongly determined by food phosphorus (P) content. In planktonic organisms population growth rates of zooplankton have repeatedly been shown to be reduced when fed with P-limited algal food sources. However, P-limitation may also affect other quality-related aspects of algae, such as biochemical composition or palatability. We studied the population growth, detailed life history and body elemental composition of the herbivorous rotifer, Brachionus calyciflorus, in response to three different food quality treatments: algae cultured in high phosphorus conditions (average algal molar C:P ≈ 112, 'HP'), algae cultured in low P conditions (molar C:P ≈ 631, 'LP') and low-P cultured algae spiked with P just before feeding (molar C:P ≈ 113, 'LP+P'). LP+P algae thus combined high P content with a history of growth under P-limited conditions. Total P content and the C:P ratio of rotifers in the LP+P treatment equaled those of rotifers in the HP treatment. Rotifer population growth rates were higher in HP than in LP and intermediate in the LP+P treatment. Similarly, many life history traits observed for animals in the LP+P treatment, such as somatic growth rate, age at maturity, and egg production rate were also intermediate to those observed in the LP and HP treatments. However, there were important deviations from this pattern: size at first reproduction and egg mortality in the LP+P treatment equaled the HP treatment, whereas size and development time of the first eggs equaled those of the LP treatment. Our results indicate that elemental limitation cannot fully explain reduced performance of consumers fed with P-limited algae and strongly suggest that indirect, non-stoichiometric effects of P-limitation, e.g., via changes in biochemical composition or morphology of the algae also play a major role. Furthermore, our study highlights that such indirect effects have a differential impact on major fitness components and may as such also determine the population dynamics and demographic structure of consumer populations.

Kinetics of growth and lipids accumulation in Chlorella vulgaris during batch heterotrophic cultivation: Effect of different nutrient limitation strategies

The present study aimed at: (1) determining the effect of sulfur addition on biomass growth and (2) assessing the effect of sulfur, phosphorus and nitrogen limitation on lipid accumulation by C. vulgaris SAG 211-11b. The sulfur cellular content was more than two-fold higher under nitrogen and phosphorus limitation (0.52% and 0.54%ww^-1, respectively) compared to sulfur requirements (0.20%ww^-1) under sulfur limiting conditions. The nitrogen needs are significantly lower (2.81-3.35%ww^-1) when compared to other microalgae and become 23% lower under nitrogen or phosphorus limitation. The microalga exhibited substrate inhibition above 30gL^-1 initial glucose concentration. Sulfur limitation had the most significant effect on lipid accumulation, resulting in maximum total lipid content of 53.43±3.93%gg_DW^-1. In addition to enhancing lipid productivity, adopting the optimal nutrient limitation strategy can result in cost savings by avoiding unnecessary nutrient additions and eliminate the environmental burden due to wasted resources.

Common and Species-Specific Effects of Phosphate on Marine Microalgae Fatty Acids Shape Their Function in Phytoplankton Trophic Ecology

The use of fatty acids (FA) to infer structure of phytoplankton assemblages and as indicators of microalgae nutritional value is acquiring relevance in modern phytoplankton ecology and new advances concerning factors influencing FA variability among microalgae are demanded. In this regard, the relationship between phosphorus and FA remains particularly little studied in marine phytoplankton. In the present study, we focus on phosphate effects on FA from a diversified set of marine microalgae and provide new insights into the applicability of FA in phytoplankton trophic ecology. Phosphate deprivation mainly induced monounsaturated FA production in eight out of nine microalgae and their changes were species-specific, with palmitoleic acid exhibiting extreme variation and discriminating between haptophyte classes. The important phosphate-induced and interspecific variability found for oleic acid was perceived as a concern for the current application of this FA as a trophic position indicator in grazers. Chloroplast C-16 and C-18 polyunsaturated FA were more affected by phosphate than C-20 and C-22 highly unsaturated FA (HUFA). The relative stability of stearidonic acid to phosphate in cryptophytes and haptophytes pinpointed this FA as a suited marker for both microalgae groups. Taken all species together, phosphate deprivation and taxonomy accounted for 20.8 and 50.7% of total FA variation, respectively. HUFA were minimally affected by phosphate indicating their suitability as indicators of phytoplankton trophic value. The asymptotic relationship between HUFA and phosphorus cell content suggested mineral composition (phosphorus) could be more important than HUFA content as attribute of marine microalgae nutritional value at the species level.

Interspecific variability in phosphorus-induced lipid remodelling among marine eukaryotic phytoplankton

The response of marine microalgal lipids to phosphorus is of central importance in phytoplankton ecology but remains poorly understood. We determined how taxonomically diverse microalgal species remodelled their lipid class profile in response to phosphorus availability and whether these changes coincided with those already known to occur in land plants and in the limited number of phytoplankton species for which data are available. The complete lipid class profile and specific lipid ratios influenced by phosphorus availability were quantified in two green microalgae and seven Chromalveolates exposed to phosphorus repletion, deprivation and replenishment. Lipid class cell quota changes in the two green microalgae resembled the currently described pattern of betaine lipids substituting for phospholipids under phosphorus depletion, whereas only two of the studied Chromalveolates showed this pattern. Sulpholipids counterbalanced phosphatidylglycerol only in Picochlorum atomus. In all other species, both lipids decreased simultaneously under phosphorus deprivation, although sulpholipids declined more slowly. Phosphorus deprivation always induced a decrease in digalactosyl-diacylglycerol. However, the ratio of digalactosyl-diacylglycerol to total phospholipids increased in eight species and remained unchanged in Isochrysis galbana. Marine phytoplankton seems to have evolved a diversified mechanism for remodelling its lipid class profile under the influence of phosphorus, with cryptophytes and particularly haptophytes exhibiting previously unobserved lipid responses to phosphorus.

Rapid Accumulation of Total Lipid in Rhizoclonium africanum Kutzing as Biodiesel Feedstock under Nutrient Limitations and the Associated Changes at Cellular Level

Increase of total lipid and the proportion of the favorable fatty acids in marine green filamentous macroalga Rhizoclonium africanum (Chlorophyceae) was studied under nitrate and phosphate limitations. These stresses were given by both eliminating and doubling the required amounts of nitrate and phosphate salts in the growth media. A significant twofold increase in total lipid (193.03 mg/g) was achieved in cells in absence of nitrate in the culture medium, followed by phosphate limitation (142.65 mg/g). The intracellular accumulation of neutral lipids was observed by fluorescence microscopy. The scanning electron microscopic study showed the major structural changes under nutrient starvation. Fourier transform infrared spectroscopy (FTIR) revealed the presence of ester (C-O-C stretching), ketone (C-C stretching), carboxylic acid (O-H bending), phosphine (P-H stretching), aromatic (C-H stretching and bending), and alcohol (O-H stretching and bending) groups in the treated cells indicating the high accumulation of lipid hydrocarbons in the treated cells. Elevated levels of fatty acids favorable for biodiesel production, that is, C16:0, C16:1, C18:1, and C20:1, were identified under nitrate- and phosphate-deficient conditions. This study shows that the manipulation of cultural conditions could affect the biosynthetic pathways leading to increased lipid production while increasing the proportion of fatty acids suitable for biodiesel production.

Evaluation of fatty acid profile and biodiesel properties of microalga Scenedesmus abundans under the influence of phosphorus, pH and light intensities

The present study dealt with biomass, lipid concentration, fatty acid profile and biodiesel properties of microalga Scenedesmus abundans under different phosphate concentrations, pH and light intensities, one at a time. Among different phosphate concentrations, higher biomass (770.10±11.0mg/L) and lipid concentration (176.87±4.6mg/L) were at the concentration of 60mg/L. Light intensity at 6000lux yielded higher biomass and lipid concentration of 742.0±9.7 and 243.15±9.1mg/L, respectively. The biomass (769.0±12.3mg/L) and lipid (179.47±5.5mg/L) concentration were highest at pH 8 and pH 6, respectively. All the culture treatments showed marked effect on the fatty acid profile and biodiesel properties of the extracted oil. FAME derived biodiesel properties were compared with European biodiesel standards (EN 14214), Indian biodiesel standards (IS 15607) and American biodiesel standards (ASTM D 6751-08) to assess the suitability of algal oil as biodiesel feedstock.

Light-Induced Changes in Fatty Acid Profiles of Specific Lipid Classes in Several Freshwater Phytoplankton Species

We tested the influence of two light intensities [40 and 300 μmol PAR / (m(2)s)] on the fatty acid composition of three distinct lipid classes in four freshwater phytoplankton species. We chose species of different taxonomic classes in order to detect potentially similar reaction characteristics that might also be present in natural phytoplankton communities. From samples of the bacillariophyte Asterionella formosa, the chrysophyte Chromulina sp., the cryptophyte Cryptomonas ovata and the zygnematophyte Cosmarium botrytis we first separated glycolipids (monogalactosyldiacylglycerol, digalactosyldiacylglycerol, and sulfoquinovosyldiacylglycerol), phospholipids (phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol, phosphatidylinositol, and phosphatidylserine) as well as non-polar lipids (triacylglycerols), before analyzing the fatty acid composition of each lipid class. High variation in the fatty acid composition existed among different species. Individual fatty acid compositions differed in their reaction to changing light intensities in the four species. Although no generalizations could be made for species across taxonomic classes, individual species showed clear but small responses in their ecologically-relevant omega-3 and omega-6 polyunsaturated fatty acids (PUFA) in terms of proportions and of per tissue carbon quotas. Knowledge on how lipids like fatty acids change with environmental or culture conditions is of great interest in ecological food web studies, aquaculture, and biotechnology, since algal lipids are the most important sources of omega-3 long-chain PUFA for aquatic and terrestrial consumers, including humans.

Interspecific competition in phytoplankton drives the availability of essential mineral and biochemical nutrients

The underlying mechanisms and consequences of competition and diversity are central themes in ecology. A higher diversity of primary, producers often results in higher resource use efficiency in aquatic and terrestrial ecosystems. This may result in more food for consumers on one hand, while, on the other hand, it can also result in a decreased food quality for consumers; higher biomass combined with the same availability of the limiting compound directly reduces the dietary proportion of the limiting compound. Here we tested whether and how interspecific competition in phytoplankton communities leads to changes in resource use efficiency and cellular concentrations of nutrients and fatty acids. The measured particulate carbon: phosphorus ratios (C:P) and fatty acid concentrations in the communities were compared to the theoretically expected ratios and concentrations of measurements on simultaneously running monocultures. With interspecific competition, phytoplankton communities had higher concentrations of the monounsaturated fatty acid oleic acid and also much higher concentrations of the ecologically and physiologically relevant long-chain polyunsaturated fatty acid eicosapentaenoic acid than expected concentrations based on monocultures. Such higher availability of essential fatty acids may contribute to the positive relationship between phytoplankton diversity and zooplankton growth, and may compensate limitations by mineral nutrients in higher trophic levels.

Linking elements to biochemicals: effects of nutrient supply ratios and growth rates on fatty acid composition of phytoplankton species

Three species of marine phytoplankton, Rhodomonas sp., Isochrysis galbana Parke, and Phaeodactylum tricornutum Bohlin, were cultivated in semicontinuous cultures to test biochemical responses (fatty acids; FAs) to five nitrogen (N):phosphorus (P) supply ratios and four growth rates (dilution rates). The characteristic FA profile was observed for each algal species (representing particular algal class), which remained relatively stable across the entire ranges of N:P supply ratios and growth rates. For all species, significant direct effects of N:P supply ratios on FAs were found at lower growth rates. The highest saturated and monounsaturated fatty acid (SFA and MUFA) contents were observed under N deficiency at the lowest growth rate in all three species, while responses of polyunsaturated fatty acids (PUFAs) revealed no consistent pattern. Total FAs (and SFAs and MUFAs) in all species showed significant negative correlations with N cell quota (QN ) under N deficiency, but PUFAs had species-specific correlations with QN . The results show that characteristic FA profiles of algal genus or species (representing particular algal classes) underlie fluctuations according to culture conditions. The significant correlation between FAs and QN under N deficiency suggests that elemental and biochemical limitation of phytoplankton should be considered mutually as determinants of food quality for zooplankton in marine ecosystems.

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.

Lipid composition of Chlorella vulgaris (Trebouxiophyceae) as a function of different cadmium and phosphate concentrations

Fatty acids are the fundamental structural components of membrane lipids, and the degree of saturation of the long hydrocarbon chains in microalgae contributes to regulation of growth, biomass production and reproduction of aquatic consumers. This research aimed at evaluating the effects of cadmium (2×10(-8); 10(-7) mol L(-1) Cd) on lipid class and fatty acid composition of the microalga Chlorella vulgaris under varying phosphate (PO(4)(3-)) concentrations (6.0×10(-7) to 2.3×10(-4) mol L(-1)). Under PO(4)(3-) limitation and Cd stress, the storage lipid class triacylglycerol (TAG) was the most accumulated among the lipid classes. Fatty acid composition revealed that the degree of saturation increased with increasing Cd stress and PO(4)(3-) limitation. Decreasing PO(4)(3-) and increasing Cd concentrations resulted in higher saturated fatty acid (SAFA) and monounsaturated FA (MUFA) concentrations. Total polyunsaturated FA (PUFA) and ω3 PUFA, and PUFA:SAFA ratios were higher in the control (2.3×10(-4) mol L(-1) PO(4)(3-)) cells than in either PO(4)(3-) limitation or Cd stress, or in the combination of both stresses. Contrasting with all the other PUFAs, 18:2n - 6 increased as PO(4)(3-) limitation increased. A significant positive relationship of PUFAs, acetone mobile polar lipids (AMPL) and phospholipids (PL) with phosphate concentration in the culture media was obtained, while TAG concentrations had a positive association with total MUFA and SAFA. Total SAFA, 14:0, 18:1n - 9 and 18:2n - 6 were positively correlated with Cd and negatively with PO(4)(3-) concentrations. The microalga responded to combined PO(4)(3-) limitation and Cd exposure by increasing its total lipid production and significantly altering its lipid composition. The FA 18:2n - 6 may be considered a stress biomarker for PO(4)(3-) limitation and Cd stress in C. vulgaris.

Effect of nutrient limitation of cyanobacteria on protease inhibitor production and fitness of Daphnia magna

Herbivore-plant interactions have been well studied in both terrestrial and aquatic ecosystems as they are crucial for the trophic transfer of energy and matter. In nutrient-rich freshwater ecosystems, the interaction between primary producers and herbivores is to a large extent represented by Daphnia and cyanobacteria. The occurrence of cyanobacterial blooms in lakes and ponds has, at least partly, been attributed to cyanotoxins, which negatively affect the major grazer of planktonic cyanobacteria, i.e. Daphnia. Among these cyanotoxins are the wide-spread protease inhibitors. These inhibitors have been shown (both in vitro and in situ) to inhibit the most important group of digestive proteases in the gut of Daphnia, i.e. trypsins and chymotrypsins, and to reduce Daphnia growth. In this study we grew cultures of the cyanobacterium Microcystis aeruginosa strain BM25 on nutrient replete, N-depleted or P-depleted medium. We identified three different micropeptins to be the cause for the inhibitory activity of BM25 against chymotrypsins. The micropeptin content depended on nutrient availability: Whereas N-limitation led to a lower concentration of micropeptins per biomass, P-limitation resulted in a higher production of these chymotrypsin inhibitors. The altered micropeptin content of BM25 was accompanied by changed effects on the fitness of Daphnia: A higher content of micropeptins led to lower IC50 values for Daphnia gut proteases and vice-versa. Following expectations, the lower content of micropeptins in the N-depleted BM25 caused higher somatic growth of Daphnia. Therefore, protease inhibitors can be regarded as a nutrient-dependent defence against grazers. Interestingly, although the P-limitation of the cyanobacterium led to a higher micropeptin content, high growth of D. magna was observed when they were fed with P-depleted BM25. This might be due to reduced digestibility of P-depleted cells with putatively thick mucilaginous sheaths. These findings indicate that both the grazer and the cyanobacterium benefit from P-reduction in light of digestibility and growth inhibition, which is an interesting starting point for further studies.

Elemental and fatty acid composition of snow algae in Arctic habitats

Red, orange or green snow is the macroscopic phenomenon comprising different eukaryotic algae. Little is known about the ecology and nutrient regimes in these algal communities. Therefore, eight snow algal communities from five intensively tinted snow fields in western Spitsbergen were analysed for nutrient concentrations and fatty acid (FA) composition. To evaluate the importance of a shift from green to red forms on the FA-variability of the field samples, four snow algal strains were grown under nitrogen replete and moderate light (+N+ML) or N-limited and high light (−N+HL) conditions. All eight field algal communities were dominated by red and orange cysts. Dissolved nutrient concentration of the snow revealed a broad range of NH⁺₄ (<0.005–1.2 mg N l⁻¹) and only low PO³⁻₄ (<18 μg P l⁻¹) levels. The external nutrient concentration did not reflect cellular nutrient ratios as C:N and C:P ratios of the communities were highest at locations containing relatively high concentrations of NH⁺₄ and PO³⁻₄. Molar N:P ratios ranged from 11 to 21 and did not suggest clear limitation of a single nutrient. On a per carbon basis, we found a 6-fold difference in total FA content between the eight snow algal communities, ranging from 50 to 300 mg FA g C⁻¹. In multivariate analyses total FA content opposed the cellular N:C quota and a large part of the FA variability among field locations originated from the abundant FAs C18:1n-9, C18:2n-6, and C18:3n-3. Both field samples and snow algal strains grown under −N+HL conditions had high concentrations of C18:1n-9. FAs possibly accumulated due to the cessation of growth. Differences in color and nutritional composition between patches of snow algal communities within one snow field were not directly related to nutrient conditions. We propose that the highly patchy distribution of snow algae within and between snow fields may also result from differences in topographical and geological parameters such as slope, melting water rivulets, and rock formation.

Independent colimitation for carbon dioxide and inorganic phosphorus

Simultaneous limitation of plant growth by two or more nutrients is increasingly acknowledged as a common phenomenon in nature, but its cellular mechanisms are far from understood. We investigated the uptake kinetics of CO(2) and phosphorus of the algae Chlamydomonas acidophila in response to growth at limiting conditions of CO(2) and phosphorus. In addition, we fitted the data to four different Monod-type models: one assuming Liebigs Law of the minimum, one assuming that the affinity for the uptake of one nutrient is not influenced by the supply of the other (independent colimitation) and two where the uptake affinity for one nutrient depends on the supply of the other (dependent colimitation). In addition we asked whether the physiological response under colimitation differs from that under single nutrient limitation.We found no negative correlation between the affinities for uptake of the two nutrients, thereby rejecting a dependent colimitation. Kinetic data were supported by a better model fit assuming independent uptake of colimiting nutrients than when assuming Liebigs Law of the minimum or a dependent colimitation. Results show that cell nutrient homeostasis regulated nutrient acquisition which resulted in a trade-off in the maximum uptake rates of CO(2) and phosphorus, possibly driven by space limitation on the cell membrane for porters for the different nutrients. Hence, the response to colimitation deviated from that to a single nutrient limitation. In conclusion, responses to single nutrient limitation cannot be extrapolated to situations where multiple nutrients are limiting, which calls for colimitation experiments and models to properly predict growth responses to a changing natural environment. These deviations from single nutrient limitation response under colimiting conditions and independent colimitation may also hold for other nutrients in algae and in higher plants.