Urediospores of rust fungi are ice nucleation active at &amp;gt; −10 °C and harbor ice nucleation active bacteria

C E Morris,A R Moukahel,F L T Gonçalves,S Asaad,E K Bigg,D C Sands,C Glaux,J Samsatly

doi:10.5194/acp-13-4223-2013

Abstract

Abstract. Various features of the biology of the rust fungi and of the epidemiology of the plant diseases they cause illustrate the important role of rainfall in their life history. Based on this insight we have characterized the ice nucleation activity (INA) of the aerially disseminated spores (urediospores) of this group of fungi. Urediospores of this obligate plant parasite were collected from natural infections of 7 species of weeds in France, from coffee in Brazil and from field and greenhouse-grown wheat in France, the USA, Turkey and Syria. Immersion freezing was used to determine freezing onset temperatures and the abundance of ice nuclei in suspensions of washed spores. Microbiological analyses of spores from France, the USA and Brazil, and subsequent tests of the ice nucleation activity of the bacteria associated with spores were deployed to quantify the contribution of bacteria to the ice nucleation activity of the spores. All samples of spores were ice nucleation active, having freezing onset temperatures as high as −4 °C. Spores in most of the samples carried cells of ice nucleation-active strains of the bacterium Pseudomonas syringae (at rates of less than 1 bacterial cell per 100 urediospores), but bacterial INA accounted for only a small fraction of the INA observed in spore suspensions. Changes in the INA of spore suspensions after treatment with lysozyme suggest that the INA of urediospores involves a polysaccharide. Based on data from the literature, we have estimated the concentrations of urediospores in air at cloud height and in rainfall. These quantities are very similar to those reported for other biological ice nucleators in these same substrates. However, at cloud level convective activity leads to widely varying concentrations of particles of surface origin, so that mean concentrations can underestimate their possible effects on clouds. We propose that spatial and temporal concentrations of biological ice nucleators active at temperatures > −10 °C and the specific conditions under which they can influence cloud glaciation need to be further evaluated so as to understand how evolutionary processes could have positively selected for INA.

Highlights

Hydrology andOver the past few years biological particles that tchEoeuraledrhtbahes baSecetyinvseataersemniecwe enducinletierienstthine atmosphere at temperatures waSrmceirethnacne−s10 ◦C
The objectives of this work were (1) to demonstrate that ice nucleation activity at temperatures > −10 ◦C is widespread among the urediospores of rust fungi, (2) to determine if this activity is due to the spores themselves or to the bacteria associated with the spores, and (3) to quantify the frequency of ice nuclei per urediospore in order to provide information useful for parameterizing atmospheric models, for comparing activity among biological ice nucleators and for enhancing understanding of the life history of the rust fungi
The frequency of ice nuclei per spore was in the range of the frequency of ice nuclei produced per bacterial cell of reference strains of P. syringae (Fig. 1)

Summary

Introduction

Hydrology andOver the past few years biological particles that tchEoeuraledrhtbahes baSecetyinvseataersemniecwe enducinletierienstthine atmosphere at temperatures waSrmceirethnacne−s10 ◦C. By catalyzing ice formation at these relatively high temperatures, ice nucleation active spores, pollen and bacterial cells transported into clouds could initiate the process of raindrop formation at of having taenmepfeferacttu(rMesoOwhlhceerereeatmanli.n,Se2r0ac0li7pe)a.nrPticacrleetisclaerse incapable of biological origin are among the only particles naturally present in the atmosphere known to be effective ice nucleators at temperatures > −10 ◦C. They have the potential to incite processes needed for rainfall that would otherwise not occur in mid- and high-latitudeScololuiddsEatasrutchh temperatures. Morris et al.: Ice nucleation activity of rust fungi urediospores quantify biological ice nucleators in the atmosphere for subsequent parameterization of cloud physics models that will contribute to estimating the impact of biological ice nucleators on rainfall (Morris et al, 2011)

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