Cli­mate change: bac­teria play an import­ant role in the long term stor­age of car­bon in the ocean

Dis­solved organ­ic mat­ter (DOM) in the ocean is a highly com­plex mix­ture of dif­fer­ent car­bon-based sub­stances which are meta­bol­ic or excret­ory products from organ­isms or have formed through decom­pos­i­tion pro­cesses. A pro­por­tion of the DOM can be con­sumed by bac­teria and remin­er­al­ized to car­bon diox­ide. How­ever, the major­ity of the DOM found in the ocean (more than 90 per­cent) is res­ist­ant to bac­teri­al util­iz­a­tion and can be 4,000 to 6,000 years old on aver­age. ​“Bac­teria have an enorm­ous rep­er­toire of enzymat­ic mech­an­isms to break down organ­ic mater­i­al and use it as a source of energy,” said Dr Oliv­er Lechten­feld of the Helm­holtz Centre for Envir­on­ment­al Research (UFZ), who car­ried out the study at the Uni­ver­sity of South Car­o­lina. ​“It’s remark­able that such a large reser­voir of DOM seems to be res­ist­ant to break­down in the ocean. And it raises the ques­tion: where does this per­sist­ent DOM actu­ally come from?” Stud­ies have shown that bac­teria in labor­at­ory exper­i­ments pro­duce DOM that can be stable for over a year. Lechten­feld: ​“We wanted to find out wheth­er the DOM pro­duced by bac­teria in the labor­at­ory is chem­ic­ally com­par­able with the per­sist­ent DOM that occurs nat­ur­ally”. For their study, the research­ers placed bac­teria from the sea in arti­fi­cial sea water, in order to ensure that the water was DOM-free at the begin­ning of the exper­i­ment. The bac­teria were fed using known car­bon sources. ​“By doing this, we knew that the DOM meas­ured in the incub­a­tions was pro­duced by the bac­teria”, explained Lechten­feld. After four weeks the DOM pro­duced by bac­teria was ana­lyzed using high-res­ol­u­tion chem­ic­al meth­ods (nuc­le­ar mag­net­ic res­on­ance spec­tro­scopy and ultra high-res­ol­u­tion mass spec­tro­metry) and com­pared with the DOM found nat­ur­ally in sea water. ​“Our study reveals that bac­teria rap­idly pro­duced com­plex DOM that was sim­il­ar in it’s chem­ic­al com­pos­i­tion to nat­ur­al DOM. The res­ults were sur­pris­ing and indic­ate that bac­teri­al meta­bol­ites are a source of the per­sist­ent molecules in the ocean,” said Dr Ron­ald Ben­ner from the Uni­ver­sity of South Car­o­lina. As a con­sequence, the research­ers got an answer to their ques­tion as to the ori­gin of per­sist­ent DOM: ​“It seems very clear that bac­teria are a major driver in keep­ing a frac­tion of the atmo­spher­ic car­bon diox­ide in the ocean for long peri­ods of time,” said Dr Norbert Her­tkorn of the Helm­holtz Centre Munich. ​“Although the per­cent­age of per­sist­ent sub­stances in our exper­i­ment was appar­ently very low, their sta­bil­ity sug­gests that they may accu­mu­late in the ocean. This is how bac­teria effi­ciently con­trib­ute to car­bon stor­age in the ocean and play an import­ant and so far under­es­tim­ated role for our cli­mate.” UFZ research­er Oliv­er Lechten­feld wants to carry out fur­ther invest­ig­a­tions to find out which chem­ic­al struc­tures and mech­an­isms are respons­ible for bac­teria being unable to break down per­sist­ent sub­stances. In doing so he will extend his focus to include the soil eco­sys­tem: ​“Even less is known about which pro­cesses in the soil are respons­ible for cap­tur­ing car­bon in the form of per­sist­ent organ­ic molecules. That is, how­ever, an import­ant aspect for agri­cul­ture and for the treat­ment of drink­ing water. As for some years, there have been increased DOM con­cen­tra­tions meas­ured in catch­ment areas of water dams,” said Lechten­feld. ​“How cli­mate change with rising aver­age tem­per­at­ures and chan­ging pre­cip­it­a­tion pat­terns is going to affect the com­pos­i­tion of bac­teri­al com­munit­ies in the sea and in soil and their abil­ity to store car­bon, requires fur­ther research efforts.”

Pub­lic­a­tion:

Oliv­er J. Lechten­feld, Norbert Her­tkorn, Yuan Shen, Mat­thi­as Witt & Ron­ald Ben­ner (2015): Mar­ine sequest­ra­tion of car­bon in bac­teri­al meta­bol­ites. Nature Com­mu­nic­a­tions 6:6711, http://​dx​.doi​.org/​1​0​.​1​0​3​8​/​n​c​o​m​m​s7711 Source: idw