Ice Ice Maybe

by Janie
 

The ability to trigger ice for­ma­tion is a prop­erty shared by cer­tain par­ticles both org­anic and in­org­anic, inc­lud­ing bac­teria, viruses, phyto­plank­ton, pollen, soot, dust. These little par­ticles help water mol­ecules come tog­ether and form ice at war­mer temp­era­tures than freez­ing point, in a process called ice nuc­lea­tion. In nature these ice nuc­lea­tors play im­por­tant roles in cloud con­dens­ation, precip­ita­tion, cli­mate, and Earth's radia­tive equil­ibrium.
 

Common ice nucleators include sev­eral bac­terial species. One such spec­ies is Pseudomonas syringae, whose special outer-membrane pro­tein engages in hydro­gen bon­ding with water to line the mol­ecules up in an or­derly lat­tice, thereby cata­lyzing the for­ma­tion of ice crys­tals. It is this unique ability that gives the bac­ter­ium a double-edged rel­ation­ship with humans: P. syringae is a not­orious crop patho­gen that attacks plants by induc­ing frost damage on leaves, but it is also this ability that makes P. syringae useful for ski resort snow machines prep­aring winter-ready slopes. Other bac­teria with sim­ilar special ice-inducing pro­teins include mem­bers of the genera Pantoea, Erwinia, and Xanthomonas. Of course, that outer mem­brane protein is exc­lusive to Gram-negatives. The basis for ice nuc­lea­tion by Gram-positive bac­teria lacking the outer mem­brane, how­ever, is an area that has been a bit quieter.
 

Figure 1. (Figure 1 of the paper) Ice nucleation activity of Lp UV mutants compared to Lp wild-type (Lp wt) based on three replicates of drop­let freez­ing assays using 30 drops per dil­ution per rep­li­cate. The X-axis shows the tem­per­a­ture and the Y-axis shows the log con­cen­tra­tion of ice nuc­lea­tion par­ticles (INP) per col­ony for­ming unit (CFU). Strains ex­hib­iting id­en­ti­cal spec­tra were com­bined into one graph. A Lp mutant UV1, B Lp mutant UV11, C Lp UV mutant 14, Lp mutant UV32, and Lp mutant UV35, D Lp mutant 131 and Lp mutant 158, E Lp mutant 168, and F Lp mutant 187. Source.

A recent ice­breaker was a 2021 paper from Failor et al. To peek into a Gram-positive mech­anism for ice nuc­lea­tion, the authors first iden­tified invol­ved genes in the Firmicute (aka Bacillota) Lysinibacillus parviboronicapiens (Lp) by com­par­ing the genome of wild-type Lp with that of mutant non-ice-nucleating Lp. These can­didate genes were win­nowed fur­ther by a UV-mutagenesis screen to two genes that res­ulted in sig­nifi­cant loss of ice-nucleating ability. Here came a sur­prise. One of these genes was pred­icted to be a type I PKS (poly­ketide syn­thase), and the other, an NRPS (non-ribosomal pep­tide syn­the­tase), both of which were in the same op­eron—marking the first time that a PKS-NRPS has been as­socia­ted with ice nuc­lea­tion.
 

The authors won­dered if the ice-nucleating par­ticle from Lp could be sec­reted via ves­icles, like fellow poly­ketide linear­mycin. It is a pos­sibil­ity; super­natant from Lp cul­tures was fil­tered and sub­jected to gra­dient ultra­centrifu­gation, and the ice-nucleating par­ticles were found in the same frac­tions as linear­mycin. Trans­mis­sion elec­tron micros­copy of Lp then rev­ealed that these par­ticles form long extra­cellular fila­ments com­posed of many little "pearls" (Figure 2) that are on average 488 nano­meters long and 5 nano­meters wide. These "pearl chains" were not found on mut­ants in­cap­able of ice-nucleation. Although how ex­actly these chains cause ice nuc­lea­tion remains un­known, this study dem­onstrates that poly­ketides, like the pro­teins of Gram-negative bac­teria, are capable of trig­ger­ing it.
 

Figure 2. (Figure 4 of the paper) Transmission el­ec­tron mic­ro­scope (TEM) im­ages of the LINA fil­a­ments ex­creted by the wild-type Lp VT1065 strain. In each panel, white ar­rows in­di­cate the LINA fil­a­ments. A–C TEM im­ages of var­ious fil­a­ments in­clud­ing fla­gella and LINA fil­a­ments. D Stitch­ing of three TEM im­ages of long LINA fil­a­ments. E, F High-resolution im­ages of the LINA fil­a­ments show­ing their “pearl chain” like struc­ture. Scale bars rep­re­sent 100 nm (A–D) and 20 nm (E and F). Source. Fron­tis­piece.

Although this is the first PKS-NRPS to be linked to the nuc­lea­tion of ice, it is not the first to be linked to nuc­lea­tion as a broader pheno­menon. For ex­ample, there is a PKS known to be inv­olved in nuc­lea­tion of cal­cium car­bon­ate in the pro­cess of bio­mineral­iza­tion. The authors sug­gest that similar polyketide-based mol­ecules may under­lie the ice-nucleating ability of pollen and of fungi like Fusarium and Mortierella, whose mech­anisms also remain enig­matic.
 

Beyond ex­amples of smaller-scale ice crystal for­mation, one might also wonder how much of a role mic­robes have played on a grander scale, in global freeze-thaw cycles throughout the history of Earth… Earth has had five major ice ages—periods in which colder temp­era­tures cause ice sheets to spread across the globe. The most rec­ent ice age 20,000 years ago was replete with woolly mam­moths and saber-toothed tigers, pop­ular­ized by the ani­mated film and fran­chise. The clim­ate of this last ice age has been recon­struc­ted using well-established methods that include sim­ula­tions of par­ticles acting as ice nuc­leators. Of these nuc­leat­ing par­ticles im­por­tant to the incep­tion of ice ages, one might wonder what pro­por­tion was bac­terial.