If you’ve ever visited a desert, you may have noticed small clumpy mounds of crusty dirt in between the stretches of sand. Within those clumps something amazing is going on. Living cells are using high-technology processes not only to survive in a harsh environment; they are helping to stabilize the entire biosphere.
Called cryptobiotic soils, or simply desert crusts, the mounds are inhabited by photosynthetic cyanobacteria. Anything that can convert sunlight into usable energy looks designed, for sure, as solar engineers surely know. But these bacteria have a problem: they get too much of it. Their environment quickly dries out from the excessive solar heat after sunrise. Excess heat can lead to the formation of reactive oxygen species, threatening the cell. How can the cyanobacterium dissipate excess energy to avoid getting burned?
New research by Europeans and Israelis is helping shed light on the strategy these “primitive” organisms use. In the Proceedings of the National Academy of Sciences (PNAS), a team of 15 scientists describe how they discovered “Changes in aggregation states of light-harvesting complexes as a mechanism for modulating energy transfer in desert crust cyanobacteria.” Here’s their summary on the significance of their findings:
All photosynthetic organisms regulate efficiency of excitation energy transfer (EET) to fit energy supply to biochemical demands. For desiccation-tolerant desert crust cyanobacteria this ability is of the utmost importance. These organisms spend most of the daytime in the desiccated state whence absorbed energy is completely quenched. Based on our observations we propose a model where in the hydrated state the organized rod structure of the light-harvesting phycobilisome supports directional EET to reaction centers with minimal losses to thermal dissipation. In the desiccated state this structure gives way to more random aggregates. The resulting EET path exhibits increased coupling to the environment and enhanced quenching. This energy dissipation mechanism is completely reliant on changes in aggregation state of phycobilisome components. [Emphasis added.]
First, what are phycobilisomes? They are the light-harvesting antennas for cyanobacteria and other photosynthetic microbes. “Phycobilisomes [PBS for short] are protein complexes (up to 600 polypeptides) anchored to thylakoid membranes,” Wikipedia says. “They are made of stacks of chromophorylated proteins, the phycobiliproteins, and their associated linker polypeptides.”
Already we are getting a glimpse of the complexity going on inside those dirt clods. 600 proteins all cooperating in a light-harvesting function! Think about that. Each polypeptide component has to be coded in DNA, but that’s not all. As Jonathan Wells noted in ID the Future, each protein “has to go to a certain place in the cell to do its job,” but “That spatial information is not in the DNA.” Additional coded information is required to assemble the pieces of the phycobilisome so that they can function as antennas to harvest light.
The simplified illustration in Wikipedia gives a hint of how the proteins are arranged. They focus their light-collecting abilities toward the reaction center. You can read on and get quickly swamped by the complexity of these proteins, but this sentence is all you need to know: “The geometrical arrangement of a phycobilisome is very elegant and results in 95% efficiency of energy transfer.”
In desert crusts, however, cyanobacteria need to shed excess light and heat. The team of scientists found how they do it: those organized structures become more randomized during the heat of the day.
The underlying mechanism proposed is based on the intermediate-length (1–2 nm) coupling interactions between PC units through the external β155 pigments. A random network of intermediate coupling through the PC aggregates will generate long and convoluted EET paths and a band gap structure with many localized states that couple well to the wide frequency of environmental thermal noise.
Figure 5 in the paper shows how the organized stacks of protein components work in the hydrated state, directing energy transfer efficiently into the thylakoid membrane where photosynthesis turns the harvested light energy into chemical energy for the cell. In the dessicated state, however, the proteins shift their relative positions, looking like they have been buckled and displaced by an earthquake. This more randomized arrangement dramatically lengthens the energy transfer path, allowing dissipation of energy into the surroundings. The process is reversible, they found:
Under laboratory conditions L. ohadii is able to retain viability for prolonged periods and regain photosynthetic activity immediately after rehydration. We have demonstrated this in the past for electron transfer activity (7). Fig. 1 demonstrates this remarkable ability for PBS function. A desiccated culture, in which PBS fluorescence was strongly quenched, regained its high fluorescence yield as soon as 2 min after rehydration. The hydration/desiccation process was completely reversible. After ∼2 h, at ambient temperature, the culture was desiccated, and fluorescence was quenched. Rehydration resulted in immediate recovery (Fig. 1).
Here’s an example of a strategy that employs reversible randomness for function! A reversible earthquake! And it works well: “The extent of quenching in these organisms far exceeds that of common laboratory model organisms,” the authors say.
Fascinating as this is, there’s more. These desert crusts play a role that benefits the whole planet. As we saw with dust in the clouds, the hidden ecosystems in desert soils are essential for the biosphere.
Deserts cover almost half of the Earth’s terrestrial surface, and although desert conditions may seem unfavorable, they are home for diverse ecosystems. Many of these ecosystems are founded on biological desert crusts, which play an essential role in stabilizing shifting sands and enriching them with nutrients. Cyanobacteria are among the first microorganisms to inhabit these crusts where one of the major sources of water is often dew deposited before dawn.
We can get another hint of the value of this role from comparative planetology. Consider Titan, which has a thick atmosphere; Mars, which has a thin atmosphere, and our moon, which has virtually no atmosphere. All three, being lifeless, have a serious problem with static electricity, even though some H2O is present. Georgia Tech describes the “electric sands” on Titan as being like “clingy packing peanuts.” NASA describes Mars and the moon as “crackling planets” with an “uncommon amount of static electricity” that will likely impede human colonization. Apollo astronauts complained about the dust that clings to everything.
This suggests that earth’s microbes play a role in neutralizing the static that might otherwise predominate in dust and sand on a lifeless earth. Having water on a planet may not be enough, as we all know from dry days when our fingers spark when touching a doorknob. By stabilizing the sand, desert crusts may reduce the static buildup that would otherwise occur due to friction. This might be a good ID research project for someone. A related question would be, to what extent does excess static impede habitability?
What’s clear is that tiny, hidden communities in dirt are suffused with enormous amounts of coded information. Both in clouds and in deserts, these complex ecosystems help lay a foundation of nutrients upon which all life depends. It’s enough to make you watch your step when you hike in the desert.