Dispersal of endolithic microorganisms in vesicular volcanic rock: Distribution, settlement and pathways revealed by 3D X‐ray microscopy

Pleistocene basanitic rocks of Vesteris Seamount in the Greenland Sea had been found to exhibit an endolithic habitat largely consisting of marine fungi that dwell within abundant vesicles, therefore representing cryptoendoliths. For the first time, we demonstrate that 3D X‐ray microscopy can unravel how microorganisms access and migrate through vesicular rock. The fossil assemblages occur within a set of vesicles connected by microcracks. Such microcracks, which are ubiquitous features in submarine volcanic rocks, enable the dispersal of marine microorganisms in the rock. This study suggests that this pathway for the colonization of marine volcanic rocks forms in consequence of early tensional stress due to variable rates of cooling of the lava flow. Subsequently, the interconnected vesicles get populated by rock‐dwelling microorganisms. This cryptoendolithic habitat exists at least since the Paleoproterozoic.

. Endoliths actively penetrating the rock-forming minerals are referred to as euendoliths, organisms settling in structural cavities or cracks induced by geologic forces are referred to as cryptoendoliths (Eickmann et al., 2009;Jones & Pemberton, 1987). In shallow marine settings, cyanobacteria are the most prevalent endolithic organisms, searching for shelter in rocks, while fungi and chemotrophic bacteria mostly settle in aphotic zones feeding on organic residues or mineral components or thriving in complex symbiotic-like relationships (Golubic et al., 2005;Wisshak, 2012). While carbonates and phosphorites are common host rock substrates for all kinds of endolithic organisms, the diversity of endoliths seems to be less in igneous rock (Bengtson et al., 2014;Golubic et al., 1981;McLoughlin et al., 2008;Wisshak, 2012).
The oldest occurrence of cryptoendoliths has been recorded in Paleoproterozoic volcanic environments, which therefore played a crucial role in the early evolution of endolithic life (Bengtson et al., 2017;Ivarsson et al., 2020;McMahon & Parnell, 2018). The oceanic crust is a long-term endolithic habitat as indicated by fossils enclosed in vesicle-fills of volcanic rocks in ophiolites and the deep sea (e.g. Ivarsson et al., 2016Ivarsson et al., , 2020Sallstedt et al., 2019). The upper volcanic part of the ocean crust is strongly fissured and rich in void space, consequently owing high porosity and permeability, making it Earth's largest aquifer and a vast microbial habitat (e.g. Bach & Edwards, 2003;Fischer & Becker, 2000;Orcutt et al., 2011).
Even though the presence of cryptoendolithic life in the igneous oceanic crust has been confirmed (Ivarsson et al., 2020), the nature of the adaptation to volcanic environments is largely unresolved. Ivarsson et al. (2015) discussed biogenicity and established a fungal affiliation of filamentous microstructures in vesicular basanite from Vesteris Seamount; these authors assumed that cryptoendoliths actively migrate into the igneous oceanic lithosphere. In this follow-up-study, we applied 3D X-ray microscopy to rock samples from Vesteris Seamount, containing similar microstructures as the rock samples studied by Ivarsson et al. (2015), to depict the distribution of cryptoendoliths and reveal their modes of dispersal. This approach unravels that synformational microcracks in lava flows are instrumental for the dispersal of cryptoendoliths in seafloor volcanic rock.

| G EOLOG IC AL S E T TING
Vesteris Seamount is a solitary volcano located in the Greenland basin, remote from any coastline or plate boundary. The Pleistocene seamount rests upon 44 Ma old basement at 3000 m water depth, and the summit rises to 137 m below sea surface (Haase & Devey, 1994;Mertz & Renne, 1995). The volcanic edifice has a stellate shape and a pronounced NNE-SSW trending central rift zone

Significance statement
Life within structural cavities of volcanic rock has been documented for the Proterozoic and much of the Phanerozoic. Therefore, cavity-dwelling, cryptoendolithic microorganisms represent a primordial mode of life, a mode of life that remained largely unchanged to this very day. Although fossil cryptoendoliths have now been commonly recognized in vesicular volcanic rock, the pathway of dispersal-how these microbes access their protected habitat in apparently isolated vesicles-has not been unravelled. For the first time, this article documents the mode of colonization of vesicular rock, which is found to progress along the connected porosity including coalescing vesicles and microcracks. Given their ubiquity, microcracks may represent a significant pathway for the microbial colonization of the igneous oceanic crust, one of the least understood habitats of the deep biosphere.
F I G U R E 1 Map of Vesteris Seamount (with sampling locations 08, 20 and 33). [Colour figure can be viewed at wileyonlinelibrary.com] more evolved mugearites sampled on ridge flanks yielded younger ages of 10-85 ka (Mertz & Renne, 1995). Vesteris Seamount has been active during the Quaternary with numerous eruptions in the last 60 ka documented by ash layers in proximal drill cores (Haase et al., 1996). with ropy to billowy surfaces. Samples of station 08 were collected 720 m southwest of a previously sampled vesicular basanite, which was reported to host abundant fungal endolithic fossils (Ivarsson et al., 2015).

| SAMPLING AND ME THODS
Characterization of volcanic rock rich in filamentous and stromatolitic microstructures was performed using a Keyence VHX-6000 microscope to capture high-resolution stacked incident light images and a Zeiss Axioscope A1 equipped with a Canon EOS1300D to obtain transmitted and reflected light images. For 3D X-ray microscopy (XRM), three exemplary fragments of TVgrab samples (Table 1) were scanned in two 360° rotation scans with the 3D X-ray microscope ZEISS Xradia 520 system at the MAPEX Center for Materials and Processes, University of Bremen, Germany. Subsequent to a scan in overview mode (approx. 2000 3 voxels, ranging between 7.5 and 9.5 μm per voxel) with a beam energy of 90 kV, and an energy flux of 89 μA (ZEISS filter LE3), a smaller volume of interest of each sample (approx. 1000 3 voxels) was scanned with 4.5 μm per voxel under the same conditions. Correction of ring artefacts and reconstruction of the spatial information on the linear attenuation coefficient in the samples was carried out using the ZEISS Reconstructor software. All subsequent processing of volume data (e.g. filtering the raw data and volume rendering) was carried out using Avizo 2020. For visualization of volume reconstructions and single slices, the Avizo filter module Recursive Exponential was used in 3D mode.

| RE SULTS
The investigated samples of vesicular basanite exhibit two types of microstructures, (1) basal crusts resembling microstromatolites and Frutexites (cf. Bengtson et al., 2014) and (2) Dixon, 1997). This mechanism was at work throughout most of the magma plumbing system, but the majority of vesicles nucleated and grew upon ascent to the seafloor at pressures of less than 1 kbar (cf. Dixon, 1997). The effect of vesicles on permeability is a function of their frequency (Saar & Manga, 1999), promoting connectivity via coalescence of vesicles. However, this mechanism is restricted to areas of high vesicle density. Microcracks like those observed by 3D X-ray microscopy (XRM) herein provide another means of enhanced permeability in volcanic rock. Indeed, the presence of such microcracks strongly affects the relationship between porosity and permeability of vesicular rocks (Macpherson, 1984;Merle et al., 2005;Petford, 2003;Saar & Manga, 1999) by greatly increasing the interconnectedness of vesicles and hence boosting permeability. The likely origin of the microcracks is tensional stress due to variable rates of cooling in the lava flow (Callow et al., 2018;McGrail et al., 2006). The aperture of the microcracks is small relative to the average diameter of the vesicles. However, even microcracks with small apertures (w) can greatly increase permeability (k) as indicated by the k ~ w 4 relationship (Saar & Manga, 1999

| CON CLUS IONS
3D X-ray microscopy (XRM) revealed a strong correlation between the distribution of fungal microfossils and microcracks within vesicular basanites of Vesteris Seamount. In vesicular rock of the igneous oceanic crust two mechanisms control the distribution of microfossils, (1) coalescing vesicles and (2) microcracks. Syngenetic microcracks are a major pathway for dispersal as this secondary permeability connects primarily isolated pore space in a lava flow.
Since microcracks are a ubiquitous feature in submarine volcanic lithologies, we suggest that this mechanism was and still is probably a key factor in the colonization of the oceanic crust since at least the Paleoproterozoic.

ACK N O WLE D G E M ENTS
Hinrich Schmid-Beurmann thanks the Landesgraduiertenförderung (HmbNFG) of the city of Hamburg for funding. We acknowledge the use of the 3D X-ray microscope ZEISS Xradia 520 system at the

DATA AVA I L A B I L I T Y S TAT E M E N T
All data of this article are available in PANGAEA Data Publisher.