Tag Archives: #biosphere

Earth-like Biospheres On Other Planets May be Rare (Planetary Science)

A new analysis of known exoplanets has revealed that Earth-like conditions on potentially habitable planets may be much rarer than previously thought. The work focuses on the conditions required for oxygen-based photosynthesis to develop on a planet, which would enable complex biospheres of the type found on Earth. The study is published today in Monthly Notices of the Royal Astronomical Society.

The number of confirmed planets in our own Milky Way galaxy now numbers into the thousands. However planets that are both Earth-like and in the habitable zone – the region around a star where the temperature is just right for liquid water to exist on the surface – are much less common.

At the moment, only a handful of such rocky and potentially habitable exoplanets are known. However the new research indicates that none of these has the theoretical conditions to sustain an Earth-like biosphere by means of ‘oxygenic’ photosynthesis – the mechanism plants on Earth use to convert light and carbon dioxide into oxygen and nutrients.

Only one of those planets comes close to receiving the stellar radiation necessary to sustain a large biosphere: Kepler−442b, a rocky planet about twice the mass of the Earth, orbiting a moderately hot star around 1,200 light years away.

The study looked in detail at how much energy is received by a planet from its host star, and whether living organisms would be able to efficiently produce nutrients and molecular oxygen, both essential elements for complex life as we know it, via normal oxygenic photosynthesis.

By calculating the amount of photosynthetically active radiation (PAR) that a planet receives from its star, the team discovered that stars around half the temperature of our Sun cannot sustain Earth-like biospheres because they do not provide enough energy in the correct wavelength range. Oxygenic photosynthesis would still be possible, but such planets could not sustain a rich biosphere.

Planets around even cooler stars known as red dwarfs, which smoulder at roughly a third of our Sun’s temperature, could not receive enough energy to even activate photosynthesis. Stars that are hotter than our Sun are much brighter, and emit up to ten times more radiation in the necessary range for effective photosynthesis than red dwarfs, however generally do not live long enough for complex life to evolve.

“Since red dwarfs are by far the most common type of star in our galaxy, this result indicates that Earth-like conditions on other planets may be much less common than we might hope,” comments Prof. Giovanni Covone of the University of Naples, lead author of the study.

He adds: “This study puts strong constraints on the parameter space for complex life, so unfortunately it appears that the “sweet spot” for hosting a rich Earth-like biosphere is not so wide.”

Future missions such as the James Webb Space Telescope (JWST), due for launch later this year, will have the sensitivity to look to distant worlds around other stars and shed new light on what it really takes for a planet to host life as we know it.

Featured image: An artistic representation of the potentially habitable planet Kepler 422-b (left), compared with Earth (right). © Ph03nix1986 / Wikimedia Commons Licence type: Attribution-ShareAlike (CC BY-SA 4.0)


Further information

The new work appears in, “Efficiency of the oxygenic photosynthesis on Earth-like planets in the habitable zone”, G. Covone, R.M. Ienco, L. Cacciapuoti and L. Inno, Monthly Notices of the Royal Astronomical Society (2021), in press (DOI: 10.1093/mnras/stab1357).


Provided by Royal Astronomical Society

Microbial Diversity Below Seafloor Is As Rich As On Earth’s Surface (Earth Science)

For the first time, researchers have mapped the biological diversity of marine sediment, one of Earth’s largest global biomes. Although marine sediment covers 70% of the Earth’s surface, little was known about its global patterns of microbial diversity.

Microbial cells in sediment: microbial cells are green, sediment particles are yellow. Image courtesy of JAMSTEC.

A team of researchers from the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), the University of Hyogo, the University of Kochi, the University of Bremen, and the University of Rhode Island delineated the global diversity of microbes in marine sediment. For the study, published in the Proceedings of the National Academy of Sciences, Tatsuhiko Hoshino, senior researcher at JAMSTEC, and his colleagues including URI Graduate School of Oceanography Professor Steven D’Hondt analyzed 299 samples of marine sediment collected as core samples from 40 sites around the globe. Their sample depths ranged from the seafloor to 678 meters below it. To accurately determine the diversity of microbial communities, the authors extracted and sequenced DNA from each frozen sample under the same clean laboratory condition.

The 16S rRNA gene sequences (approximately 50 million sequences) obtained through comprehensive next-generation sequencing were analyzed to determine microbial community composition in each sample. From these 50 million sequences, the research team discovered nearly 40,000 different types of microorganisms in marine sediment, with diversity generally decreasing with depth. The team found that microbial community composition differs significantly between organic-rich sediment of continental margins and nutrient-poor sediment of the open ocean, and that the presence or absence of oxygen and the concentration of organic matter are major factors in determining community composition.

A deep-frozen sediment core which was frozen on board immediately after sampling for microbiological analysis. DNA was extracted from the frozen core in a clean room. Photo courtesy of JAMSTEC.

By comparing their results to previous studies of topsoil and seawater, the researchers discovered that each of these three global biomes–marine sediment, topsoil, and seawater–has different microbial communities but similar total diversity. “It was an unexpected discovery that microbial diversity in the dark, energy-limited world beneath the seafloor is as diverse as in Earth’s surface biomes,” said Hoshino.

Furthermore, by combining the estimates of bacterial and archaeal diversity for these three biomes, the researchers found that bacteria are far more diverse than archaea–microbes distinct from bacteria and known for living in extreme environments–on Earth.

“In this respect as well, microbial diversity in the dark realm of marine sediment resembles microbial diversity in the surface world,” said D’Hondt. “It’s exciting to glimpse the biological richness of this dark world.”

References: Tatsuhiko Hoshino, Hideyuki Doi, Go-Ichiro Uramoto, Lars Wörmer, Rishi R. Adhikari, Nan Xiao, Yuki Morono, Steven D’Hondt, Kai-Uwe Hinrichs, Fumio Inagaki, “Global diversity of microbial communities in marine sediment”, Proceedings of the National Academy of Sciences Oct 2020, 201919139; DOI: 10.1073/pnas.1919139117

Provided by University Of Rhode Island