Island Biogeography of Cryoconite Hole Bacteria in Antarctica's Taylor Valley and Around the World

Abstract

Cryoconite holes are holes in a glacier's surface caused by sediment melting into the glacier. These holes are self-contained ecosystems that include abundant bacterial life within their sediment and liquid water, and have recently gained the attention of microbial ecologists looking to use cryoconite holes as "natural microcosms" to study microbial community assembly. Here, we explore the idea that cryoconite holes can be viewed as "islands," in the same sense that an island in the ocean is an area of habitat surrounded by a barrier to entry. In the case of a classic oceanic island, the ocean is a barrier between islands, but in the case of cryoconite holes, the ocean is comprised of impermeable solid ice. We test two hypotheses, born out of island biogeographic theory, that can be readily applied to cryoconite hole bacteria. First, we ask to what extent the size of a cryoconite hole is related to the amount of bacterial diversity found within it. Second, we ask to what extent cryoconite holes exhibit distance decay of similarity, meaning that geographically close holes are expected to harbor similar bacterial communities, and distant holes are expected to harbor more different bacterial communities. To test the island size hypothesis, we measured the sizes of cryoconite holes on three glaciers in Antarctica's Taylor Valley and used DNA sequencing to measure diversity of bacterial communities within them. We found that for two of these glaciers, there is a strong relationship between hole size and bacterial phylogenetic diversity, supporting the idea that cryoconite holes on those glaciers are "islands." The high biomass dispersing to the third glacier we measured could explain the lack of size-diversity relationship, remaining consistent with island biogeography. To test the distance decay of similarity hypothesis, we used DNA sequence data from several previous studies of cryoconite hole bacteria from across the world. Combined with our Taylor Valley data, those data showed that cryoconite holes have strong spatial structuring at scales of one to several hundred kilometers, also supporting the idea that these dirty holes on glaciers are really islands in the cryosphere.

Extreme weather event results in the removal of invertebrates from cryoconite holes on an Arctic valley glacier (Longyearbreen, Svalbard)

“…Despite such harsh survival conditions, microbial and invertebrate assemblages have adapted to life in these ecosystems (Edwards et al, ; Mueller, Vincent, Pollard, & Fristen, ; Shain et al, ; Wharton, McKay, Simmons, & Parker, ). The largest biodiversity hotspots on glaciers worldwide are found within cryoconite holes (Darcy, Gendron, Sommers, Porazinska, & Schmidt, ; Mueller et al, ; Wharton et al, ), which are round hollows in a glacier's surface. Cryoconite holes form during the summer when accumulation of windblown dust and microorganisms inhabiting ice decrease the albedo at a spot on the glacial surface, resulting in ice melting and water reservoir formation (Fountain, Tranter, Nylen, Lewis, & Mueller, ; Hodson et al, ; Wharton et al, ).…”

Dormancy dampens the microbial distance-decay relationship

“…The DDR characterizes how community similarity changes across increasing geographical distance [20], often reflecting the influence of dispersal limitation and spatially autocorrelated environmental conditions [21-24]. The DDR has attracted attention among microbial ecologists for testing predictions of island biogeography [25], for elucidating scale-dependency of beta-diversity [26], and for revealing the relative influence of geographical isolation versus environmental conditions [27-30]. Because it can decrease environmental filtering and dispersal limitation, microbial dormancy is predicted to flatten the slope of the DDR [19, 31].…”

Fine-scale spatial heterogeneity of invertebrates within cryoconite holes

“…The ecology and biology of cryoconite holes as ecosystems have been the subject of intensive studies (Cook et al 2015;Darcy et al 2018;Pittino et al 2018;Poniecka et al 2018;Zawierucha et al 2018). The majority of recent works focused on the diversity, physiology and metabolism of bacteria and algae (Yallop et al 2012;Grzesiak et al 2015;Gawor et al 2016) in the processes forming cryoconite granules (Takeuchi et al 2001b;Uetake et al 2019), on interactions between surface ice structures, cryoconite holes, weathering crust and organisms, on the socalled biocryomorphology (Cook et al 2016a), and on the contamination of supraglacial ecosystems (Ferrario et al 2017).…”

“…Studies on the cryosphere ranged from those performed on a large scale, e.g. concerning glaciers (Smith et al 2017;Darcy et al 2018), to the ones conducted on a small-scale, e.g. investigating cryoconite granules (Takeuchi et al 2001a, b;Uetake et al 2019).…”