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17/01/2022

Plants absorb more CO2 than we thought, but …
Through burning fossil fuels, humans are rapidly driving up levels of carbon dioxide in the atmosphere, which in turn is raising global temperatures.

But not all the CO2 released from burning coal, oil and gas stays in the air. Currently, about 25% of the carbon emissions produced by human activity are absorbed by plants, and another similar amount ends up in the ocean.

To know how much more fossils fuels we can burn while avoiding dangerous levels of climate change, we need to know how these “carbon sinks” might change in the future. A new study led by Dr. Sun and colleagues published in the US journal Proceedings of the National Academy of Sciences shows the land could take up slightly more carbon than we thought.

But it doesn’t change in any significant way how quickly we must decrease carbon emissions to avoid dangerous climate change.

Models overestimate CO2
The new study estimates that over the past 110 years some climate models over-predicted the amount of CO2 that remains in the atmosphere, by about 16%.

Models are not designed to tell us what the atmosphere is doing: that’s what observations are for, and they tell us that CO2 concentrations in the atmosphere are currently over 396 parts per million, or about 118 parts per million over pre-industrial times. These atmospheric observations are in fact the most accurate measurements of the carbon cycle.

But models, which are used to understand the causes of change and explore the future, often don’t match perfectly the observations. In this new study, the authors may have come up with a reason that explains why some models overestimate CO2 in the atmosphere.

Looking to the leaves
Plants absorb carbon dioxide from the air, combine it with water and light, and make carbohydrates — the process known as photosynthesis.

It is well established that as CO2 in the atmosphere increases, the rate of photosynthesis increases. This is known as the CO2 fertilisation effect.

But the new study shows that models may not have quite right the way they simulate photosynthesis. The reasons comes down to how CO2 moves around inside a plant’s leaf.

Models use the CO2 concentration inside a plant’s leaf cells, in the so called sub-stomatal cavity, to drive the sensitivity of photosynthesis to increasing amounts of CO2. But this isn’t quite correct.

The new study shows that CO2 concentrations are actually lower inside a plant’s chloroplasts — the tiny chambers of a plant cell where photosynthesis actually happens. This is because the CO2 has to go through an extra series of membranes to get into the chloroplasts.

This means that photosynthesis takes place at lower CO2 than models assume. But counterintuitively, because photosynthesis is more responsive to increasing levels of CO2 at lower concentrations, plants are removing more CO2 in response to increasing emissions than models show.

Photosynthesis increases as CO2 concentrations increase but only up until a point. At some point more CO2 has no effect on photosynthesis, which stays the same. It becomes saturated.

But if concentrations inside a leaf are lower, this saturation point is delayed, and growth in photosynthesis is higher, which means more CO2 is absorbed by the plant.

The new study shows that when accounting for the issue of CO2 diffusivity in the leaf, the 16% difference between modelled CO2 in the atmosphere and the real observations disappear.

It is a great, neat piece of science, which connects the intricacies of leaf level structure to the functioning of the Earth system. We will need to re-examine they way we model photosynthesis in climate models and whether a better way exists in light of the new findings.

Does this change how much CO2 the land absorbs?
This study suggests that some climate models models under-simulate how much carbon is stored by plants, and in consequence over-simulate how much carbon goes into the atmosphere. The land sink might be a little bigger — although we don’t know yet how much bigger.

If the land sink does a better job, it means that for a given climate stabilisation, we would have to do a little bit less carbon mitigation.

But photosynthesis is a long, long way before a true carbon sink is created, one that actually stores carbon for a long time.

About 50% of all CO2 taken in by photosynthesis goes back to the atmosphere soon after through plant respiration.

Of what remains, more than 90% also returns back to the atmosphere through microbial decomposition in the soils and disturbances such as fire over the following months to years — what stays, is the land sink.

Good news, but not time for complacency
The study is a rare and welcome piece of possible good news, but they need to be placed in context.

The land sink has very large uncertainties, they have been well quantified, and the reasons are multiple.

Some models suggest that the land will continue to absorb more carbon all throughout this century, some predict it will absorb more carbon up to a point, and some predict that the land will start releasing carbon — becoming a source, not a sink.

The reasons are multiple and include limited information on how the thawing of permafrost will effect large carbon reservoirs, how the lack of nutrients could limit the further expansion of the land sink, and how fire regimes might change under a warmer world.

These uncertainties put together are many times bigger than the possible effect of the leaf CO2 diffusion. The bottom line is that humans continue to be in full control of what’s happening to the climate system over the coming centuries, and what we do with greenhouse emissions will largely determine its trajectory.

17/01/2022

To shift away from fossil fuels, we need to copy plants
Most of the energy that fuels our lives comes from plants. Whether it is a fossil fuel that was formed hundreds of millions of years ago or the food we eat, all carbon-borne energy has its ultimate origins in plant photosynthesis.

By burning fossil fuels (the fossilised remains of ancient plants) and releasing the carbon stored within into the atmosphere, we are warming the earth, with potentially devastating consequences.

Photosynthesis, put simply, uses the energy in sunlight to convert water and carbon dioxide into plant structures. It also creates an energy store.

Unsurprisingly, given our need to quickly move away from fossil fuel use, researchers world-wide are frantically investigating methods of turning plant material into more useful forms of fuel. One way is to mimic the process plants use to make energy, through artificial photosynthesis.

And a recent discovery may be a big step in generating new, climate-friendly fuels.

Beating plants at their own game
Plant-based photosynthesis is actually fairly inefficient in terms of gathering and storing solar energy. It’s unlikely that the energy needs of the world could be supplied solely from plant sources such as biofuels — there just isn’t enough space to grow enough plants.

Another approach is to understand the fundamental processes that take place in photosynthesis and mimic them in an industrial setting — a concept known as artificial photosynthesis.

If artificial photosynthesis can be developed that is more efficient than plant-based photosynthesis, a large fraction of our fuel needs could conceivably be supplied from these “solar fuel” factories. They could develop wherever sunshine and water are plentiful, and the atmosphere supplies the rest.

But artificial photosynthesis is still a work in progress. Researchers have developed various approaches to tackle the challenges of making artificial photosynthesis work on a large-scale.

As always, the devil is in the details: by delving deep into how natural photosynthesis works we can see that plants have evolved a multi-step process.

How photosynthesis works
First, the energy from light needs to be harvested. In plants, this is done by a light-harvesting complex called antennae molecules found in the chloroplasts in a leaf.

Then, plants use the harvested energy from light to create free energy in the form of electrons and split water molecules to gain oxygen molecules. This reaction is “catalysed” by a manganese oxide compound.

These electrons are then passed to a different site and given a further energy boost from another photon of sunlight. This generates new carbon molecules from carbon dioxide.

The key to this process are the catalysts. These catalysts have to be able to absorb light and free up energy in the form of electrons, and pass them on to split carbon dioxide molecules.

A combination of materials is clearly required to carry out these different tasks. And to work effectively the different materials need to sit in very close contact with one another, so all of this needs to be shrunk down to the size of a nano-particle.

Getting closer to the answer
Our group recently made significant advances, in the form of a new catalyst developed by researcher Dr Haitao Li in the Monash laboratories of the ARC Centre of Excellence for Electromaterials Science (ACES). The discovery was published recently in Advanced Energy Materials.

Dr Li took tiny copper oxide spheres (Cu2O) (about 1 micrometre across, or a thousandth of a millimetre) and attached tiny carbon dots (about 2 nanometres across) at random points across its surface.

The copper oxide very effectively absorbs light to free up energy in the form of electrons, as well as attaching CO2 to its surface. The carbon dots help split the water molecules.

Once exposed to sunlight the catalyst steadily absorbs and converts CO2. The product of this process is methanol — an extremely useful liquid fuel that could be used to run cars, heat homes, or generate electricity.

Carbon capture — the capture of CO2 from power station flues — could accelerate the development of artificial photosynthesis technology. At the moment, there is no application for this CO2 and so it must be “sequestered” in geological formations, at significant economic cost.

Since artificial photosynthesis could make direct use of this concentrated CO2, it will make carbon capture technology more economic. This CO2 would be the perfect feed-stock for the high efficiency artificial photosynthesis process.

Approaches to artificial photosynthesis are being investigated in a variety of directions in research groups around the world and there is a growing movement, led by Prof Tom Faunce at ANU, to try to coordinate these efforts at a high level to accelerate our progress (see also here).

17/01/2022

Flower pharmacies help bees fight parasites
Search for information on ‘self-medication,’ and you’ll likely find descriptions of the myriad ways that we humans use drugs to solve problems. In fact, the consumption of biologically active molecules — many of which come from plants — to change our bodies and minds seems a quintessentially human trait.

But plants feature prominently in the diets of many animals too. A growing body of research suggests some animals may derive medicinal benefit from plant chemistry, and perhaps even seek out these chemicals when sick. Chimpanzees eat certain leaves that have parasite-killing properties. Pregnant elephants have been observed eating plant material from trees that humans use to induce labor. You may have even seen your pet dog or cat eat grass – which provides them no nutrition – in what’s believed to be an effort to self-treat nausea by triggering vomiting.

In my research, I’ve looked at how bumble bees are affected by these kinds of biologically active compounds. With colleagues, I’ve found that certain plant chemicals naturally present in nectar and pollen can benefit bees infected with pathogens. Bees may even change their foraging behavior when infected so as to maximize collection of these chemicals. Could naturally occurring plant chemicals in flowers be part of a solution to the worrying declines of wild and managed bees?

Why do plants make these chemicals?
On top of the compounds plants make to carry out the ‘primary’ tasks of photosynthesis, growth and reproduction, plants also synthesize so-called secondary metabolite compounds. These molecules have many purposes, but chief among them is defense. These chemicals render leaves and other tissues unpalatable or toxic to herbivores that would otherwise chomp away.

Many studies of coevolution center on plant-herbivore interactions mediated by plant chemistry. An ‘arms race’ between plants and herbivores has played out over long time scales, with the herbivores adapting to tolerate and even specialize in toxic plants, while plants appear to have evolved novel toxins to stay ahead of their consumers.

For monarch larvae, swamp milkweed is both kitchen cupboard and medicine cabinet. Leif Richardson, CC BY-NC-ND
Herbivores may experience benefits, costs or a combination of both when they consume plant secondary metabolites. For example, monarch butterfly larvae are specialized herbivores of milkweeds, which contain toxic steroids called cardenolides. While monarchs selectively concentrate cardenolides in their own bodies as defense against predators such as birds, they may also suffer slowed growth rate and increased risk of mortality as a consequence of exposure to these toxic compounds.

Interestingly, secondary metabolites are not only found in leaves. They’re also present in tissues whose apparent function is to attract rather than repel – including fruits and flowers. For example, it has long been known that floral nectar commonly contains secondary metabolites, including non-protein amino acids, alkaloids, phenolics, glycosides and terpenoids. Yet little is known of how or whether these chemicals affect pollinators such as bees.

Bees could use some reliable self-remedies. Daniel Krieg, CC BY
Could secondary metabolites influence plants’ interactions with pollinators, just as they affect interactions with herbivorous consumers of leaf tissue? Similar to other herbivores, could bees also benefit by consuming these plant compounds? Could secondary metabolite consumption help bees cope with the parasites and pathogens implicated in declines of wild and managed bees?

Plant compounds decrease parasites in bees
With colleagues in the labs of Rebecca Irwin at Dartmouth College and Lynn Adler at University of Massachusetts, Amherst, I investigated these questions in a new study. We found that a structurally diverse array of plant secondary metabolite compounds found in floral nectar can reduce parasite load in bumble bees.

Bumble bees in the lab colony. Leif Richardson, CC BY-NC-ND
In a lab setting, we infected the common eastern bumble bee (Bombus impatiens) with a protozoan gut parasite, Crithidia bombi, which is known to reduce bumble bee longevity and reproductive success. Then we fed the bees daily either a control sucrose-only nectar diet or one containing one of eight secondary metabolite compounds that naturally occur in the nectar of plants visited by bumble bees in the wild.

A bee consumes an experimental nectar solution containing plant chemicals. Leif Richardson, CC BY-NC-ND
After one week, we counted parasite cells in bee guts. Overall, a diet containing secondary metabolites strongly reduced a bee’s disease load. Half the compounds had a statistically significant effect on their own. The compound with the strongest effect was the to***co alkaloid anabasine, which reduced parasite load by more than 80%; other compounds that protected bees from parasites included another to***co alkaloid, ni****ne, the terpenoid thymol, found in nectar of basswood trees, and catalpol, an iridoid glycoside found in nectar of turtlehead, a wetland plant of eastern North America.

We expected that bees might also incur costs when they consumed these compounds. But we found that none of the chemicals had an effect on bee longevity. Anabasine, the compound with the strongest anti-parasite benefit, imposed a reproductive cost, increasing the number of days necessary for bees to mature and lay eggs. Despite this delay, however, there were no differences in ultimate reproductive output in our experiment.

This research clearly demonstrates that wild bees can benefit when they consume the secondary metabolites naturally present in floral nectar. And bees’ lifetime exposure to these compounds is likely even greater, since they also consume them in pollen and as larva.

The author studying nectar chemistry effects on bees in a field experiment. Adrian Carper, CC BY-NC-ND
In other research, we’ve uncovered evidence that some of the compounds with anti-parasite function are sought after by bees when they have parasites, but not when they are healthy. At least in some contexts – including a field experiment with wild bees naturally infected with Crithidia bombi – bumble bees make foraging choices in response to parasite status, similar to other animals that self-medicate.

Rx for struggling bee populations?
So what about practical applications: could this research be leveraged to help declining bee populations? We don’t know yet. However, our findings suggest some interesting questions about landscape management, pollinator habitat gardening and farm practices.

In future work, we plan to investigate whether planting particular plants around apiaries and farms would result in healthier bee populations. Are native plants important sources of medicinal compounds for bees with which they share long evolutionary histories? Can farms that depend on wild bee pollinators for delivery of the ‘ecosystem service’ of pollination be better managed to support bee health?

Delivery of nectar and pollen secondary metabolites to diseased bees is likely not the only tool necessary to promote long-term sustainability of these ecologically and economically important animals. But it appears that this could be at least part of the solution. Agriculture may come full circle, acknowledging that in order to benefit from an ecosystem service delivered by wild animals, we must consider their habitat requirements.

17/01/2022

17/01/2022