Sea spray aerosol as a unique source of ice nucleating particles.

Ice nucleating particles (INPs) are vital for ice initiation in, and precipitation from, mixed-phase clouds. A source of INPs from oceans within sea spray aerosol (SSA) emissions has been suggested in previous studies but remained unconfirmed. Here, we show that INPs are emitted using real wave breaking in a laboratory flume to produce SSA. The number concentrations of INPs from laboratory-generated SSA, when normalized to typical total aerosol number concentrations in the marine boundary layer, agree well with measurements from diverse regions over the oceans. Data in the present study are also in accord with previously published INP measurements made over remote ocean regions. INP number concentrations active within liquid water droplets increase exponentially in number with a decrease in temperature below 0 °C, averaging an order of magnitude increase per 5 °C interval. The plausibility of a strong increase in SSA INP emissions in association with phytoplankton blooms is also shown in laboratory simulations. Nevertheless, INP number concentrations, or active site densities approximated using "dry" geometric SSA surface areas, are a few orders of magnitude lower than corresponding concentrations or site densities in the surface boundary layer over continental regions. These findings have important implications for cloud radiative forcing and precipitation within low-level and midlevel marine clouds unaffected by continental INP sources, such as may occur over the Southern Ocean.

Measurement of ice nucleation-active bacteria on plants and in precipitation by quantitative PCR.

Ice nucleation-active (INA) bacteria may function as high-temperature ice-nucleating particles (INP) in clouds, but their effective contribution to atmospheric processes, i.e., their potential to trigger glaciation and precipitation, remains uncertain. We know little about their abundance on natural vegetation, factors that trigger their release, or persistence of their ice nucleation activity once airborne. To facilitate these investigations, we developed two quantitative PCR (qPCR) tests of the ina gene to directly count INA bacteria in environmental samples. Each of two primer pairs amplified most alleles of the ina gene and, taken together, they should amplify all known alleles. To aid primer design, we collected many new INA isolates. Alignment of their partial ina sequences revealed new and deeply branching clades, including sequences from Pseudomonas syringae pv. atropurpurea, Ps. viridiflava, Pantoea agglomerans, Xanthomonas campestris, and possibly Ps. putida, Ps. auricularis, and Ps. poae. qPCR of leaf washings recorded ∼10(8) ina genes g(-1) fresh weight of foliage on cereals and 10(5) to 10(7) g(-1) on broadleaf crops. Much lower populations were found on most naturally occurring vegetation. In fresh snow, ina genes from various INA bacteria were detected in about half the samples but at abundances that could have accounted for only a minor proportion of INP at -10°C (assuming one ina gene per INA bacterium). Despite this, an apparent biological source contributed an average of ∼85% of INP active at -10°C in snow samples. In contrast, a thunderstorm hail sample contained 0.3 INA bacteria per INP active at -10°C, suggesting a significant contribution to this sample.

Persistence and Latency of Clavibacter michiganensis subsp. sepedonicus in Field-Grown Seed Potatoes.

Whole seed tubers were inoculated with different inoculum doses of Clavibacter michiganensis subsp. sepedonicus and planted in the field. Ring rot symptom development and the persistence of C. michiganensis subsp. sepedonicus, in the absence of symptoms, was determined for four generations following introduction of inoculum. Seed tubers were not cut during propagation to preclude redistribution of inoculum. Detection of C. michiganensis subsp. sepedonicus was positively related to inoculum dose (r = 0.97) and rapidly declined with increasing generations. Although the frequency of detection was much less following inoculation with ≤102 cells versus inoculation with ≥104 cells, all inoculum doses established latent infections that were undetected for at least one, and up to three, generations. Latency for three generations, characterized by C. michiganensis subsp. sepedonicus detection or ring rot symptom development during the fourth generation, was the longest time interval tested. The occurrence of latent C. michiganensis subsp. sepedonicus infections and the persistence of inadvertent inoculum sources may, at least partially, explain our inability to eradicate ring rot based on visible symptom expression.