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Comprehensive characterization of an aspen (Populus tremuloides) leaf litter sample that maintained ice nucleation activity for 48 years

Abstract. Decaying vegetation was determined to be a potentially important source of
atmospheric ice nucleation particles (INPs) in the early 1970s. The bacterium
Pseudomonas syringae was the first microorganism with ice nucleation
activity (INA) isolated from decaying leaf litter in 1974. However, the ice
nucleation characteristics of P. syringae are not compatible with
the characteristics of leaf litter-derived INPs since the latter were found
to be sub-micron in size, while INA of P. syringae depends on much
larger intact bacterial cells. Here we determined the cumulative ice
nucleation spectrum and microbial community composition of the historic leaf
litter sample 70-S-14 collected in 1970 that conserved INA for 48 years. The
majority of the leaf litter-derived INPs were confirmed to be sub-micron in
size and to be sensitive to boiling. Culture-independent microbial community
analysis only identified Pseudomonas as potential INA.
Culture-dependent analysis identified one P. syringae isolate, two
isolates of the bacterial species Pantoea ananatis, and one fungal
isolate of Mortierella alpina as having INA among 1170 bacterial
colonies and 277 fungal isolates, respectively. Both Pa. ananatis
and M. alpina are organisms that produce heat-sensitive sub-micron
INPs. They are thus both likely sources of the INPs present in sample 70-S-14
and may represent important terrestrial sources of atmospheric INPs, a
conclusion that is in line with other recent results obtained in regard to
INPs from soil, precipitation, and the atmosphere.
