Fear is in the soil

Leaf litter and other plant debris are broken down in the soil by fungi and microbes, which decompose the organic matter into molecules which they can use.  We know that all living matter is connected through complex ecosystem interactions.  Changes in predator populations affect the prey populations which may then cause changes to the plants they eat.  Changes in plant populations would alter the amount of decomposition of leaf litter, the soil microbes and the amount of carbon dioxide released by the microbes.  This interconnection between predator and soil decomposition is not unexpected and seems fairly logical.  However, as reported by Hawlena et al. in Science last week, there is another way in which predators can affect the belowground communities.

Fear changes molecular composition of prey

When predators are present in an ecosystem, the prey are afraid and stressed out.  The stress actually causes a slight change in the ratio of carbon to nitrogen (C:N) in the bodies of the prey.  This has been reported before, but let’s try to understand why this would happen.  There are two very important types of molecules that affect the C:N ratio.

1) Sugars/carbohydrates: sugars are our main source of food and energy.  They are made of carbon, oxygen and hydrogen, but not nitrogen.  

 2) Proteins: proteins have many functions in the cell, but are usually not used as a direct source of energy. Proteins are made up of amino acids, each of which has at least one nitrogen atom.

Therefore, the amount of carbon is most affected by concentrations of sugars and the amount of nitrogen is most affected by the presence of proteins.

When prey are afraid and on guard against attack, they consume more carbs, because that’s the best source of energy.  They make less protein, because that process takes up precious energy that is needed to escape predators.  They also breakdown their remaining proteins into glucose to fuel the cells.  In other words, the prey increase their ratio of sugar to protein, which means they will have an increased C:N ratio.

When the prey with altered C:N content die in the soil, will this affect the soil microbes’ ability to decompose other materials?

Scare the grasshoppers, harm the soil

The authors set out to test this question in various settings.  They did an experiment in the field and did a number of others in a pseudo field/lab setting, so they could control the variables.  They raised grasshoppers in the field with or without the risk of spider predation.  The spiders scared the grasshoppers (understandably) and raised the grasshopper C:N ratio by just 4%.  The spiders couldn’t actually kill the grasshoppers because their mouths had been glued shut, so the authors sacrificed the scared grasshoppers and the controls and took them back to the lab.  They added the grasshoppers to large natural soil samples to let them decompose.  Both types of grasshoppers decomposed at roughly the same rates (measured by the amount of CO2 released by the soil microbes). 

The authors then added grass litter to the soils which had previously broken down the grasshoppers.  Remember there was only a 4% change in C:N ratio of the scared grasshoppers.  This small change caused a threefold decrease in decomposition of the plant material!  Small changes to the amount of carbon and nitrogen that is entering the soil can have a major impact on how well the soil breaks down plant litter.  Wow!

It’s the protein

The nitrogen content in the soil is very important to the functioning of decomposers.  Nitrogen is used by the microbes to produce enzymes that catalyze the degradation of complex carbohydrates (found in high concentrations in plants).  Therefore, the high C:N ratio in the scared grasshoppers is most likely hurting the soil microbes by offering less nitrogen.  To test this idea, Hawlena et al. created “artificial grasshoppers”, basically a mass of chitin (the shell of insects), carbohydrates and proteins.  They could change the ratio of carbon to nitrogen and specifically the amount of protein.  The more protein they added to the artificial grasshopper, the more the grass was broken down later on and CO2 emitted.  In other words, the microbes need nitrogen from the proteins of the dead herbivores in order to decompose the plant litter.  Even a small change in nitrogen content can throw everything way out of whack.

A grasshopper in the presence of predators (like the spider) will make less proteins.  When the grasshopper dies, less nitrogen (from protein) will enter the soil ecosystem.  The decomposers will not be able to make key enzymes necessary to break down plants, so the levels of decomposition will drop.

So a predator in an ecosystem can have effects all the way down to the way the tiny microbes in the soil decompose organic matter.

What makes a bloody butcher taste so damn good?

Ever wonder what makes a good tomato taste so great?  Well, so do the plant geneticists trying to produce the “better” tomato.  The amount of sugar has a lot to do with it, but what about that tomato smell?  Our perception of taste is enhanced by how food smells before we put it in our mouths and as we chew it.  The chemicals that produce the tomato aroma are called volatiles.  The food industry assumed that the volatiles that are found in the highest concentrations are the ones that make a tomato a tomato, and these should be the targets of genetic manipulation.  A paper by Tieman et al, which appeared this week in Current Biology, challenges this thinking by systematically investigating what combination of chemicals found naturally in tomatoes makes a delicious tomato.

Mass produced tomatoes are relatively homogenous, so the authors decided to examine the chemical composition of 152 heirloom varieties.  I think it’s worth noting the names of a few of these varieties: Bloody Butcher, Giant Oxheart, Crimson Sprinter, Tasti-Lee, Mr. Stripey, and Mexico Midget.  

Bloody Butcher variety of heirloom tomato. (Credit: Totally Tomatoes)

After figuring out the concentration of chemicals in the tomatoes, the researchers asked consumers to rate the flavor of the tomato varieties.  Surprisingly, a number of generic supermarket tomatoes scored quite high.  There was no simple pattern of chemicals that defined a good tomato.  As you would expect, the flavor profile of a tomato is quite complex, but the authors were able to pull out some new interesting information from their analysis.

1) The volatiles that are the most concentrated in tomatoes do not necessarily correlate with perceived flavor intensity.  In other words, some of the odors that are in the highest concentrations are not associated with flavor intensity.  Take them or leave them, either way the consumer wouldn’t notice.  The authors proved this by testing the flavor of mutant tomatoes that cannot enzymatically produce some of the volatiles that are normally found in high concentrations.  There was no difference in preference between the mutants and normal tomatoes.

2) Some of the volatiles contributed to the perception of sweetness.  In particular, an odor called geranial was positively correlated with sweetness.  To investigate this further, they used a mutant tomato that could not make geranial but still had the same amount of sugars and acids.  Consumers rated these mutants as being less sweet even though the sugar:acid ratio was exactly the same as the normal tomato.  Think about that for a minute... a smell increased the sweetness of a food.  We could replace excess sugars in processed foods with geranial to lower the calories without affecting the overall taste of the food!

So what makes a bloody butcher tomato taste so good?  High levels of geranial and other volatiles that trick your taste perception into thinking you’ve bitten into a slice of heaven.

Read more about this research on wired.

Magneto...nope!

We have all heard about birds being able to sense the Earth’s magnetic field in order to navigate during migration, but how can they do that?  There have been a number of studies that implicated iron-rich cells in the upper beak of birds as magnetoreceptors.  Just like there are neurons that can sense mechanical stimuli, the idea was that the iron in these magnetosensitive neurons is affected by the Earth’s magnetic field in such a way that the neurons then become activated, signaling a change in the intensity of the field. 

There is no doubt that birds can sense a magnetic field, but a recent paper published online in Nature disputes that there are magnetosensitive neurons.  It’s unusual to find a paper based mainly on negative results published in a prestigious journal.  I like controversies in biology, so let’s take a look at this one.

First, the authors, Treiber et al., very systematically mapped the location of all the iron-rich cells in the beak.  You can find these cells by simply staining thin sections of the beak with a dye called Prussian blue, which labels ferric iron.  They found that the number and distribution of iron-positive cells was extremely variable from pigeon to pigeon of the same age and sex.  Sensing the magnetic field is very important to birds, so you would think there would be a near constant number and location of iron-rich cells, if these were in fact sensing the field.

In order for these cells to be acting as sensors, they would really need to be neurons and feed into the bird’s brain.  The authors stained the iron cells in the beak with markers for neurons and saw almost no overlap.  In other words, the iron-rich cells are not neurons.  Well, what are they then? 

Macrophages!

Here's a macrophage engulfing some bad bacteria.

Macrophages are cells of the immune system.  They eat up bacteria and other pathogens and destroy them.  It turns out that macrophages also store iron that is released from hemoglobin when old red blood cells are recycled.  It makes complete sense, then, that the iron-rich cells in the beak could be macrophages just doing their normal storage job.  The iron-rich cells in the beak look like macrophages, but the authors confirm this by positively staining these cells for a macrophage-specific marker.  These cells are definitely not involved in sensing the magnetic field, since they can’t send signals to the brain.

This paper brings us back to square one: how do birds sense the Earth’s magnetic field?  All we know is that they don’t do it with the iron rich cells in their beaks.

UPDATE: Birds may not have magnetoreceptors in their beaks, but they do have neural correlates of magnetic fields in their brains.  In a recent report published in Science, researchers recorded from neurons in pigeon brains while varying a local magnetic field around them.  They found that individual cells in the brainstem would get activated when the magnetic field was pointing a particular direction.  Some neurons preferred 15 degrees, others 90 degrees, etc.  This is a strategy that the brain uses for other sensations, such as vision and audition.  Individual neurons are "tuned" to a particular stimuli.  We still don't know how birds are able to sense magnetic fields, but now we know that their brains are set up to interpret that information coming in from the mystery receptors.

The good, the bad and the noisy

I thought I would take a break from the complicated molecular pathways for a week and do an ecology paper.  It was interesting to read an ecology paper because it was written in a totally different style.  I made it half way through the methods section before I realized it wasn’t the results section.  The paper is by Francis et al. and was published in the Proceedings of the Royal Society B (what royal society?  B for Biology?  I think so).

The authors were looking at the effects of noise pollution on an ecological system.  Many papers have examined how increased noise can affect the behaviors of individual species, but this paper focuses on how changes in one species can alter the ecosystem.  The results are not surprising at all, but what I found most interesting were the methods.  There were two main experiments: one looking at hummingbirds and pollination, the other examining seed dispersal by scrub jays.

Noise increases pollination

Turns out hummingbirds like noise.  This may seem surprising at first, but lets think about it.  Have you ever heard a hummingbird sing?  Vocalization doesn’t seem to be that important to them, so from their perspective, noisy areas are good because it drives away their competitors and predators.  Since hummingbirds are important pollinators, it goes to reason that a noisy area would attract more hummingbirds, which would pollinate more flowers, leading to an increased population and diversity of flowering plants. 

How do the authors actually test this, though?  For all their experiments, they went to New Mexico, which has natural gas wells.  Some of these wells are quiet and some of them have noisy compressors.  This provides a great experimental setting where the only variable is noise, because the type of human activity and the vegetation features are nearly the same at all the wells.  In order to measure how often flowers are visited by hummingbirds and if pollination has occurred, the authors set up some artificial flowers.  In this way, they can control for random variations in flowers at the noisy wells versus the quiet wells.  From what I gathered from their description, they used pipettes filled with sweet nectar that was replenished each day.  They “decorated” the pipettes with colored tape to imitate the colors of flowers.  Yarn and colored tape?  That sounds fun!  The hummingbirds totally fell for it and started visiting the artificial flowers.  As expected, the hummingbirds went to the flowers near the noisy wells more often than the quiet control wells.

When hummingbirds visit flowers, some of the pollen (containing sperm) brushes off onto their bodies.  As they fly to a new flower, they bring the pollen with them, which may fall off onto the female parts of another flower, thus completing pollination.  To test if the increased visits to noisy wells would lead to more pollination, the researchers put fluorescent dyes onto some of the artificial flowers.  They followed the transfer of the dyes between different flower patches, and found that near noisy wells, more of the dye was transferred from flower to flower.  In other words, there would most likely be an increase in pollination of these flowers near noisy areas. 

Noise impairs seed dispersal

Many trees depend on animals to carry their seeds away to new environments where they can germinate.  For this experiment, the authors focused on the seeds of the pine Pinus edulis.

The authors scattered seeds under pine trees and set up motion-triggered cameras.  Every time a seed was removed, a photo of the culprit was taken.  They found that one species of mice preferentially took seeds from trees near the noisy wells.  The mice probably like noisy areas for the same reason as hummingbirds: less predators around (like owls).  Unfortunately for the pine, the mice mostly ate the seeds and didn’t help disperse them.  On the other hand, scrub jays collect lots of seeds, hide them and then forget about them; this is the optimal situation for the pine seeds, because they don’t get eaten and they get to grow in new places.  Scrub jays, unlike the mice, avoid the noisy wells, probably because they rely a lot on vocal communication.  The unhelpful mice are more prominent at noisy wells and the helpful jays avoid the noisy wells, so all this leads to decreased seed dispersal and fewer pine seedlings growing near the noisy areas.

Changes in the number of the hummingbird-pollinated flowers (positively) and pine seedlings (negatively) can have all sorts of other effects on the ecosystem near noisy wells.  As we all expected, noise may only affect one species directly, but it can have long reaching consequences for all the integral members of the environment.  Bad news in our industrial world.