Is the presence of oxygen on an exoplanet a reliable biosignature?

We revisit the validity of the presence of O(2) or O(3) in the atmosphere of a rocky planet as being a biosignature. Up to now, the false positive that has been identified applies to a planet during a hot greenhouse runaway, which is restricted to planets outside the habitable zone (HZ) of the star that are closer to the star. In this paper, we explore a new possibility based on abiotic photogeneration of O(2) at the surface of a planet that could occur inside the HZ. The search for such a process is an active field of laboratory investigation that has resulted from an ongoing interest in finding efficient systems with the capacity to harvest solar energy on Earth. Although such a process is energetically viable, we find it to be a very unlikely explanation for the observation of O(2) or O(3) in the atmosphere of a telluric exoplanet in the HZ. It requires an efficient photocatalyst to be present and abundant under natural planetary conditions, which appears unlikely according to our discussion of known mineral photochemical processes. In contrast, a biological system that synthesizes its constituents from abundant raw materials and energy has the inherent adaptation advantage to become widespread and dominant (Darwinist argument). Thus, O(2) appears to continue to be a good biosignature.

Efficient microprojectile bombardment-mediated transformation of rice using gene cassettes.

This study was aimed at determining whether gene cassettes (promoter-coding sequence-terminator) can be efficiently used in microprojectile acceleration-mediated co-transformation of rice in the place of whole plasmids, and to what extent their use influences the integration and expression of the co-transferred gene of interest. Two non-linked marker genes ( yfp and hph) were co-introduced by microprojectile bombardment into cells of embryogenic calli in three separate experiments. Three different DNA structures were compared for their ability to transiently and stably transform rice cells: supercoiled or linearized whole-plasmid DNA, gene cassette DNA and single-stranded gene cassette DNA coated with Escherichia coli single-stranded binding (SSB) proteins. Our results demonstrate that microprojectile bombardment-mediated transformation of rice using gene cassettes is possible without significantly reducing transformation efficiency in comparison to the use of whole-plasmid DNA. Furthermore, no obvious difference in transgene integration pattern and inheritance was observed among plants transformed with gene cassettes compared to those transformed with the whole plasmid, except that concatemerization of molecules prior to integration was rarely observed in gene cassette transformants.

Optical data storage in microfibers.

Tests of optical data storage in arrays of microfibers confirm its applicability and potential for higher storage densities than those achievable with conventional holographic data storage. Arrays of single-mode microfibers, spaced 0.78 microm apart and 60 microm long, were generated in a photopolymer film with four laser beams and simultaneously inscribed with Lippmann-Bragg fringes by use of a counterpropagating beam. Following the curing steps, spectra of white light retroreflected from a single fiber exhibit the reconstructed spectral lines of the multiwavelength laser used in the recording step; 10(11) bits/cm(2), or 10(13) bits on a compact disk, appear to be recordable.