Every­body Is Al­ways Talk­ing About the Weather...

by Elio

...but bac­te­ria are ac­tu­ally do­ing some­thing about it. For starters, we re­al­ize that bac­te­ria play an im­por­tant role in the cli­mate of this planet. Mi­crobes were the orig­i­nal source of all at­mos­pheric oxy­gen; they are ac­tive in the ni­trogen cy­cle and in pro­duc­ing and re­mov­ing car­bon di­oxide. As we are painfully re­minded every day, to change the at­mos­phere is to also change the cli­mate. The chan­ges due to mi­crobes may not be as much in the news as the cur­rent man-made ones, but the mi­crobes have been re­spon­si­ble for shap­ing this Earth and they con­tinue to do their im­por­tant work.

That much is com­monly known. What is less well ap­pre­ci­ated is that bac­te­ria may play a role in the weather, that is in the day-to-day changes.

Some ma­rine bac­te­ria make large amounts of di­methyl sul­fide (DMS), a gas that rises up into the at­mos­phere. There it be­comes ox­i­dized into sul­fates and other com­pounds that cause wa­ter par­ti­cles to nu­cle­ate, thus form­ing clouds.

Where does DMS come from? Bac­te­ria make it from di­methyl sul­fo­nio­pro­pi­onate (DMSP), a non-volatile com­pound pro­duced in abun­dance by ma­rine al­gae, big and small, who use it to counter the high os­motic pres­sure of sea­wa­ter. When these al­gae are un­der stress or die, they re­lease their DMSP, some of which is taken up by the bac­te­ri­o­plank­ton who me­tab­o­lize it by one of two path­ways. One path­way in­volves demethy­la­tion of the DMSP and makes some of the sul­fur avail­able for bac­te­r­ial pro­tein syn­the­sis. The other path­way con­verts the DMSP into volatile DMS, for what pur­pose we know not. How­ever, this abil­ity to con­vert DMSP to DMS is wide­spread among ma­rine bac­te­ria. Re­cently, it has been pro­posed that the path­way for mak­ing DMS is not a di­rect degra­da­tion by a lyase en­zyme, but rather in­volves the ad­di­tion of acyl coen­zyme A and the sub­sequent con­ver­sion into DMS.

In prin­ci­ple, this should con­sti­tute a neg­a­tive feed­back loop: the more DMS is made, the more clouds are formed, the less sun­light shines on the ocean, the less pho­to­syn­the­sis is car­ried out, the less DMSP is made, the less DMS is pro­duced, etc. And the re­verse should be true as well. This mode of cli­mate reg­u­la­tion has been called the CLAW hy­poth­e­sis (for the ini­tials of its au­thors), and is still be­ing de­bated. Of course, large sul­fur emis­sions re­sult from the burn­ing of fos­sil fu­els – that be­ing the pre­dom­i­nant source, at least in the more industrial­ized North­ern hemi­sphere. This sounds like an ar­gu­ment for burn­ing more fos­sil fuel and emit­ting more sul­fur. How­ever, this would not mean­ing­fully coun­ter­act the green­house ef­fect of car­bon diox­ide and other gases. Still, sul­fur com­pounds in the at­mos­phere are far from in­signif­i­cant. They amount to one mil­lion met­ric tons and are pos­tu­lated to have a turnover time of a few weeks.

DMS is an in­ter­est­ing gas for ol­fac­tory rea­sons as well. It at­tracts small crus­taceans, such as cope­pods, and seabirds, such as pe­trels to ar­eas where the food sup­ply is abun­dant. As we ex­pe­ri­ence in our bath­rooms, DMS of in­testi­nal ori­gin has an odor that is un­pleas­ant to hu­mans. How­ever, in a dif­fer­ent con­text, it con­tributes to the com­plex fra­grance of sea breezes of­ten lauded in ro­man­tic rhymes.

With the su­perb ed­i­to­r­ial help of Merry Youle. Pho­tos cour­tesy of the Na­tional Oceanic and At­mos­pheric Ad­min­is­tra­tion.