Metabolic and morphological changes of an oil accumulating trebouxiophycean alga in nitrogen-deficient conditions

Oil-rich algae have promising potential for a next-generation biofuel feedstock. Pseudochoricystis ellipsoidea MBIC 11204, a novel unicellular green algal strain, accumulates a large amount of oil (lipids) in nitrogen-deficient (–N) conditions. Although the oil bodies are easily visualized by lipophilic staining in the cells, little is known about how oil bodies are metabolically synthesized. Clarifying the metabolic profiles in –N conditions is important to understand the physiological mechanisms of lipid accumulations and will be useful to optimize culture conditions efficiently produce industrial oil. Metabolome and lipidome profiles were obtained, respectively, using capillary electrophoresis- and liquid chromatography-mass spectrometry from P. ellipsoidea in both nitrogen-rich (+N; rapid growth) and –N conditions. Relative quantities of more than 300 metabolites were systematically compared between these two conditions. Amino acids in nitrogen assimilation and N-transporting metabolisms were decreased to 1/20 the amount, or less, in –N conditions. In lipid metabolism, the quantities of neutral lipids increased greatly in –N conditions; however, quantities of nearly all the other lipids either decreased or only changed slightly. The morphological changes in +N and –N conditions were also provided by microscopy, and we discuss their relationship to the metabolic changes. This is the first approach to understand the novel algal strain’s metabolism using a combination of wide-scale metabolome analysis and morphological analysis.

     

Changes in the accumulation of alkenones and lipids under nitrogen limitation and its relation to other energy storage metabolites in the haptophyte alga <Emphasis Type="Italic">Emiliania huxleyi</Emphasis> CCMP 2090

Alkenones are long-chain methyl/ethyl ketones (mainly in length of C₃₇-C₃₉) with two to four trans-unsaturated bonds produced by several kinds of marine haptophytes such as Emiliania huxleyi (coccolithophore). The physiological functions and metabolic profile of alkenones are not well known yet. In this study, we focused on elucidating how alkenones contribute to energy storage and cellular carbon partitioning in relation to other cellular components. For the purpose, we analyzed the changes in carbon allocation among various cell components like lipids, alkenones, proteins, and polysaccharides between cells exposed to N-sufficient (+N) and N-limited conditions (?N) in E. huxleyi CCMP 2090. Finally, the alkenones were found to function as main storage lipids and their accumulation was clearly increased by ?N, whereas triacylglycerols (TAGs) were barely detected under any N conditions. The mobilization of carbons into alkenones was stimulated by ?N from 15% under +N to 27% under ?N. However, photosynthetic C allocation into other components was suppressed by ?N, showing that percent C allocation into fatty acids, proteins, and polysaccharides was decreased from 9, 46, and 6.8% under +N to 7, 25, and 4.5% under ?N, respectively. In addition, fatty acids such as 16:0, 18:0, 18:1, and 18:2 became dominant under ?N while 18:5 became dominant under +N conditions, with no significant change in 22:6. This study revealed that alkenones function as primary carbon storage pools especially under ?N condition in E. huxleyi CCMP 2090 and that N supply triggers a dynamic change in carbon metabolism by modifying membrane lipid composition and regulating carbon allocation preferences.     

Monitoring oil production for biobased feedstock in the microalga <Emphasis Type="Italic">Nannochloropsis</Emphasis> sp.: a novel method combining the BODIPY BD-C12 fluorescent probe and simple image processing

A simple reliable method with fast response for lipid detection and quantification is proposed, combining a new highly lipophilic fluorescent probe BODIPY BD-C12 and image analysis to determine the algal lipid content and the lipid production in the microalgae Nannochloropsis sp. Lipid bodies stained with BODIPY BD-C12 have a characteristic multicolor fluorescence, and their volumes were determined using a sphere volume approach. The method developed was applied in the evaluation of lipid accumulation by Nannochloropsis sp. under different cultivation conditions (varying nitrate and salinity concentrations and combined effect of these two variables). The results show an increase of lipid content in Nannochloropsis sp. cultivated in nitrogen replete and depleted conditions, from 9.4 to 40.8 μm³ cell^?1 and 35.5 to 73.5%, respectively. The findings are also compared with conventional methods for determination of neutral lipids and with results obtained from the dyes Nile Red and BODIPY 505/515. A reasonable agreement between neutral lipid production measured by BODIPY BD-C12 and gravimetric methods (correlation coefficient of 0.98) was obtained. The neutral lipids production decreased from 964.6 to 244.8 mg L^?1 and from 809.1 to 396.7 mg L^?1, as the nitrate concentration increased from 0 to 0.3 g L^?1. It is observed that, with the two commercially available dyes, lipid quantification using Nile Red leads to an overestimation of lipids, while the use of BODIPY 505/515 promoted unreliable measures due to rapid bleaching of the chromophore. The method proposed shows excellent potential to become a standard, yet advanced, strategy for rapid evaluation and quantification of intracellular lipids in microalgae, a crucial step of the scaling-up process involved in the production of biobased products.     

An alternative high-throughput staining method for detection of neutral lipids in green microalgae for biodiesel applications

A simple and high-throughput method for determining in situ intracellular neutral lipid accumulation in Chlorella ellipsoidea and Chlorococcum infusionum with flow cytometry and confocal microscopy was established by employing different solvents and a lipophilic dye, Nile red. Seven different organic solvents, acetic acid, dimethyl sulfoxide (DMSO), acetone, methanol, ethanol, n-hexane, and chloroform at different concentrations ranging from 0 to 80% (v/v) were tested. The fluorescence signal for neutral lipids was collected with a 586/42 emission filter (PE-A) and the maximum fluorescence intensity (% grandparent) was measured as 74.01 ± 4.82% for Chlorella and 70.1 ± 5.52% for Chlorococcum at 30% acetic acid (v/v). The statistical analysis of Nile red-stained cells showed a high coefficient of variation (CV), standard deviation (SD), mean, and median values in the acetic acid-based staining method, followed by DMSO, n-hexane and chloroform. Confocal microscopy revealed a high rate of accumulation of cytosolic neutral lipids when stained with Nile red and other organic solvents. Higher lipid accumulation in Fesupplemented conditions was also detected and a maximum lipid content of 57.36 ± 0.41% (4-fold) in Chlorella and 48.20 ± 0.43% (4-fold) in Chlorococcum were measured at 0.001 g/L of ferrous sulfate (FeSO₄). High fluorescence intensity (75.16 ± 0.24% in Chlorella and 72.24 ± 1.07% in Chlorococcum) in Fe-treated cells confirmed the efficiency of the staining procedure.     

System-level network analysis of nitrogen starvation and recovery in <Emphasis Type="Italic">Chlamydomonas reinhardtii</Emphasis> reveals potential new targets for increased lipid accumulation

Background

Nitrogen starvation is known to cause drastic alterations in physiology and metabolism leading to the accumulation of lipid bodies in many microalgae, and it thus presents an important alternative for biofuel production. However, despite the importance of this process, the molecular mechanisms that mediate the metabolic remodeling induced by N starvation and especially by stress recovery are still poorly understood, and new candidates for bioengineering are needed to make this process useful for biofuel production.

Results

We have studied the molecular changes involved in the adaptive mechanisms to N starvation and full recovery of the vegetative cells in the microalga Chlamydomonas reinhardtii during a four-day time course.High throughput mass spectrometry was employed to integrate the proteome and the metabolome with physiological changes. N starvation led to an accumulation of oil bodies and reduced Fv/Fm.. Distinct enzymes potentially participating in the carbon-concentrating mechanism (CAH7, CAH8, PEPC1) are strongly accumulated. The membrane composition is changed, as indicated by quantitative lipid profiles. A reprogramming of protein biosynthesis was observed by increased levels of cytosolic ribosomes, while chloroplastidic were dramatically reduced. Readdition of N led to, the identification of early responsive proteins mediating stress recovery, indicating their key role in regaining and sustaining normal vegetative growth.Analysis of the data with multivariate correlation analysis, Granger causality, and sparse partial least square (sPLS) provided a functional network perspective of the molecular processes. Cell growth and N metabolism were clearly linked by the branched chain amino acids, suggesting an important role in this stress. Lipid accumulation was also tightly correlated to the COP II protein, involved in vesicle and lysosome coating, and a major lipid droplet protein. This protein, together with other key proteins mediating signal transduction and adaption (BRI1, snRKs), constitute a series of new metabolic and regulatory targets.

Conclusions

This work not only provides new insights and corrects previous models by analyzing a complex dataset, but also increases our biochemical understanding of the adaptive mechanisms to N starvation in Chlamydomonas, pointing to new bioengineering targets for increased lipid accumulation, a key step for a sustainable and profitable microalgae-based biofuel production.

     

Metabolic characteristics and lipid composition of yeastlike cells and mycelium of <Emphasis Type="Italic">Mucor circinelloides</Emphasis> var. <Emphasis Type="Italic">lusitanicus</Emphasis> INMI grown at a high glucose content in the medium

It is shown that the fungus Mucor circinelloides var. lusitanicus INMI grown under aerobic conditions in a medium with a high glucose concentration (20%) is capable of both yeastlike and mycelial growth. In the mycelium, the activity of NAD-dependent isocitrate dehydrogenase was more than twice as high as in yeastlike cells, whereas the isocitrate lyase activity was lower. A number of significant differences were found in the lipid composition of the cells of two different morphological variants. Yeastlike cells contained more polar lipids and free fatty acids and less principal reserve lipids (triacylglycerides) than mycelial cells; the content of γ-linolenic acid and the degree of lipid unsaturation were significantly lower in these cells than in the mycelium. In yeastlike cells, glycolipids composed the bulk of polar lipids; the proportion of phospholipids (primarily phosphatidylserine, phosphatidylcholine, phosphatidylethanolamine, and cardiolipin) was lower. The relationship between cellular metabolism and the lipid composition of fungal cells of different morphotypes grown at high concentrations of glucose, one of the main inducers of dimorphic growth, is discussed.     

Impact of nitrogen limitation on biomass,photosynthesis, and lipid accumulation in <Emphasis Type="Italic">Chlorella sorokiniana</Emphasis>

Induction of oil accumulation in algae for biofuel production is often achieved by withholding nitrogen. However, withholding nitrogen often reduces total biomass yield. In this report, it is demonstrated that Chlorella sorokiniana will not only accumulate substantial quantities of neutral lipids when grown in the absence of nitrogen but will also exhibit unimpeded growth rates for up to 2 weeks. To determine the physiological basis for the observed increase in oil and biomass accumulation, we compared photosynthetic and respiration rates and chlorophyll, lipid, and total energy content under ammonia replete and deplete conditions. Under N-depleted growth conditions, there was a 64 % increase in total energy density and a ～20-fold increase in oil accumulation relative to N-replete growth leading to a 1.6-fold greater total energy yield in N-depleted than in N-replete cultures. We propose that the higher energy accumulation in N-depleted cultures is due to enhanced photosynthetic energy capture and conversion associated with reduced chlorophyll levels and reduced self-shading as well as a shift in metabolism leading to the accumulation of oils.     

Effects of neutrophils peptide-1 transgenic <Emphasis Type="Italic">Chlorella ellipsoidea</Emphasis> on the gut microbiota of male Sprague–Dawley rats,as revealed by high-throughput 16S rRNA sequencing

Rabbit neutrophils peptide-1 (NP-1) is a type of defensin that possesses a broad spectrum of antimicrobial activity. Chlorella ellipsoidea is a new eukaryotic expression system for exogenously producing NP-1. The NP-1 transgenic C. ellipsoidea can be directly added into feed as antimicrobial agent without any purification procedure for the NP-1 peptide. However, the effects of C. ellipsoidea and NP-1 on the host gut microbiota should be explored before application. In this study, diets containing different concentrations (1.25, 2.5, and 5 %) of C. ellipsoidea and NP-1 transgenic C. ellipsoidea were administered to male Sprague–Dawley rats. Compared with the chow diet control group, none of the experimental groups showed any significant differences in their growth indices, and no histopathological damage was observed. The phylotypes of gut microbiota in the control group, the 5 % C. ellipsoidea diet group and the 5 % NP-1 transgenic C. ellipsoidea diet group were determined by 16S rRNA sequencing. The results showed that both 5 % experimental groups had shifted community memberships of gut microbiota. In particular, the 5 % NP-1 transgenic C. ellipsoidea diet exhibited an increased abundance of most Gram-positive bacterial taxa and a reduced abundance of most Gram-negative bacterial taxa, and it promoted the growth of some lactic acid bacterial genera. Lactic acid bacteria, especially the Bifidobacterium and Lactobacillus, have been widely reported to be benefic effects on the host. Thus NP-1 transgenic C. ellipsoidea is promising feed additive and gut regulator, as it have the potential to increase the abundance of Bifidobacterium and Lactobacillus in gut microbiota of animal.     

Transformation of Lipid Bodies Related to Hydrocarbon Accumulation in a Green Alga,Botryococcus braunii (Race B)

The colonial microalga Botryococcus braunii accumulates large quantities of hydrocarbons mainly in the extracellular space; most other oleaginous microalgae store lipids in the cytoplasm. Botryococcus braunii is classified into three principal races (A, B, and L) based on the types of hydrocarbons. Race B has attracted the most attention as an alternative to petroleum by its higher hydrocarbon contents than the other races and its hydrocarbon components, botryococcenes and methylsqualenes, both can be readily converted into biofuels. We studied race B using fluorescence and electron microscopy, and clarify the stage when extracellular hydrocarbon accumulation occurs during the cell cycle, in a correlation with the behavior and structural changes of the lipid bodies and discussed development of the algal colony. New accumulation of lipids on the cell surface occurred after cell division in the basolateral region of daughter cells. While lipid bodies were observed throughout the cell cycle, their size and inclusions were dynamically changing. When cells began dividing, the lipid bodies increased in size and inclusions until the extracellular accumulation of lipids started. Most of the lipids disappeared from the cytoplasm concomitant with the extracellular accumulation, and then reformed. We therefore hypothesize that lipid bodies produced during the growth of B. braunii are related to lipid secretion. New lipids secreted at the cell surface formed layers of oil droplets, to a maximum depth of six layers, and fused to form flattened, continuous sheets. The sheets that combined a pair of daughter cells remained during successive cellular divisions and the colony increased in size with increasing number of cells.     

Biochemical and physiological responses of <Emphasis Type="Italic">Selenastrum gracile</Emphasis> (Chlorophyceae) acclimated to different phosphorus concentrations

Algae are able to adjust their metabolism according to their environment, maximizing growth rate and production of biomolecules under adverse conditions such as pulses of excess of a contaminant or limitation of a nutrient. In order to evaluate the effects of phosphorus (P) availability on the biochemical composition of the freshwater microalga Selenastrum gracile, we acclimated the microalgae to different phosphorus concentrations. After acclimation, exponentially growing cells were inoculated and after 120 h, samples were processed for the determination of carbohydrate, lipid, fatty acid, chlorophyll, cell density, growth rate, and dry weight. Cell density, growth rate, and dry weight decreased with less P, while chlorophyll a, carbohydrates, lipids, and fatty acids per cell increased under P limitation. According to our lipid class and fatty acid results, algae alter their metabolism and membrane configuration to avoid more structural or metabolic damage under limitation, especially at 23 μmol P L^?1. The most sensitive parameters under P limitation were chlorophyll a, lipids, and poly- and monounsaturated fatty acids. The changes in fatty acids contributed to the fluorescence and photosynthesis changes under P limitation, and they occurred before changes were detected in other parameters, such as growth rate. Furthermore, we suggest that prior acclimation to different P affected microalgal physiology and metabolism.     

Lipid composition of the three co-existing <Emphasis Type="Italic">Calanus</Emphasis> species in the Arctic: impact of season,location and environment

Arctic species of Calanus are critical to energy transfer between higher and lower trophic levels and their relative abundance, and lipid content is influenced by the alternation of cold and warm years. All three species of Calanus were collected during different periods in Kongsfjorden (Svalbard, 79°N) and adjacent shelf during the abnormally warm year of 2006. Lipid composition and fatty acid structure of individual lipid classes were examined in relation with population structure. Wax esters dominated the neutral lipid fraction. Phosphatidylcholine (PC) dominated the structural lipids followed by phosphatidylethanolamine (PE). PC/PE ratios of 3–6 suggested an increase in PC proportions compared to earlier studies. Depending on the time scale, fatty acids of wax esters illustrated either trophic differences between fjord and offshore conditions for C. hyperboreus and C. finmarchicus or trophic differences related to seasonality for C. glacialis. Similarly, seasonality and trophic conditions controlled the changes in fatty acids of triglycerides, but de novo synthesis of long-chain monoenes suggested energy optimization to cope with immediate metabolic needs. Polar lipids fatty acid composition was species specific and on the long-term (comparison with data from the past decade) composition appears related to changes in trophic environment. Fatty acid composition of PC and PE indicated relative dominance of 20:5n-3 in PC and 22:6n-3 in PE for all three species. The combination of PE and PC acyl chain and phospholipid head group restructuring indicates an inter-annual variability and suggests that membrane lipids are the most likely candidate to evaluate adaptive changes in Arctic copepods to hydrothermal regime.     

Lipid profiles reveal different responses to brown planthopper infestation for pest susceptible and resistant rice plants

Introduction

Brown planthopper (BPH) is the most destructive insect pest for rice, causing major reductions in rice yield and large economic losses. More than 31 BPH-resistance genes have been located, and several of them have been isolated. Nevertheless, the metabolic mechanism related to BPH-resistance genes remain uncharacterized.

Objectives

To elucidate the resistance mechanism of the BPH-resistance gene Bph6 at the metabolic level, a Bph6-transgenic line R6 (BPH-resistant) and the wild-type Nipponbare (BPH-susceptible) were used to investigate their lipid profiles under control and BPH treatments.

Methods

In conjunction with multivariate statistical analysis and quantitative real-time PCR, BPH-induced lipid changes in leaf blade and leaf sheath were investigated by GC–MS-based lipidomics.

Results

Forty-five lipids were identified in leaf sheath extracts. Leaf sheath lipidomics analysis results show that BPH infestation induces significant differences in the lipid profiles of Nipponbare and R6. The levels of hexadecanoic acid, methyl ester, linoleic acid, methyl ester, linolenic acid, methyl ester, glycidyl palmitate, eicosanoic acid, methyl ester, docosanoic acid, methyl ester, beta-monolinolein, campesterol, beta-sitosterol, cycloartenol, phytol and phytyl acetate had undergone enormous changes after BPH feeding. These results illustrate that BPH feeding enhances sterol biosynthetic pathway in Nipponbare plants, and strengthens wax biosynthesis and phytol metabolism in R6 plants. The results of quantitative real-time PCR of 5 relevant genes were consistent with the changes in metabolic level. Forty-five lipids were identified in the leaf blade extracts. BPH infestation induces distinct changes in the lipid profiles of the leaf blade samples of Nipponbare and R6. Although the lipid changes in Nipponbare are more drastic, the changes within the two varieties are similar. Lipid profiles in leaf sheath brought out significant differences than in leaf blade within Nipponbare and R6. We propose that Bph6 mainly affects the levels of lipids in leaf sheath, and mediates resistance by deploying metabolic re-programming during BPH feeding.

Conclusion

The results indicate that wax biosynthesis, sterol biosynthetic pathway and phytol metabolism play vital roles in rice response to BPH infestation. This finding demonstrated that the combination of lipidomics and quantitative real-time PCR is an effective approach to elucidating the interactions between brown planthopper and rice mediated by resistance genes.

     

The augmented lipid productivity in an emerging oleaginous model alga <Emphasis Type="Italic">Coccomyxa subellipsoidea</Emphasis> by nitrogen manipulation strategy

The lipid productivity controlled by both of biomass and lipid content was really crucial for economic-feasibility of microalgae-based biofuels production. This study attempted at augmenting lipid productivity in an emerging oleaginous model alga Coccomyxa subellipsoidea by different nitrogen manipulation including one-stage continuous N-sufficiency (OCNS), N-deprivation (OCND), N-limitation (OCNL), and also two-stage batch N-starvation (TBNS). Amongst four tested nitrogen manipulation strategies, OCNS performed remarkable promoting effect on cell metabolic growth and the maximum biomass was achieved by 7.39 g/L. Whereas TBNS regime induced the highest lipid content (over 50.5%). Only OCNL treatment augmented the lipid productivity by 232.37 mg/L/day, representing 1.25-fold more than TBNS and even as much as 5.06-fold more than that of OCND strategy. OCNL also strengthened the proportions of saturated (C16:0 and C18:0) and monounsaturated fatty acid (C18:1) which were inclined to high-quality biofuels-making. This might be due to that most part of energy and metabolic flux (e.g. acetyl-CoA) derived from TCA cycle and glycolysis flowed into fatty acids biosynthesis pathway (especially C18:1) response to OCNL manipulation. This study represented a pioneering work of utilizing OCNL for lipids production by C. subellipsoidea and clearly implied that OCNL might be a feasible way for algal lipid production on a commercial scale and also promoted the potential of C. subellipsoidea as an ideal biodiesel feedstock.     

The enhanced biomass and lipid accumulation in <Emphasis Type="Italic">Coccomyxa subellipsoidea</Emphasis> with an integrated treatment strategy initiated by brewery effluent and phytohormones

Brewery effluent (BE) as an appreciable and sustainable resource presented new possibilities in low-cost algal biomass production, whereas the relatively low essential macronutrients hindered extensive applications as growth medium for microalgae cultivation. The objective of this study was to investigate the feasibility of an integrated treatment strategy initiated by BE coupling phytohormones in augmenting biomass and lipid accumulation in Coccomyxa subellipsoidea. Results revealed that BE coupling synthetic 1-naphthaleneacetic acid (NAA) accomplished the favorable lipid productivity of 481.76 mg/L/days, representing 6.80- to 9.71-fold more than that of single BE as well as standard Basal media. BE coupling NAA feeding also heightened the proportions of C16–C18 fatty acids (over 96%) and mono-unsaturated C18:1 (approximate 45%) which were prone to high-quality biofuels-making. Such profound lipids accumulation might be attributable to that BE coupling NAA treatment drove most of metabolic flux (i.e. acetyl-CoA) derived from TCA cycle and glycolysis flowing into lipid accumulation pathway. Concurrently, the complete removal of total nitrogen and total phosphorus by C. subellipsoidea with assistance of NAA were easily complied with the permissible dischargeable limits for BE. These present results strongly demonstrated that BE coupling NAA was a potential feeding strategy in boosting algal lipid productivity and further provided great possibilities in linking affordable algal biomass production with high-efficient biological contaminants removal.     

Metabolic responses to salinity changes in the subantarctic notothenioid teleost <Emphasis Type="Italic">Eleginops maclovinus</Emphasis>

Eleginops maclovinus is an endemic, subantarctic Notothenioidei species. This study examined the influence of different environmental salinities (5, 15, and 45 psu; and 32 psu as a control) on energy metabolism in E. maclovinus over a period of 14 days. Metabolite contents and enzymatic activities related to carbohydrate, amino acid, and lipid metabolisms were evaluated in metabolic (liver) and osmoregulatory (gill and kidney) tissues. At extreme salinities (5 and 45 psu), the liver showed a high consumption of energy reserves, mainly as amino acids and carbohydrates. Carbohydrate metabolism in the gills did not change under different salinities, but increased lactate levels were found, suggesting that this tissue may use lactate as an energy substrate. Amino acid metabolism in the gills decreased at 5 psu but increased at 45 psu, and lipid metabolism increased at 5 and 15 psu during the first days of the trial, indicating a possible use of lipids as energy. Kidney carbohydrate catabolism and amino acid metabolism increased after 14 days at 45 psu, while lipid metabolism did not vary in relation to salinity changes. Together, these results suggest that the liver is most affected by salinity changes, probably due to its role as a supplier of energetic substrates. The gills and kidney, osmoregulatory tissues, maintained their energy metabolism levels with minor modifications. In conclusion, E. maclovinus exhibits metabolic adjustments to adapt to different salinities, showing the best responses in isosmotic environmental salinities.     

Cyanobacteria <Emphasis Type="Italic">Nostoc Punctiforme</Emphasis> from Abyssal Benthos of Lake Baikal: Unique Ecology and Metabolic Potential

A strain of Nostoc punctiforme was isolated from the bottom sediments of the oil seep at Gorevoy Utes (Central Baikal) at a depth of 890 m. The Baikal strain is highly similar (98–99%) to the N. punctiforme CCAP 1453/9 strain and the typical N. punctiforme PCC 73103 strain isolated from soil ecotopes. Based on the analysis of functional genes and mass spectrometry data, we determined that the strain can produce bioactive peptides and polyketides, but does not produce known cyanobacterial toxins, saxitoxin or its analogs, or microcystins. The peptides aeruginosinamide, aeruginosin 606, aeruginosin 98-A, kasumigamide C, and microginin 91-D were recorded in the metabolic profile of the strain. The major ion found in the MALDI mass spectrum is most likely to be an ion of a polyketide substance with unknown function.