Estimating the contribution of lightning to microbial evolution: guidance from the Drake equation: comment on "Lightning-triggered electroporation and electrofusion as possible contributors to natural horizontal gene transfer" by Tadej Kotnik.

The possibility that lightning-mediated horizontal gene transfer (HGT) has contributed to evolution has been raised before [1–5], but not vigorously pursued. Kotnik now thoughtfully argues for doing so [6]. In addition to applauding his presentation, I suggest that estimating the significance of lightning’s contribution to HGT in microbial evolution is fundamentally a quantitative issue. For this reason aspects of electroporation (EP) and the Drake equation are briefly discussed. Fig. 1 is based on experimental exposure conditions, mainly for mammalian cells, and indicates both Supra-EP and Conventional EP [7]. The former involves creation of about 100-fold more pores during nsPEF (nanosecond pulsed electric fields) than conventional EP (pulse duration greater than about 1 μs), but nsPEF pores are mostly small. Lightning-induced electric fields in the sea and soil lie within this strength-duration space, with smaller bacteria requiring ten-fold larger strengths for pulse durations greater than ∼ 1 μs. The most relevant feature of lightning is its generation of an electric current density, J , in sea or soil. The spatially distributed J leads to electric fields that can electroporate prokaryote membranes, with these field pulse strengths, durations and repetitions defining three hemispherical zones [5,6]: lethal, HGT possible and no EP effect. Here discussion focuses on DNA delivery by electroporation without electrofusion. This seems to be the more general possibility, requiring neither bacterial protoplasts nor membrane contact.