Natural selection is a rigorous master, demanding tough choices and efficiency of all living organisms. Even when things are good and the living is easy, trade-offs are required.
For example, as a microbe is enjoying a nutritionally replete buffet, it begins to grow, laying the groundwork for proteins that will build biomolecular scaffolds and ultimately generate a new cell through replication. But as quickly as this process begins – as DNA is transcribed into the RNA that will serve as the template for proteins – it all comes grinding to a halt. The cell has run out of phosphorous.
Take a look outside your window. How many species do you see?
This question of how geography influences biodiversity has bedeviled biologists for centuries. But according to a new study led by Marcell Peters from the University of Wurzburg, the number of distinct species you’re seeing – or, more accurately, the number you would see in the nearest natural environment – depends most strongly on temperature.
Several hypotheses have percolated through the scientific literature over the years:
1) The temperature hypothesis, in which the greater rate of biological processes, interactions, and evolution associated with higher temperatures is the main determinant of diversity.
2) The water availability hypothesis, proposing that water supply underlies primary productivity and enables more diversity at higher trophic levels.
3) The productivity hypothesis, linking maximal diversity with a greater abundance of life’s required nutrients.
4) The area and geometric constraints hypotheses, which link diversity and the availability of distinct niches to more space or geographic gradients.
5) The plant diversity hypothesis, connecting animal diversity to the number of consumable plant species.
All of these different possibilites have remained in the discussion for so long because diversity studies typically examine a single type of organism. So while vascular plants are subject to the water availability hypothesis, arthropod distributions are best described by the plant diversity hypothesis.
After waking up this morning, your body began a well-established set of procedures to prepare for the day: heart rate ramped up, body temperature increased, and hormone levels shifted. And tonight, you’ll probably start to feel sleepy around the same time you did last night, as other physiological changes start to take place. This predictable cycle, of course, is your circadian rhythm, the hard-wired biological changes that help your body achieve its full set of diurnal activities, from sleep at night to optimal coordination around 2:30 PM and greatest strength at roughly 5:00 PM.
To qualify as a “circadian,” a biological cycle must be endogenous (self-programmed, rather than merely a response to environmental conditions), temperature resistant (despite the changing kinetics of different thermal regimes), and re-settable (think about your latest transition to a new timezone, jetlagged as it may have been). This doesn’t just happen in animals that sleep at night: plants, bacteria, and fungi have all been shown to prepare biochemically for daily cycles.
When NASA’s New Horizons mission flew by Pluto last year, it sent back images that recalled a past era – a time when a few photos of initial reconnaissance could drastically change our sense of an entire celestial body. For The Moon and Mars, these initial glimpses happened decades ago; satellites of the gas giants, like Europa, Enceladus, and Titan were ready for their close-ups next. And after that? Well, it seemed as if the geologically active, charistmatic targets in our Solar System were spoken for. As illuminating as subsequent investigations have been – and to be sure, there are fundamental aspects of all of these bodies that still await discovery – at least we knew what kind of world we were dealing with.
That all changed with Pluto. Expectations were low, given the outer Solar System’s reputation as an icy wasteland, but when initial photos showed potential cryovolcanoes and recent resurfacing, the questions came fast and furious. Many of them centered around the heart-shaped feature a thousand kilometers across located at 25 degrees north latitude.
How hot is too hot for life to survive? Ever since microbes were discovered squirming around in hydrothermal springs several decades ago, the limit of heat-loving (thermophilic) organisms has been a moving target. The current record-holder is “strain 121,” an archaeon isolated from the Mothra hydrothermal vent deep beneath the surface of the northeast Pacific Ocean; suitably enough, it can grow at a blistering temperature of 121 °C.
There are some physical limitations of biological activity at high temperature. Enzymes unfold, as rapid molecular bond vibrations tear intricate molecular structures apart, obliterating their functional capabilities. Individual amino acids lose their “handedness” as geometrical arrangements of molecular structures equilibrate with heat; since all known biological amino acids are “right-handed,” organisms must spend a lot more energy re-shaping amino acids to fit the template. DNA bases “A” and “G” falter at exponentially higher rates as the temperature goes up, leading to murderous rates of mutations.
Despite these assessments of biomolecule stability and the search for the thermal outliers, there haven’t been a lot of systematic studies of how habitability at seafloor sites changes with temperature. Sure, an impressive organism or two can eke out a living at 121 °C in the lab, but is that also true in the real world, where conditions could be much more variable? And are viable communities around continuously up to that thermal limit?
In 2007, the Mars Exploration Rover (MER) Spirit came across a slightly raised platform, roughly pentagonal in shape and 90 meters across, which scientists named Home Plate. The rocky outcrop had a base of solidified ash, with nearby deposits of gas-filled basalts. Next to the plateau, nubby, nodular chunks of rock showed up, and light-colored soil just beneath the surface was exposed by the rover’s wheels. Mineralogical spectra of the bright soil were beamed back to Earth, revealing, to the scientists’ surprise, that it was composed almost entirely of silica.
When the geological puzzle pieces were assembled, two main options emerged: Home Plate may have been a volcanic fumarole, spewing sulfuric acid at basaltic rocks and leaving silica behind, or it could signify the remnants of a mineral rich hot spring, whose silica-saturated water generated the knobby rocks. Either way, water and heat were likely involved, and the discovery led to an onslaught of new questions and exciting plans for further studies.
But then, the Spirit rover went silent, forcing MER scientists to get creative. To pursue the Home Plate mystery, they’ve scoured the Earth for mineralogical signals most similar to those found on Mars. By determining the conditions that best recapitulate the martian data, the thinking goes, we might be able to piece together the events of Mars’ ancient past.
Astronauts flying long-duration missions in space have been known to suffer substantial health effects: they grow nauseous, lose bone density, and watch their muscles atrophy. These large, human-scale changes are pretty easy to observe, as trillions of cells’ responses to microgravity are compiled into the physiological response of one organism. But what’s happening at the cellular scale? How do single-celled life forms respond when launched into space?
Gravity is an aspect of life that we – and all other organisms on our planet – have taken for granted for all but the last 50 years of our 3+-billion year evolutionary journey. This makes it hard to think about ways in which gravity could be hard-wired into biology, as it never represented an actionable variable for evolutionary pressures.
By taking life into space, biologists are seeing how gravity affects the cellular environment in fundamental ways. It’s been suspected for years: past experiments have shown that microbial cultures tend to form biofilms more easily, and pathogens become stronger. But a genetic, mechanistic understanding of these processes hadn’t been explored.
The search for life beyond Earth has inspired many strategies, from examining microfossils with elemental analyzers, to sequencing putative genetic material, or studying the composition of distant exoplanets. But a recent paper from Jay Nadeau, a Scientific Researcher at the California Institute of Technology, proposes a surprisingly new approach of looking for life by, well, looking for it.
Nadeau and her colleagues suggest that “rapid and meaningful” cell movement is “an unambiguous biosignature that makes no assumptions about the chemical composition of the organisms under study.” They propose using a microscope to track particles moving through a field of view, but the two qualifiers are critical: after all, dust grains that move by diffusion, or are pushed by wind or water currents, can easily be mistaken for cells.
“Rapid” movement can be explored using the Stokes-Einstein equation to determine how fast a non-living sphere might move through water. A sphere one millionth of a meter (a micron) in diameter moves about 0.01 microns per second; microbes of the same size can swim at 10 to 100 microns per second. “Meaningful” movement distinguishes a putative cell from its medium: under controlled conditions, abiotic diffusing particles tend to move in a straight line, while microbes tumble around in wandering paths, occasionally with a chemical target in mind.
Natural gas is an increasingly important energy source, as vast reservoirs are being accessed through fracking, coal exploitation, and, in the not-too-distant future, subseafloor hydrate ice mining. But how exactly all of this fuel is being made deep underground is not very well known; figuring out will tell geologists how carbon moves through the planet and whether or not we should depend on natural gas reserves as a long term energy source.
Methane is the dominant ingredient of natural gas, a result of complex, gooey organic molecules being chopped up into smaller pieces by industrious microbes or the pressure cooker burial of geological activity. The microbial instigators are members of the archaeal domain, producing methane from just a few precursors: carbon dioxide, methanol, methylamines, and dimethylsulfide.
Growth rate is a fundamental aspect of life, a metric that can separate biology’s winners and losers. Even a small advantage can lead to complete dominance in short order, given the exponential scaling patterns of biological growth. Calculating growth rate is pretty straightforward when you’re looking at plants or animals, where it’s possible to measure an organism’s mass with relative ease. But what about microbes? How can minuscule changes in a cell’s mass be measured when the whole organism weighs just a picogram (10-12 grams)?
Over the last few years, physicists, biologists, and nanoscale engineers have joined forces to tackle this question. One camp has examined cells under the microscope, tracking their expanding diameters over time, but this method makes broad assumptions about cell geometry and takes a long time. A different approach pioneered by Scott Manalis, a Professor of Biological Engineering at MIT, combines microfluidics and resonating cantilevers to calculate a cell’s mass as it flows through tiny channels just three microns across.