Whereas the shortest way between two points is a straight connection, it might not be the most efficient path to follow. Complex currents often affect the motion of microswimmers and make it difficult for them to reach their destination. At the same time, making use of these currents to navigate as fast as possible is a certain evolutionary advantage. Whereas such strategies allow biological microswimmers to better access food or escape a predator, microrobots could this way be directed to perform specific tasks.

The optimal path in a given current can readily be determined mathematically, yet fluctuations perturb the motion of microswimmers and deviate them from the optimal route. Thus, they have to readjust their motion in order to account for environmental changes. This typically requires the help of an external interpreter and takes away their autonomy.

“Thanks to evolution, some microorganisms have developed autonomous strategies that enable directed motion towards larger concentration of nutrients or light,” first author of the study Lorenzo Piro explains. Inspired by this idea, the researchers from the Department of Living Matter Physics at the MPI-DS designed strategies that allow microswimmers to navigate optimally in a nearly autonomous way.

Light as a guide for autonomous navigation

When an external interpreter defines the navigation pattern, microswimmers on average follow a well-defined path. Thus, it is a good approach to guide the microswimmer along that path within the current. This can be achieved autonomously via external stimuli, despite the presence of fluctuations. This principle could be applied to swimmers that respond to variation of light, such as certain algae, in which case the optimal path can simply be illuminated. Remarkably, the resulting performances are comparable to externally supervised navigation. “These new strategies can moreover conveniently be applied to more complex scenario such as navigation on curved surfaces or in presence of random currents,” concludes Ramin Golestanian, director at MPI-DS.

Possible applications of the study thus range from targeted drug delivery at the microscale to the optimal design of autonomous micromachines.

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Materials provided by Max Planck Institute for Dynamics and Self-Organization. Note: Content may be edited for style and length.

Up to 70 percent of mothers develop postnatal depressive mood, also known as baby blues, after their baby is born. Analyses show that this can also affect the development of the children themselves and their speech. Until now, however, it was unclear exactly how this impairment manifests itself in early language development in infants.

In a study, scientists at the Max Planck Institute for Human Cognitive and Brain Sciences in Leipzig have now investigated how well babies can distinguish speech sounds from one another depending on their mother’s mood. This ability is considered an important prerequisite for the further steps towards a well-developed language. If sounds can be distinguished from one another, individual words can also be distinguished from one another. It became clear that if mothers indicate a more negative mood two months after birth, their children show on average a less mature processing of speech sounds at the age of six months. The infants found it particularly difficult to distinguish between syllable-pitches. Specifically, they showed that the development of their so-called Mismatch Response was delayed than in those whose mothers were in a more positive mood. This Mismatch Response in turn serves as a measure of how well someone can separate sounds from one another. If this development towards a pronounced mismatch reaction is delayed, this is considered an indication of an increased risk of suffering from a speech disorder later in life.

“We suspect that the affected mothers use less infant-directed-speech,” explains Gesa Schaadt, postdoc at MPI CBS, professor of development in childhood and adolescence at FU Berlin and first author of the study, which has now appeared in the journal JAMA Network Open. “They probably use less pitch variation when directing speech to their infants.” This also leads to a more limited perception of different pitches in the children, she said. This perception, in turn, is considered a prerequisite for further language development.

The results show how important it is that parents use infant-directed speech for the further language development of their children. Infant-directed speech that varies greatly in pitch, emphasizes certain parts of words more clearly — and thus focuses the little ones’ attention on what is being said — is considered appropriate for children. Mothers, in turn, who suffer from depressive mood, often use more monotonous, less infant-directed speech. “To ensure the proper development of young children, appropriate support is also needed for mothers who suffer from mild upsets that often do not yet require treatment,” Schaadt says. That doesn’t necessarily have to be organized intervention measures. “Sometimes it just takes the fathers to be more involved.”

The researchers investigated these relationships with the help of 46 mothers who reported different moods after giving birth. Their moods were measured using a standardized questionnaire typically used to diagnose postnatal upset. They also used electroencephalography (EEG), which helps to measure how well babies can distinguish speech sounds from one another. The so-called Mismatch Response is used for this purpose, in which a specific EEG signal shows how well the brain processes and distinguishes between different speech sounds. The researchers recorded this reaction in the babies at the ages of two and six months while they were presented with various syllables such as “ba,” “ga” and “bu.

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The researchers have been able to show that quantum waves behave significantly differently from their classical counterparts and can be used to increase the precision of distance measurements. Their findings also add to the discussion on physical origin of the anomalous focusing behaviour. The results are now published in the journal of Nature Photonics.

“Interestingly, we started with an idea based on our earlier results and set out to structure quantum light for enhanced measurement precision. However, we then realised that the underlying physics of this application also contributes to the long debate about the origins of the Gouy phase anomaly of focused light fields.,” explains Robert Fickler, group leader of the Experimental Quantum Optics group at Tampere University.

Quantum waves behave differently but point to the same origin

Over the last decades, methods for structuring light fields down on the single photon level have vastly matured and led to a myriad of novel findings. In addition, a better of optics’ foundations has been achieved. However, the physical origin of why light behaves in such an unexpected way when going through a focus, the so-called Gouy phase anomaly, is still often debated. This is despite its widespread use and importance in optical systems. The novelty of the current study is now to put the effect into the quantum domain.

“When developing the theory to describe our experimental results, we realised (after a long debate) that the Gouy phase for quantum light is not only different than the standard one, but its origin can be linked to another quantum effect. This is just like what was speculated in an earlier work,” adds Doctoral researcher Markus Hiekkamäki, leading author of the study.

In the quantum domain, the anomalous behaviour is sped up when compared to classical light. As the Gouy phase behaviour can be used to determine the distance a beam of light has propagated, the speed up of the quantum Gouy phase could allow for an improvement in the precision of measuring distances.

With this new understanding at hand, the researchers are planning to develop novel techniques to enhance their measurement abilities such that it will be possible to measure more complex beams of structured photons. The team expects that this will help them push forward the application of the observed effect, and potentially bring to light more differences between quantum and classical light fields.

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Fossil findings of insect feeding damage on plants are evidence that insects have been using plants as a food source for more than 400 million years. Researchers led by Roy Kirsch and Yannick Pauchet from the Department of Insect Symbiosis are investigating how herbivorous insects are able to break down the hard-to-digest components of their plant food in the first place. In earlier work, they had already shown that pectin-degrading enzymes are widespread in herbivorous beetle species. They were also able to demonstrate that these beetle enzymes were invariably of microbial origin. The question of the current study was therefore how important these enzymes are for the nutrition and fitness of the insect, in this case the mustard leaf beetle Phaedon cochleariae.

“Our goal was to better understand how herbivorous insects deal with the plant cell wall, which makes the bulk of their diet. Pectin is the matrix embedding cellulose and hemicellulose fibers within the plant cell wall, and it is the main constituent of the cell-connecting middle lamellae. Consequently, pectin must be digested first so that cellulase and hemicellulase enzymes can access their substrates and the plant cells are finally freed from their protective cell wall. In this context, the action of pectinases, i.e. pectin-degrading enzymes, are key to efficient digestion of a leaf beetle’s diet,” says first author Roy Kirsch.

To study the role of the pectinases, the researchers generated beetle lines in which these enzymes were not present. This initially turned out to be more difficult than expected.” “Not even a drastic reduction of pectinase activity through knockdown RNAi experiments in leaf beetle larvae was sufficient to effectively inhibit pectin digestion. We were only able to completely knock-out pectinase-encoding genes in the leaf beetles by applying CRISPR/Cas9 genome editing,” says study leader Yannick Pauchet, underlining the importance of the “genetic scissors,” a technology for which Emmanuelle Charpentier and Jennifer Doudna were awarded the 2020 Nobel Prize in Chemistry.

“Pectinase-null mutant” larvae generated in this way turned out to have low survival capacity. A further question now was whether the degradation products of pectin digestion would improve the survival rate of these beetles when applied orally to the beetle larvae. However, as feeding experiments showed, this was not the case. “On the one hand, this result was a bit of a surprise for us. But on the other hand, it was an important indicator that leaf beetles do not digest pectin to use its breakdown products for their own metabolism, but rather disrupt pectin, and likely other plant cell wall polysaccharides, to access the protein-rich cytoplasm of plant cells,” Roy Kirsch summarizes the results.

Research in recent years, including from Yannick Pauchet’s project group, has shown that the acquisition of microbial enzymes via horizontal gene transfer contributed to the abundance of leaf beetle species, but also of representatives of other insect families, such as weevils, bark beetles and stick insects. The appropriation of enzymes from other microbial organisms is like an evolutionary shortcut; in any case, it created the conditions for insects to open up pectin-rich plants as a food source in the first place. “There is perhaps some irony in the fact that pectinases probably first evolved in plants to dynamically adapt the pectin structure to the changing needs of the plant. These enzymes were then acquired by plant pathogenic microbes, which in turn donated them to herbivorous insects. Plant adaptation strategies apparently always involve risks and side effects,” says Roy Kirsch.

However, there are also leaf beetle species that harbor symbiotic bacteria that produce pectinases for their hosts. So-called “knockin” experiments, in which a pectinase gene is introduced into the beetle genome using CRISPR-Cas9, are now planned to show how this new beetle trait affects not only interactions between plants and insects, but also interactions between insects and their bacterial partners. Future studies will also focus on other enzymes that do not degrade pectin but other plant cell wall components. The work of Roy Kirsch, Yannick Pauchet and their team shows that for an insect to survive on a host plant, it is not only the adaptation to plant defenses, such as toxins and feeding inhibitors and other secondary metabolites, which is crucial. The scientists rather focus on the primary metabolism, which has been neglected in research on plant-insect interactions so far. “The ability of an insect to digest these primary metabolites is equally important for the evolutionary success of herbivorous insects,” says Yannick Pauchet.

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Perhaps that will change soon. Because the position and shape of the cavities are like a kind of fingerprint: they can be used to identify fossil plant remains. “Until now, it was not known how these cavities were formed,” explains Mahdieh Malekhosseini from the Institute of Geosciences at the University of Bonn. “For example, it was believed that they came from algae or pollen from other plants that somehow got onto the leaf during fossilization. But after analyzing hundreds of these structures, we can rule that out. Instead, we were able to show that calcium oxalate crystals are responsible for the depressions.”

Microlenses for better photosynthesis?

Calcium oxalate is formed by very many living plants; it is considered one of the most common biominerals. What functions it fulfills has not yet been conclusively clarified. However, it is suspected that the crystals serve as calcium stores. In addition, because they are formed in the leaf but often penetrate the leaf surface as they grow, they probably repel pests. “Many insects have an aversion to calcium oxalate — they don’t like to walk on it,” explains Prof. Dr. Jes Rust, who supervised the study. “Some plants also seem to use the crystals as microlenses to use sunlight more efficiently for photosynthesis.”

The crystals are very sensitive to acid. They therefore dissolve during fossilization and can no longer be detected in the millions of years old finds. Often, however, imprints remain in the places where they have sat (in biology one speaks of “druses”). Sometimes organic material or other minerals also accumulate in these depressions, which then sit like tiny beads in the fossil leaf.

“We studied the microstructure of the pits and their distribution on fossil leaves whose species affiliation we knew,” Malekhosseini explains. “In addition, we looked at calcium oxalate crystals in the leaves of present-day plants. We found clear parallels in closely related species. For example, the crystal imprints in a fossil ginkgo leaf strongly resemble the calcium oxalate deposits of a present-day ginkgo in distribution and structure.”

Important insights into evolution

It was already known from the fossils of bare-seeded plants such as firs or pines that they sometimes show imprints of calcium oxalate crystals. However, this was not known of angiosperms — which are most flowers and deciduous trees. “This is a completely new field of research,” explains Jes Rust. “Among other things, we now want to investigate how the ability to form calcium oxalate crystals has developed over the course of evolution.” In doing so, the researchers want to focus on periods when environmental conditions changed rapidly — such as temperature or the intensity of UV radiation. “If the distribution of the drusen also changes after such incisions, then we can draw conclusions about the biological function of the crystals,” says Rust.

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