Much like the ocean, the atmosphere is teeming with what seem to be millions of forms of microscopic life. These airborne microbes, collectively referred to as "aeroplankton," can transmit diseases and directly influence the weather and climate by seeding the formation of clouds. And much like plankton in the ocean can be carried long distances by waves and currents, aeroplankton can be transported up to thousands of kilometers by seasonal winds, storms, and even clouds of smog.

Given the versatility of their travel arrangements, the recent discovery of microfossils in eruption deposits from Yellowstone and Mount Tarawera in New Zealand prompted some researchers to wonder whether it was possible for microbes to hitch a ride on volcanic eruptions. The surprising answer, the findings from a new Geology study show, is yes.

To tackle this intriguing question, the researchers examined the fossil record of diatoms contained within rocks erupted by the Taupo volcano in New Zealand 25.4 thousand years ago. What set this eruption apart from others in the area is that it happened under a lake (Lake Huka) and therefore involved the large-scale mixing of magma and water replete with phytoplankton. The resulting deposits—complex mixtures of volcanic ash, pumice, and rock fragments—were carried up to hundreds of kilometers away by moisture-rich atmospheric plumes.

In total, the authors analyzed 22 deposit samples from 11 sites, some of them up to 850 km away from the volcano. They identified at least 300 whole and partial diatom valves in each sample to establish the composition of the individual diatom populations. To determine whether these samples matched the ones originally found in Lake Huka, the researchers also examined sediment samples collected from the lake floor. Finally, in order to rule out the possible influence of additional microfossil inputs from the surrounding environment, they analyzed the microfossil record of sediment layers deposited immediately before and after the eruption.

The results of their analyses clearly showed that the deposit-hosted diatoms were remarkably similar to those found in the Lake Huka sediment samples, pointing to a common origin. The deposit-hosted organisms were also highly consistent between samples—even those collected from sites hundreds of kilometers apart—and were distinct from those at the site where the debris landed. Moreover, of the three most abundant species discovered within both sample types, two are commonly found in lake waters globally, but one is only found in the deep waters of lakes in the volcanic region of New Zealand's North Island.

Based on their findings, the authors posit that the interaction between magma and Lake Huka water during the eruption entrained significant quantities of freshwater microbes from both the water column and a roughly 300 meter-thick layer of lake-floor sediments. They estimate that the eruption deposits contain almost 0.6 cubic kilometers of diatom remains.

While the authors can't confidently say whether the diatoms would survive their explosive voyage, they point to evidence that some species have been shown to successfully endure periods of prolonged dessication and temperature extremes—the very same conditions that these deposit-hosted diatoms would likely have been exposed to. They also hypothesize that the processes of condensation, freezing and particle aggregation that occur in moisture-rich volcanic plumes could encase the diatoms (or hardier microbes like thermophilic bacteria) in protective coatings made of fine ash, water, or ice.

Any microbes that survive the trip could go on to seed new populations in previously inaccessible locations. Over time, these migrations could help reshape environments and even lead to the evolution of new species. This study suggests the possibility that these "wet" eruptions may have enabled the large-scale dispersal of an enormous variety of microorganisms through geological space and time.

From a (somewhat) more practical standpoint, this study also demonstrates the utility of microbes as biological markers of volcanic eruptions. The analysis of the microfossil records contained within deposits could yield valuable insights into the location, timing, and dynamics of individual eruptions—much in the same way that recent studies of aeroplankton are allowing scientists to track the source region and age of air masses.

Geology, 2013. DOI: 10.1130/G34829.1.

Applied and Environmental Microbiology, 2013. DOI: 10.1128/AEM.03029-12. (About DOIs)