The microstructure, biogeochemistry and aggregation of Arctic cryoconite granules

Cryoconite granules are hydrous microaggregates, principally composed of microorganisms, organic matter and mineral particles, that reside upon glacier surfaces. Whilst recent research has highlighted the diverse microbial community within cryoconite granules and their role as biogeochemical reactors, little is known about their microstructure, the cell–mineral interaction within them, or their aggregation mechanisms. Knowledge of these is crucial for the understanding of autotrophic organic matter production and cycling, the entrainment of particulate matter and its consequential effect on glacier melt, and the life cycle of cryoconite granules and its impact on proglacial soil development. These studies find that cryoconite granules are heterogeneous, demonstrating spatially variable microorganism and organic matter contents, containing significant quantities of filamentous cyanobacteria and exhibiting a fine groundmass of clay-sized particles enmeshed within various extracellular polymeric substances (EPS). The importance of photoautotrophy, EPS production and cation-bridged EPS–mineral interactions as biological ‘forming factors’ is demonstrated. The cyanobacterial filament and carbohydrate contents of cryoconite material explain 83% of the measured variability in aggregate size and stability upon Longyearbreen glacier. Geospatial investigations of these ‘forming factors’ reveal spatial patterns, with a zone of excess EPS production evident towards the snowline and increased pigment production evident in stable, down-glacier locations. A variety of mineral particles can co-aggregate with EPS-producing cyanobacteria, with ionic strength, temperature and growth phase all affecting efficiency. Spectroscopic studies reveal evidence for chiefly polymer-based modes of attachment within cyanobacteria–EPS–mineral aggregates, with complex EPS able to overcome surface charge and interact with mineral surfaces in a variety of ways. This thesis presents the first detailed study of cryoconite granule microstructure and aggregation, finding that photoautotrophy and EPS production are vital to the development of stable aggregates, providing a matrix that attracts aeolian particulates, stabilises granules and promotes biogeochemical interaction and the development of microenvironments.