Seasonal restructuring of heterotrophic microbial communities is differentially affected by glacier type in Greenland fjords

Seasonal alterations in heterotrophic microbial communities within Greenland fjords vary between glacier types. The Greenland Ice Sheets' rapid retreat is causing enhanced freshwater influx into coastal fjords, causing shifts from marine-terminating glacier (MTG) to land-terminating (LTG) systems. These distinctions significantly influence water column characteristics, subsequently affecting the structure and functioning of fjord ecosystems.

Through high-throughput imaging and DNA metabarcoding methods, researchers examined seasonal (spring and summer) disparities in pelagic microbial communities in two contrasting fjord systems (MTG and LTG) in southwest Greenland, focusing on bacterial and heterotrophic protist populations, which have been underexplored in Greenland fjords.

In spring, both fjord systems exhibited a pronounced relationship between heterotrophic community composition and the diatom spring bloom. These communities were predominantly composed of typical phytoplankton-associated and copiotroph bacterial groups capable of utilizing algal-derived organic matter, as well as protists grazing on phytoplankton.

However, during summer, heterotrophic communities exhibited substantial differences between the two fjords. This divergence was likely attributed to the varied impact of MTG and LTG glaciers on the water column structure and chemistry.

The summer period in both fjords saw a notable rise in pico- and nano-sized heterotrophs, parasitic protists (dinoflagellate Syndiniales), and bacterial groups indicative of low-nutrient environments. Furthermore, bacterial, phytoplankton, and heterotrophic protist communities became more strongly coupled, resulting from either more intense interactions or stronger environmental filtering affecting all groups collectively.

These findings suggest a transition from resource-abundant systems in spring to resource-scarce systems in summer. Nevertheless, this transition was less evident in the MTG fjord, where subglacial upwelling facilitated the persistence of diatom bloom-related communities into summer.

Additionally, the research uncovered the existence of a unique, ice melange-associated inner fjord microbial community within the inner part of the MTG-impacted fjord. This community exhibited similarities with both supraglacial and sea-ice associated communities.