Shining Light on Clean Energy Superbugs

Overcoming some of the obstacles that have hindered petri-dish-to-gas-pump schemes in the past, scientists are finding ways to produce high-octane fuel and even pure hydrogen from co-opted algae.
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Overcoming some of the obstacles that have hindered petri-dish-to-gas-pump schemes in the past, scientists are finding ways to produce high-octane fuel and even pure hydrogen from co-opted algae.

Over the past year, amid falling oil prices, an ongoing food vs. fuel controversy, and a few over-anxious market predictions, the bioenergy bandwagon may have picked up a few scratches and dings. But James Liao and Anastasios Melis say a new kind of photosynthetic biofuel could provide the spark for a clean energy revolution.

President Barack Obama announced earlier this year that by 2022, he wants to "more than double the amount of biofuel produced in the United States" to an annual rate of 36 million barrels. But the most popular biofuels on the market today, such as ethanol and biodiesel, come with significant drawbacks.

Ethanol requires massive investments in land and resources, and competition for these resources sets off worries of a food crisis. Plus, Liao, a professor of chemical and biomolecular engineering at the University of California, Los Angeles, says ethanol makes a poor gasoline substitute. An alcohol with a low molecular weight bonding together only two carbon atoms (compared with gasoline's eight or more bonded carbons), ethanol lacks the octane to deliver.

Biodiesel at least compares to petrodiesel in its performance, but in Liao's assessment, whether one considers the fermentation of mass quantities of plant material to produce ethanol or the intensive manhandling required to squeeze lipids (the oils) from algae for biodiesel, even the best biofuels in today's market rely on a series of cumbersome brute force operations that add unnecessary costs to energy.

Melis and Liao believe that photosynthesis — the splitting of water molecules using sunlight and carbon dioxide in a biological system — can provide the hydrocarbons we need without the back-breaking effort. They envision photosynthetic microbes that spew ready-made fuel as they grow as the clean energy equivalent of the money tree. And the seeds they've planted are beginning to sprout in the lab.

Melis, a biologist at UC Berkeley's Department of Plant and Microbial Biology, wonders whether a biological oddity observed more than 70 years ago might point to a clean, renewable source of fuel. In 1939, researchers discovered that, for brief periods when starved for oxygen, certain single-celled algae called cyanobacteria, (also known as blue-green algae) generate hydrogen gas as a product of photosynthesis. However, after mere seconds, the scientists observed the organisms would revert to their normal photosynthetic activity, adding mass in the form of fatty acids, sugars and cellulose while releasing oxygen as a byproduct.

While the discovery demonstrated that photosynthesis could be malleable, scientists chalked up the phenomenon as a biological curiosity.

The push for clean energy, however, renewed interest in the decades-old experiment. Melis, for one, envisions the reaction observed in 1939 as a model for a convenient source of clean energy — provided the peculiar mode of photosynthesis could be sustained.

By manipulating environmental conditions for experimental, genetically engineered algae cultures (chlamydomonas reinhardtii), in 2001, Melis succeeded in doing just that. He found that when he restricted the availability of sulfur, which normally plays an important role in photosynthesis in these organisms, the algae percolated tiny bubbles of pure hydrogen, continuously, over an extended period of time. (The theory of endosymbiosis suggests bodies in the algae, chloroplasts, developed from similar bodies in cyanobacteria.)

Overcoming the social-psychological challenge of the Hindenburg disaster, Melis lined the shelves of his lab with flasks containing strains of hydrogen generating algae and continued tweaking their genes and environmental conditions to boost hydrogen output.

Encouraged by inquiries from venture capitalists, he built a scaled-up version of his biohydrogen generator in his backyard. Built from simple plastic tubing and aquarium supplies, he called the model his "doughnut reactor." A cubic meter in volume, and filled with algae, water and a dash of baking soda as a carbon source, the reactor could liberate enough hydrogen through its handy plastic tap to provide clean energy, sufficient enough to supply an individual household, anyplace the sun shines.

"If I can do it, anybody can," Melis said.

And he suggests similar technology could be set up on non-arable land as easily as on fertile fields, providing a perfect fit for energy-poor households in the developing world.

Melis' effervescent hydrogen bioreactor soon became a cultural event in academic circles. In 2004, it even made an appearance — as a work of art under the banner "Future Farmers" — at a gallery at the University of the Pacific in Stockton, Calif.

However, before rushing down to the big box store to buy up its stock of plastic tubing and baking soda, Melis cautions there are a few technical issues to be resolved before "doughnut reactor" becomes a household phrase. Aside from algae husbandry issues, such as controlling sulfur concentrations and keeping colonies well fed and free of microbial invaders through use of large doses of antibiotics, there is a fundamental problem: The algae, even the genetically engineered variety, do not like producing hydrogen when there's oxygen around.

In fact, the organism's hydrogen-producing enzyme shuts down in its presence, and, since oxygen is the major byproduct of photosynthesis, solving that problem presents a formidable challenge.

Although interrupting photosynthesis at regular intervals to dissipate the taint of oxygen provides a temporary remedy, Melis believes the procedure wastes precious daylight that could otherwise go into generating hydrogen.

But there's more to the story. According to Melis, the availability of sunlight represents another limiting factor in fuel production through photobiolysis.

Algae, he says, generally use only the small portion of light in the visible spectrum to support their growth. This leaves huge quantities of energy going untapped in the infrared wavelengths.

And Melis, as stated earlier, doesn't like to see light wasted.

He decided to add purple bacteria to the mix; these bugs are capable of charging up their metabolic processes on invisible infrared radiation from the deep purple end of the spectrum. Melis reported that the two organisms played well together, boosting the solar energy conversion and hydrogen output within their shared terrarium.

But he finds that bacteria are sometimes too green — shade created by vigorous growth presents a problem of its own. Melis said in his doughnut reactor sunlight can penetrate no deeper than about 5 centimeters into the dense, green canopy of suspended algae, leaving the bulk of the colony in darkness and taking a huge bite out of its photosynthetic potential.

However, Melis noted that naturally occurring mutant algal cells containing a bare minimum of pigmentation could successfully carry out photosynthesis and survive, so he surmises that the productivity of algae might not be tied strictly to the degree of pigmentation within the cells. He regards the green part of the cell as an antenna that is tuned to absorb light from the sun to drive photosynthesis, but he believes this antenna could be shortened considerably without ruining reception.

With backing from the U.S. Department of Energy, Melis is working on a project to isolate, clone and reintroduce antennae-shrinking genes into a hydrogen-generating strain, which he hopes can eventually populate reactors with bacteria in transparent shades of chartreuse that permit hydrogen production down to their very core.

As exciting as the prospect of a photosynthetic hydrogen economy might be, transitioning to this paradigm will take considerable time. What might photobiolytic biofuel offer in the interim?

The grayish hydrocarbon haze that envelops urban settings is well known by a non-scientific term: smog; and it's part of the problem. But what of the misty veil that likewise shrouds pristine forests in bands of blue and violet? Hydrocarbons as well, emanating from heat-stressed trees, are, according to some environmental authorities, also part of the problem.

Leaving the pollution question aside, one of the summertime emissions of poplars and pines is known to science as isoprene, and it is a powerful precursor to many chemicals used in industry and transport on a daily basis. As a testament to isoprene's versatility, Goodyear unveiled the world's first bio-isoprene tire, fabricated from using "renewable biomass," at the December Copenhagen Climate Conference. Isoprene also constitutes a potent feedstock for a potential gasoline substitute known as isobutanol, high in octane and easily managed. Could the fuel of the future be growing on trees?

Probably not.

While scooping high-octane hydrocarbons from the sky would likely be futile, Melis and Liao propose replicating nature's inaccessible largesse with genetically altered microbes, providing a model for what they see as the next generation of biofuels.

Undaunted that in their natural state algae have zero ability to make isoprene, Melis surveyed the plant kingdom to find a set of genes that when transcribed into his bugs would give them that ability. His search led to the ubiquitous, and noxious kudzu vine, which carries an isoprene synthase gene that appeared conveniently matched to the genetic makeup of synechocystis cyanobacteria, a much studied species of cyanobacteria.

Through a meticulous process of engineering the cloned and transplanted genes from kudzu, Melis arrived at a viable strain of synechocystis cyanobacteria, that, when stimulated by sunlight, produced the target chemical isoprene, albeit in miniscule quantities. More than 90 percent of photosynthetic product constituted the usual sugars and biomass that algae generally live off ; a mere 3 percent went to isoprene. The breakthrough, nonetheless, drew notoriety in the scientific community, representing the first engineered platform for photosynthetic isoprene production.

In November 2009, Liao announced that he had successfully spliced genes from a bacteria used by cheese makers, along with genes found in E.coli, into Synechococcus elongatus, thereby commandeering photosynthesis in that strain of bacteria to produce the valuable chemical and fuel stocks isobutyraldehyde and isobutanol, directly from CO2.

With isobutyraldehyde wafting from his bacterial colonies, Liao said he reached his benchmark goal on one of his first attempts: matching the production rate of fuels from algae biodeisel facilities. Although Liao said it is difficult to make direct comparisons between lab results and industrial processes, he says his results show "that we are in the ballpark." Moreover, he says his bacteria survived high concentrations of the target chemicals by releasing them in gaseous form and remained "productive up to eight days" — and there may be room for improvement. "We have not optimized the bacteria for isobutyraldehyde yet."

One barrier to large-scale production of photosynthetic biofuels, Liao says, lies in industrial design. "What's sorely needed now is an economical bio-reactor containment vessel," he says, hoping for "something with a lot of surface area that can deliver light, with some easy way to distribute nutrients and collect the final product."

Although the two scientists say they are approaching biosynthetic fuels from different angles, Liao says their work "complement each other." To which Melis responds, "I like what he's doing."