It’s good to see a story from a branch of science with which I am familiar make the headlines, although the headlines themselves have been somewhat surprising. “Stanford researchers find electrical current stemming from plants”, says the University press release. They didn’t find it, really. We knew it was there. “Ultrasharp Nano-Electrode Harnesses Electric Current from Plant Cell?” is better, although ‘harnesses’ implies the current could be put to some sort of useful work, which is unlikely, given the vanishingly small amount of current. “Stealing electricity from algae” is perhaps a better headline, as that is exactly what was achieved.
And what was achieved was, in my opinion, something remarkable.
Plants make for marvellous experimental subjects. No ethics approval required, no need to consider the health and well being of the plant. Genomes have been cracked, distantly related species interbred. Plants have been bombarded with radiation to create mutations, and genes introduced from entirely different kingdoms of life into plants, such as a gene encoding an anti-freeze protein from fish into tomatoes. The science of plant breeding is thousands of years old, the study of plants fascinated the Greeks, and taxonomy and plant anatomy bloomed in the Enlightenment. Despite this long history, there are aspects of plant science that remain technically challenging.
One such challenge is phloem analysis. Plants carry the nutrients produced through photosynthesis through specialised tissues called phloem. The sieve tube elements in the phloem, cells that are arranged end to end, with sieve plates at the adjoining walls, form a tube capable of transporting photosynthates like sucrose. While we know in general terms what comprises the phloem sap, accurately identifying the specific components carried in the sieve elements is a surprisingly difficult task. When a sieve element is cut, allowing phloem sap to exude, the contents of other cells are also released, contaminating the sample. The sap can be diluted by water from the xylem. Sap exudation rates are rapidly limited when sealing mechanisms in the plants are activated, and by the loss of the turgor pressure that drives phloem sap translocation. Collecting pure phloem sap is tricky. The solution to the phloem sap collection problem is by necessity ingenious, and uses aphids.
Aphids feed on the sap in phloem, and they do it so effectively – without interrupting sap flow – that their bodies overfill with it, excreting the excess as honeydew. Aphids are able to insert their stylet into the sieve element without reducing the turgor pressure in the phloem, maintaining a continuous flow of sap.
The aphid’s ability to do so neatly solves the problem for scientists of how to collect phloem sap, and only phloem sap: one cuts off the aphid stylet from the aphid, once it is inserted into the sieve element, with a laser. The stylets continue exuding sap even when separated from the insect. It’s an ingenious solution to a tricky problem, and one that came to mind on reading about about the Stanford research.
"We believe we are the first to extract electrons out of living plant cells," said the lead author, WonHyoung Ryu. It is incredibly tricky to directly analyse phloem sap, even though it can be obtained in millilitre quantities. One can imagine it to be orders of magnitude more difficult to directly intercept electron flow between protein complexes, located inside organelles, inside single plant cells. And all without the aid of a ready made nanoprobe like the aphid’s stylet.
The electrons in question are produced during the process of photosynthesis. Light energy is absorbed by photosynthetic reaction centers and then used to split H2O, generating H+ and electrons. The H+ generates a pH gradient across the thylakoid membrane which, together with high energy electrons (e-), reduces inorganic carbon to sugars and polysaccharides. These are the electrons we are interested in. When a molecule of chlorophyll in the reaction center of PSII absorbs a photon, an electron in the chlorophyll is excited to a higher energy state. The state of the electron is highly unstable, so it is rapidly transferred into what is called the electron transfer chain, a series of molecules that flow electrons from one photosystem to another. In fact, as a consequence of the high energy reactions taking place in the photosystem II reaction center, the D1 protein at the core is rapidly degraded and replaced; photoinhibition occurs when the rate of D1 breakdown exceeds the rate of D1 repair, a condition that can occur in high light conditions. Generating high energy electrons is not without cost.
Of course, the electrons are there for a reason. One electron transport chain provides energy for the synthesis of ATP, via chemiosmosis, while the 2e- that reach the end of that electron transport chain reduce the chlorophyll in PSI. When light excites chlorophyll in PSI, its electrons are transferred via another electron transport chain to NADP+ reductase, creating 2NADPH. Both ATP and NADPH are essential molecules, used for transporting chemical energy and the synthesis of lipids and nucleic acid, respectively.
By inserting an Au nanoelectrode into the algal chloroplast, the researchers were able to extract electrons directly from the electron transport chain. Molecules in the electron transport chain donated electrons to the nanoelectrode; electrons were extracted from either plastoquinone (the PQ pool) or Ferrodoxin (Fd), at the point when the electrons were at their highest energy level immediately after being excited by light. The figure below shows the two possible locations of the electrode and the two possible electron donors.
Electrons have been harvested from the electron transport chain before, using chemical mediators such as p-benzoquinone (BQ). To do that, the organelles that contain the photosynthetic apparatus, the thlyakoid membranes, are bathed in a solution of BQ and illuminated. When the light comes on, electrons produced in photosystem II are accepted by BQ, which is reduced to p-hydroquinone (QH2).
Directly intercepting electrons from the electron transport chain, with an electrode, and without the use of a mediator like BQ, is what makes this such an achievement.
Nothing like this had been done before, and a number of obstacles had to be overcome to do it this time. The nanoprobe had to be designed, it had to be able to penetrate through cell membranes without killing the cell, and the algal cell had to remain stationary. A mutant of Chlamydomonas that has no motility was crossed with a strain of Chlamydomonas with a cell wall minus phenotype, to obtain an algal cell that wouldn’t swim around and, without cell walls, could be easily penetrated by the nanoprobe. The nanoprobe itself was just 100 nm thick, and had to be carefully positioned within the thylakoid stack (see inset box in the figure).
It makes for a fine piece of basic research. However, the reporting on the story focused not so much on the aspects discussed above but on the potential for generating bioelectricity. It’s like visiting a restaurant that makes great steaks, but only ever tries to sell the sizzle. Yet far from this being a story blown out of proportion by the press, bioelectricity generation is the motivation given for the research. The abstract concludes with the following: “This result may represent an initial step in generating “high efficiency” bioelectricity by directly harvesting high energy photosynthetic electrons.” It’s only fair, then, to examine the sizzle.
As their introduction states, “abundant solar energy is stored and converted into chemical bond energy by photosynthesis… one approach for extracting energy from the photosynthetic conversion process is to harvest the biomass stored as polysaccharide and convert it to ethanol, longer chain alcohols or hydrogen.”
True, and there is a theoretical limit to how much solar energy can be converted this way to polysaccharide; around 25-27 %, although in the real world this conversion efficiency is considerably lower.
So what can be done about it? “To increase the efficiency of light energy conversion, we evaluated the feasibility of generating bioelectricity by extracting e- from the photosynthetic electron transport (PET) chain before they are used to fix CO2… This approach potentially reduces energy losses associated with the multistep transformation of solar energy into products used for the production of biodiesel and bioelectricity.”
Ryu described the amount of electricity drawn from the cell, one picoampere, as “so tiny that they would need a trillion cells photosynthesising for one hour just to equal the amount of energy stored in an AA battery”. And there’s the rub. It may be a more efficient way of tapping solar energy than burning biofuels, but it isn’t any where near as effective. Solar energy isn’t a limiting factor for plant life and it isn’t complicated to grow a crop. Biofuels as a means of utilising solar energy may not always be economical, and may even be carbon positive, but they do work, and they work at meaningful scales of power generation.
In contrast, this isn’t a technique for producing useful bioelectricity, and nor will it be anytime soon. Stealing electrons from the process of photosynthesis in algal cells was not without consequence, as the cells died after about an hour. The amount of current harvested didn’t exceed the energy used in running the experiment (that is, the voltage applied to the electrode), so there was no net gain. Even the suggestion of scaling up the size of the chloroplast and electrode, to be able to capture more electrons, won’t solve the problem of the vanishingly small amounts of current involved, the technical challenges of fixing the cells so they don’t move, the difficulty of correctly inserting an electrode into each cell, and then having to start all over after an hour when the cell dies. It may be a more efficient conversion of solar energy to electricity than biofuels, but that’s just sizzle.
The meat of the story can be found, at least for me, in just a single line in their introduction:
“In addition, the system allows direct monitoring of specific charge transfer reactions in live cells, leading to broad applications for investigating developmental processes and the responses of cells and organelles to light and chemical stimuli.”
It didn’t need to be sold as a breakthrough in clean energy. For photosynthesis research, the effort to elucidate one of the most important biological reactions on Earth, this technique is a great investigative tool. It’s as innovative as the aphid’s stylet, a true milestone in scientific discovery and an exciting piece of research. That’s more than good enough.