Adaptation of soil microbial growth to temperature: Using a tropical elevation gradient to predict future changes

Terrestrial biogeochemical feedbacks to the climate are strongly modulated by the temperature response of soil microorganisms. Tropical forests, in particular, exert a major influence on global climate because they are the most productive terrestrial ecosystem. We used an elevation gradient across tropical forest in the Andes (a gradient of 20°C mean annual temperature, MAT), to test whether soil bacterial and fungal community growth responses are adapted to long‐term temperature differences. We evaluated the temperature dependency of soil bacterial and fungal growth using the leucine‐ and acetate‐incorporation methods, respectively, and determined indices for the temperature response of growth: Q₁₀ (temperature sensitivity over a given 10oC range) and T_min (the minimum temperature for growth). For both bacterial and fungal communities, increased MAT (decreased elevation) resulted in increases in Q₁₀ and T_min of growth. Across a MAT range from 6°C to 26°C, the Q₁₀ and T_min varied for bacterial growth (Q_10–20 = 2.4 to 3.5; T_min = ?8°C to ?1.5°C) and fungal growth (Q_10–20 = 2.6 to 3.6; T_min = ?6°C to ?1°C). Thus, bacteria and fungi did not differ significantly in their growth temperature responses with changes in MAT. Our findings indicate that across natural temperature gradients, each increase in MAT by 1°C results in increases in T_min of microbial growth by approximately 0.3°C and Q_10–20 by 0.05, consistent with long‐term temperature adaptation of soil microbial communities. A 2°C warming would increase microbial activity across a MAT gradient of 6°C to 26°C by 28% to 15%, respectively, and temperature adaptation of microbial communities would further increase activity by 1.2% to 0.3%. The impact of warming on microbial activity, and the related impact on soil carbon cycling, is thus greater in regions with lower MAT. These results can be used to predict future changes in the temperature response of microbial activity over different levels of warming and over large temperature ranges, extending to tropical regions.     

The visual response from the compound eye of Oncopeltus fasciatus: Effects of temperature and sensory adaptation

The effects of temperature (10–30°C) on response latency and amplitude were determined in the dark- and light-adapted compound eye of the milkweed bug, Oncopeltus fasciatus. Response amplitude was an inverse function of temperature under dark-adapted conditions, but was nearly independent of temperature when superimposed on a background field. Response amplitude to a test stimulus (S₂) was maximum at 15–20°C when presented 10 sec after an adapting stimulus (S₁). The optimum temperature (10°C or less) for maximum response amplitude from dark-adapted eyes was far below the temperature at which the animals were grown (19–25°C) and lower than previously reported for insect compound eyes. These results are discussed in terms of the adaptive implications for diurnal, terrestrial ectotherms.Response latency from dark-adapted eyes was an inverse exponential function of temperature with an apparent activation energy of 13·6 kcal mole⁻¹. No change in activation energy could be attributed to light adaptation.Reductions of S₂ response latency and amplitude, during the adaptation régimes employed in this study, were different functions of temperature. Adapting stimuli, which were equally effective at reducing S₂ latency, had different effects on S₂ response amplitude. Recovery of S₂ response latency and amplitude were not related at either 20 or 10°C. Therefore, changes in S₂ response latency as a function of adaptation appeared independent of changes in S₂ response amplitude.     

EVOLUTIONARY ADAPTATION TO TEMPERATURE. IV. ADAPTATION OF ESCHERICHIA COLI AT A NICHE BOUNDARY

Following an environmental change, the course of a population's adaptive evolution may be influenced by environmental factors, such as the degree of marginality of the new environment relative to the organism's potential range, and by genetic factors, including constraints that may have arisen during its past history. Experimental populations of bacteria were used to address these issues in the context of evolutionary adaptation to the thermal environment. Six replicate lines of Escherichia coli (20°C group), founded from a common ancestor, were propagated for 2000 generations at 20°C, a novel temperature that is very near the lower thermal limit at which it can maintain a stable population size in a daily serial transfer (100-fold dilution) regime. Four additional groups (32/20, 37/20, 42/20, and 32–42/20°C groups) of six lines, each with 2000 generation selection histories at different temperatures (32, 37, 42, and daily alternation of 32 and 42°C), were moved to the same 20°C environment and propagated in parallel to ascertain whether selection histories influence the adaptive response in this novel environment. Adaptation was measured by improvement in fitness relative to the common ancestor in direct competition experiments conducted at 20°C. All five groups showed improvement in relative fitness in this environment; the mean fitness of the 20°C group after 2000 generations increased by about 8%. Selection history had no discernible effect on the rate or final magnitude of the fitness responses of the four groups with different histories after 2000 generations. The correlated fitness responses of the 20°C group were measured across the entire thermal niche. There were significant tradeoffs in fitness at higher temperatures; for example, at 40°C the average fitness of the 20°C group was reduced by almost 20% relative to the common ancestor. We also observed a downward shift of 1–2°C in both the upper and lower thermal niche limits for the 20°C selected group. These observations are contrasted with previous observations of a markedly greater rate of adaptation to growth near the upper thermal limit (42°C) and a lack of trade-off in fitness at lower temperatures for lines adapted to that high temperature. The evolutionary implications of this asymmetry are discussed.     

Thermal preferences of subtropical Aedes aegypti and temperate Ae. japonicus mosquitoes

Temperature is an important determinant affecting the capacity of disease vectors like mosquitoes (Culicidae) to transmit disease agents. Although the impact of temperature on vector-borne disease dynamics has been studied intensively, the actual temperature encountered by the vector in a heterogeneous landscape is rarely taken into account. If disease vectors have temperature preferences and therefore select specific microhabitats, this would substantially influence key life history traits that determine transmission intensity. The thermal preferences of subtropical Aedes aegypti and temperate Ae. japonicus mosquitoes were investigated in a temperature gradient set-up consisting of a Plexiglas box on top of an aluminium plate on two thermal regulators. Blood-fed (one day after feeding) and unfed (non-blood-fed) mosquitoes were released in small (15–20 °C, 20–25 °C, 25–30 °C) and large (15–30 °C, 30–45 °C) temperature gradients to assess their thermal preferences after 15 min. Additionally, the effect of humidity was investigated in a two-choice chamber setup. Both mosquito species avoided higher temperatures, pronouncedly dangerously high temperatures in the 30–45 °C gradient. At lower temperatures, blood-fed mosquitoes preferred the cooler sides of the 20–25 °C and 25–30 °C gradient, which were all below their rearing temperature. In the lowest gradient of 15–20 °C, no preferences were found. The thermal preference of unfed mosquitoes was similar to that of the blood-fed mosquitoes. No humidity preference or effect of humidity on temperature preferences was observed within the tested range (40–90%). The set-up allows for assessing the thermal preference of mosquitoes under controlled conditions. The observed preference of mosquitoes for cooler temperatures would increase their longevity and slow down pathogen development. If similar microhabitat selection is observed in the field, vector borne disease models should be adjusted accordingly.     

EVOLUTIONARY ADAPTATION TO TEMPERATURE. V. ADAPTIVE MECHANISMS AND CORRELATED RESPONSES IN EXPERIMENTAL LINES OF ESCHERICHIA COLI

We previously demonstrated temperature-specific genetic adaptation in experimental lines of Escherichia coli. Six initially identical populations were propagated for 2000 generations under each of five regimes: constant 20°C, 32°C, 37°C, and 42°C, and a daily switch between 32°C and 42°C. Glucose was the sole carbon source in all cases. Here, we examine the physiological bases of adaptation to determine whether the same mechanisms evolved among the replicate lines within each thermal regime and across different regimes. Specifically, we investigate whether changes in glucose transport may account for the temperature-specific adaptation. We compared each line's direct response of fitness to glucose with its correlated response to maltose; glucose and maltose enter the cell by different pathways, but their catabolism is identical. Except for lines maintained at the ancestral temperature (37°C), almost all derived lines had significantly different fitnesses (relative to their common ancestor) in glucose and maltose, supporting the hypothesis that adaptation involved changes in glucose transport. An alternative explanation, that maltose transport decayed by genetic drift, appears unlikely for reasons that are discussed. Although most lines showed evidence of temperature-specific adaptation in glucose transport, several different mechanisms may underlie these improvements, as indicated by heterogeneity in correlated responses (across temperatures and substrates) among replicate lines adapted to the same regime. This heterogeneity provides a latent pool of genetic variation for responding to environmental change.     

Temperature requirements for growth and maturation of the warm temperate kelp Eckloniopsis radicosa (Laminariales,Phaeophyta)

The annual kelp Eckloniopsis radicosa is distributed along Japanese coasts and occurs within the area with a February isotherm ranging 15–18°C and August isotherm ranging 25–28°C. In this study, the effects of temperature on the gametophyte growth and maturation, and the young sporophyte growth of E. radicosa were examined and the results are discussed in relation to the distribution of other warm‐adapted kelp species and the potential effects of climate change. The optimal temperature ranges for growth of male and female gametophytes were 23–27°C and 20–26°C, respectively. The upper survival temperature for gametophyte growth was 31°C for males and 30°C for females, respectively. The optimal temperature range for maturation of female gametophytes was ≤23°C. The optimal temperature range for growth of young sporophytes was 14–22°C. It was clarified that E. radicosa has the most warm‐adapted characteristics for growth and maturation of gametophytes among members of the Laminariales studied so far. The natural seawater temperature ranges during the growth and maturation seasons for gametophytes of E. radicosa, as well as the growth season for young sporophytes near to the northern and southern distribution limits (Izu‐Oshima: 14.9–24.5°C, Ichiki‐kushikino: 17.1–29.6°C), agreed with the experimentally determined temperature requirements. The warm‐adapted gametophyte stage and annual lifecycle are major factors enabling survival of E. radicosa in warm waters near tropical regions along the Japanese coast.     

Temperature-Sensitive Responses in Blepharisma

Temperature sensitivity of Blepharisma cultured at 23°C was investigated in a temperature range between 18.5°C and 33.5°C. The cells accumulated in an optimal temperature (ca. 27°C) region when they were placed in a chamber with a temperature gradient, although a certain population of the cells accumulated at much higher temperatures. The quantitative analysis of behavioral responses exhibited by the cells revealed that three types of thermal response were responsible for thermoaccumulation of the cells in an optimal temperature: (1) an increase in the frequency of thermophobic response in the cells swimming away from the optimal temperature region; (2) acceleration of forward swimming velocity of the cells swimming toward the optimal temperature region; and (3) higher frequency of spontaneous ciliary reversal of the cells in higher temperature regions.     

ADAPTATION OF PHOTOSYNTHESIS IN LAMINARIA SACCHARINA (PHAEOPHYTA) TO CHANGES IN GROWTH TEMPERATURE1

The effect of growth temperature on photosynthetic metabolism was studied in the kelp Laminaria saccharina (L.) Lamour. Photosynthesis was subject to phenotypic adaptation, with almost constant photosynthetic rates being achieved at growth temperatures between 0 and 20° C. This response involved: (1) an inverse relationship between growth temperature and photosynthetic capacity, (2) a reduction in the Q₁₀ value for photosynthesis of L. saccharina grown at 0 and 5° C compared with 10, 15 and 20° C grown sporophytes, and (3) an acquired tolerance of photosynthesis to temperatures between 15–25° C (which inhibited photosynthesis in 0 and 5° C grown L. saccharina) in sporophytes grown at 10, 15 and 20° C. The physiological basis of these adaptations is discussed in terms of observed changes in activities and kinetics of the Calvin cycle enzyme ribulose-1, 5-bisphosphate carboxylase (oxygenase) and efficiency of light harvesting-electron transport systems.     

Adaptivity of the bivalve Mytilus trossulus larvae to short-and long-term changes in water temperature and salinity

Adaptivity to short-term and long-term changes in water temperature and salinity was studied in larvae of the bivalve mollusk Mytilus trossulus. It was shown that water temperature of 4°C mostly suppressed growth and development of larvae. A temperature of 20°C promoted an enhanced larval growth and development. Though a temperature of 20°C caused enhanced larval growth, the temperature was not optimal, while its effect caused quality diversity of larval development, owing to the difference in their growth rates. Such diversity was not observed at moderate temperatures of 10 and 15°C. At 20°C, fast-growing mussel larvae were very sensitive to temperature drops. Growth of slowly-growing individuals did not depend on temperature in the range of 10 to 20°C. Daily temperature variations by 3–8°C did not markedly affect growth and development of the larvae. A continuous 24-h exposure to temperature drops by 3–8°C did not influence these very important physiological characteristics either. A salinity drop down to 8‰ exerted an adverse effect only on early larvae. Later on, the larvae showed their ability to adapt to such a strong desalination. The negative effect of reduced salinity (to 8‰) upon mussel larvae was increased at a temperature increase to 20°C.     

Low-temperature limitation of primary photosynthetic processes in Antarctic lichens <Emphasis Type="Italic">Umbilicaria antarctica</Emphasis> and <Emphasis Type="Italic">Xanthoria elegans</Emphasis>

Temperature response curves of chlorophyll a fluorescence parameters were used to assess minimum sub-zero temperature assuring functioning of photosynthetic photochemical processes in photosystem II (PS II) of Antarctic lichens. Umbilicaria Antarctica and Xanthoria elegans were measured within the temperature range from −20 to +10°C by a fluorometric imaging system. For potential (F _V/F _M) and actual (Φ _II) quantum yields of photochemical processes the minimum temperature was found to be between −10 and −20°C. Non-photochemical quenching (NPQ) of absorbed excitation energy increased with temperature drop reaching maximum NPQ at −15°C. Image analysis revealed intrathalline heterogeneity of chlorophyll a fluorescence parameters with temperature drop. Temperature response of Φ _II exhibited an S-curve with pronounced intrathalline differences in X. elegans. The same relation was linear with only limited intrathalline difference in U. antarctica. The results showed that Antarctic lichen species were well adapted to sub-zero temperatures and capable of performing primary photosynthesis at −15°C.     

Proteomic analysis of the adaptation to warming in the Antarctic bacteria Shewanella frigidimarina

Antarctica is subjected to extremely variable conditions, but the importance of the temperature increase in cold adapted bacteria is still unknown. To study the molecular adaptation to warming of Antarctic bacteria, cultures of Shewanella frigidimarina were incubated at temperatures ranging from 0 °C to 30 °C, emulating the most extreme conditions that this strain could tolerate. A proteomic approach was developed to identify the soluble proteins obtained from cells growing at 4 °C, 20 °C and 28 °C. The most drastic effect when bacteria were grown at 28 °C was the accumulation of heat shock proteins as well as other proteins related to stress, redox homeostasis or protein synthesis and degradation, and the decrease of enzymes and components of the cell envelope. Furthermore, two main responses in the adaptation to warm temperature were detected: the presence of diverse isoforms in some differentially expressed proteins, and the composition of chaperone interaction networks at the limits of growth temperature. The abundance changes of proteins suggest that warming induces a stress situation in S. frigidimarina forcing cells to reorganize their molecular networks as an adaptive response to these environmental conditions.     

Effects of prolonged exposure to cold on the spontaneous activity of two different types of filiform sensilla in Pyrrhocoris apterus

We recorded the spontaneous activity of T₁ and T₂ filiform sensilla from October to May in Pyrrhocoris apterus acclimatized to outdoor conditions. The aim of the study was to determine how prolonged exposure to cold affects two closely related mechanosensitive sensilla. We recorded the activity at seven temperatures from 5 to 35 °C. In both sensilla types the activity level was reduced during winter, which correlated to changes in acclimatization temperature (r = 0.7), the reduction was greater at high recording temperatures, and the effects of exposure to cold were reversed by transferring the animals indoors. However, T₁ activity always increased monotonically, if the recording temperature was increased from 5 to 35 °C, whereas T₂ activity in cold-acclimatized animals increased to temperatures between 20 and 30 °C and then started decreasing. As a result, the temperature sensitivity of the activity was reduced more profoundly in T₂ sensilla (in T₂ Q ₁₀ was reduced from 3.5 in October to 1.4 in January, whereas in T₁ it was reduced from 2.5 to 2.2). In conclusion, we have shown that prolonged exposure to cold does affect filiform sensilla; however, the effect is significantly different in the two sensilla types.     

Environmental change in the Colombian subandean forest belt from 8 pollen records: the last 50 kyr

We present a reconstruction of forest history and climatic change based on 11 pollen records from eight sites, all located in the lower montane forest belt of the northern Andes in Colombia. We compared records from the Popayán area in southern Colobia, Timbio (1750 m), Genagra (1750 m) and Pitalito (1300 m) and the new Piagua (1700 m) record with the records from Lusitania (1500 m), Libano (1820 m), Pedro Palo (2000 m) and Ubaqué (2000 m) from Central Colombia. The changes of the altitudinal position of the lower/upper montane (= subandean/Andean, S/At) forest belt transition were used to estimate temperature change for the last 50 kyr. We infer a Last Glacial Maximum (LGM) temperature drop of 6°–7°C at 1700 m, and a steeper LGM lapse rate of 0.76°C/100 m compared to today (ca. 0.6°C/100 m). Around 50 uncal. kyr B.P. the temperature at 1700 m was ca. 3°C lower than today. Until 20 uncal. kyr B.P. the temperature oscillated and gradually decreased. During the LGM, temperature was down to ca. 6°–7°C lower than today. After the LGM, temperature increased and ca. 14 uncal. kyr B.P. it was 2°–3°C lower than today (S/At at ca. 1800 m, 500 m below present elevation; Susacá interstadial). An unquantified cooling (Ciega stadial) followed. During ca. 12.3–11.7 uncal. kyr B.P. the S/At shifted upslope to 2100 m indicating a temperature of 1°–2°C cooler than today (Guantiva interstadial). From 11.7–10.9 uncal. kyr B.P. the S/At was at 1800 m indicating that the temperature was ca. 3°C lower than today and wet conditions prevailed (partly coinciding with the El Abra stadial). The period 10.9–9 uncal. kyr B.P. was also cool, but drier. During 9–7.5 uncal. kyr B.P. temperature was ca. 1°C warmer relative to today (mid Holocene hypsithermal). During the last 5 kyr the presence of cultivated plants demonstates human colonization of the lower montane zone in Colombia. Received June 14, 2000 / Accepted December 19, 2000     

Supraoptimal Temperature Effects upon Agrostis palustris

Agrostis palustris turfs cut weekly at 1.3 cm were subjected to successive four-week periods with day-night temperature regimes of 20–10, 25–15, 30–20, 35–25 and 40–30°C. Plants grown at 40–30°C exhibited a growth character distinctly different from those grown at 20–10°C. They were more upright and bristle-like in growth habit. The percentage dry weight of leaf blade tissue increased 67% and weight per unit area increased 53% between 20–10 and 40–30°C. Reduced leaf blade width was noted first at 30–20°C while leaf blade length reduction first occurred at 35–25°C. Weekly yields were significantly reduced at the supraoptimal temperature regimes of 35–25 and 40–30°C. Chlorophyll content was lowest at 20–10 and 40–30°C, the lowest and highest temperature regimes studied. Shoot density appeared to decrease under the 35–25°C regime, but no dead plants were observed. The apparent decrease in shoot density was attributed to the upright growth habit. Density decreased at 40–30°C upon death of individual plants. A community of grass plants maintained as a turf was found to change in form quite rapidly in response to temperature.     

Indirect selection of thermal tolerance during experimental evolution of Drosophila melanogaster

Natural selection alters the distribution of a trait in a population and indirectly alters the distribution of genetically correlated traits. Long‐standing models of thermal adaptation assume that trade‐offs exist between fitness at different temperatures; however, experimental evolution often fails to reveal such trade‐offs. Here, we show that adaptation to benign temperatures in experimental populations of Drosophila melanogaster resulted in correlated responses at the boundaries of the thermal niche. Specifically, adaptation to fluctuating temperatures (16–25°C) decreased tolerance of extreme heat. Surprisingly, flies adapted to a constant temperature of 25°C had greater cold tolerance than did flies adapted to other thermal conditions, including a constant temperature of 16°C. As our populations were never exposed to extreme temperatures during selection, divergence of thermal tolerance likely reflects indirect selection of standing genetic variation via linkage or pleiotropy. We found no relationship between heat and cold tolerances in these populations. Our results show that the thermal niche evolves by direct and indirect selection, in ways that are more complicated than assumed by theoretical models.     

The role of antennae in the thermopreference and biting response of haematophagous bugs

Insects sense thermal cues mainly through thermoreceptors located in the antenna. To analyse the impact of antennectomy on the thermal behaviour of the haematophagous bug Triatomainfestans, we studied the distribution of intact and antennectomised bugs in an experimental arena where a temperature gradient was established, as well as the biting response of insects with and without antennae to objects at the temperature of a potential host. Antennectomy did not abolish thermopreference, but modified the temperature at which the insects preferred to stay. In the arena, antennectomised insects chose to remain at a higher temperature (ca. 3 °C higher in unfed bugs), and exhibited a larger dispersion around that preferred temperature, than intact bugs. In addition, ablated insects temporarily lost their ability to bite an object at the temperature of a potential host, but that ability was gradually recovered after the fifth day post-antennectomy. Results presented here show that thermoreceptors other than those located on the antennae can also guide thermal behaviours. We conclude that the function of antennal thermoreceptors can be taken over by other receptors located in different regions of the body. Those receptors have a different sensitivity and confer the insects with a different responsiveness.     

Functional response of Telenomus busseolae (Hym.: Scelionidae) an egg parasitoid of the sugarcane stem borer,Sesamia nonagrioides (Lep.: Noctuidae) at different temperatures

Telenomus busseolae Gahan is the most important egg parasitoid of sugarcane stem borers in Iran. Laboratory experiments were conducted to evaluate the functional response of T. busseolae to egg densities of Sesamia nonagrioides (Lefebvre) under four different temperatures (20, 25, 30 and 35°C). Results of logistic regression revealed a type III functional response for all temperatures tested and type of functional response was not affected by temperature. Roger's random parasite equation was fitted to observed data at 35°C. In the other data sets (collected data under 20, 25 and 30 °C) Roger's equation could not provide meaningful parameters, therefore Holling's disk equation was used to fit the data. The attack rate for T. busseolae varying from 0.001784 to 0.10704, 0.0026 to 0.152, 0.0024 to 0.144 and 0.000866 to 0.05196 per h and estimated handling times were 0.59, 0.50, 0.53 and 0.29 h at 20, 25, 30 and 35°C, respectively. Based on asymptotic 95% confidence intervals, functional response parameters did not differ significantly from 20 to 30°C, which indicates that handling time and attack rate are least affected by the temperature changes. The results suggest a similar performance of T. busseolae against S. nonagrioides eggs at temperatures of 20–30°C. These results show that T. busseolae is well adapted to temperature changes during the sugarcane growing season.     

INTERACTIVE EFFECTS OF IRRADIANCE AND TEMPERATURE ON THE PHOTOSYNTHETIC PHYSIOLOGY OF THE PENNATE DIATOM PSEUDO‐NITZSCHIA GRANII (BACILLARIOPHYCEAE) FROM THE NORTHEAST SUBARCTIC PACIFIC1

This study examined how light and temperature interact to influence growth rates, chl a, and photosynthetic efficiency of the oceanic pennate diatom Pseudo‐nitzschia granii Hasle, isolated from the northeast subarctic Pacific. Growth rates were modulated by both light and temperature, although for each irradiance tested, the growth rate was always the greatest at ～14°C. Chl a per cell was affected primarily by temperature, except at the maximum chl a per cell (at 10°C) where the effects of light were noticeable. At both ends of the temperature gradient, cells displayed evidence of chlorosis even at low light intensities. Chl fluorescence data suggested that cells at 8°C were significantly more efficient in their photosynthetic processes than cells at 20°C, despite having comparable concentrations of chl. Cells at low temperature showed photosynthetic characteristics similar to high‐irradiance‐adapted cells. The decline of growth rates beyond the optimum growth temperature coincided with the cell's inability to accumulate chl in response to increasing temperature. The decline in photosynthetic ability at 20°C was likely due to a combination of high‐temperature stress on cellular membranes and a decline in chl. Our results highlight the important interactions between light and temperature and the need to incorporate these interactions into the development of phytoplankton models for the subarctic Pacific.     

A negative staining study of natural and synthetic L-α-lysophosphatidylcholine micelles,macromolecular aggregates and crystals

A comparative assessment has been made by transmission electron microscopy of negatively stained specimens, of the micellar, aggregated and crystalline states of palmitoyl, oleoyl and ex ovo L-α-lysophosphatidylcholine present in aqueous suspensions. Micelle formation from dry lysophosphatidylcholines is shown to be temperature dependent. The presence of the unsaturated fatty acid in oleoyl L-α-lysophosphatidylcholine and some degree of unsaturation in L-α-lysophosphatidylcholine (ex ovo) promotes micelle formation at low temperatures (4 and 22°C), whereas crystalline palmitoyl L-α-lysophosphatidylcholine is essentially insoluble at low temperatures and requires incubation at 60°C to produce a micellar suspension.It is suggested that the micellar conformation is not spherical, a cylindrical or discoid shape is more compatible with the images presented. Both palmitoyl and ex ovo L-α-lysophosphatidylcholine produce flexibel rod-like micellar aggregates ca 6 nm in diameter and larger (20–60 nm dia) stacked-disc aggregates, again with a temperature dependency. The thickness of the disc-like L-α-lysophosphatidylcholine of a phospholipid bilayer (ca 6–7 nm). This, together with the ability of palmitoyl L-α-lysophosphatidylcholine to crystallize as multi-lamellar hexagonal particles which remain stable in aqueous suspensions at 4°C, suggests that, as with other phospholipids, the L-α-lysophosphatidylcholines possess the property of forming lamellar structures, but that these become increasingly unstable at higher temperatures depending on the fatty acid unsaturation. Ammonium molybdate and sodium phosphotungstate have been found to be more satisfactory than uranyl acetate for negative staining of aqueous suspensions of L-α-lysophosphatidylcholines.