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Sim­pler than ex­pec­ted: A mi­cro­bial com­munity with re­duced di­versity cleans up after algal blooms

Jul 26, 2019
Bac­terial de­grad­a­tion of algal blooms seems sur­pris­ingly simple
Bre­men, Ger­many

Algae blooms regularly make for pretty, swirly satellite photos of lakes and oceans. They also make the news occasionally for poisoning fish, people and other animals. What's less frequently discussed is the outsize role they play in global carbon cycling. A recent study now reveals surprising facts about carbon flow in phytoplankton blooms. Unexpectedly few bacterial clades with a restricted set of genes are responsible for a major part of the degradation of algal sugars.

 

Heligoland is Germany’s only true offshore island, famous for its seabirds, seals and duty-free shopping rather than for microscopic algae. But what the MPI-scientists were interested in was the fate of the organic matter once the algae die. (© Max Planck Institute for Marine Microbiology, Naomi Esken)
Heligoland is Germany’s only true offshore island, famous for its seabirds, seals and duty-free shopping rather than for microscopic algae. But what the MPI-scientists were interested in was the fate of the organic matter once the algae die. (© Max Planck Institute for Marine Microbiology, Naomi Esken)

Al­gae take up car­bon di­ox­ide (CO2) from the at­mo­sphere and turn the car­bon into bio­mass while re­leas­ing the oxy­gen back to the at­mo­sphere. Fast algal growth dur­ing phyto­plank­ton blooms leads to a massive trans­fer of car­bon di­ox­ide into algal bio­mass. But what hap­pens to the car­bon next?

“Once the al­gae die, the car­bon is re­min­er­al­ized by mi­croor­gan­isms con­sum­ing their bio­mass. It is thus re­turned to the at­mo­sphere as car­bon di­ox­ide. Al­tern­at­ively, if the dead al­gae sink to the sea­floor, the or­ganic mat­ter is bur­ied in the sed­i­ment, po­ten­tially for a very long time”, ex­plains first au­thor Karen Krüger from the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy in Bre­men. “The pro­cesses be­hind the re­min­er­al­iz­a­tion of algal car­bon are still not fully un­der­stood.”

Thus, Krüger and her col­leagues in­vest­ig­ated mi­croor­gan­isms dur­ing spring algal blooms in the south­ern North Sea, at the is­land of Heligo­land. They spe­cific­ally looked at the bac­terial use of poly­sac­char­ides – sug­ars that make up a sub­stan­tial frac­tion of the algal bio­mass. To­gether with col­leagues from the Max Planck In­sti­tute, the Uni­versity of Gre­if­swald and the DOE Joint Gen­ome In­sti­tute in Cali­for­nia, Krüger car­ried out a tar­geted meta­ge­n­omic ana­lysis of the Bacteroidetes phylum of bac­teria, since these are known to con­sume lots of poly­sac­char­ides. In de­tail, the sci­ent­ists looked at gene clusters called poly­sac­char­ide util­isa­tion loci (PULs), which have been found to be spe­cific to a par­tic­u­lar poly­sac­char­ide sub­strate. If a bac­terium con­tains a spe­cific PUL, that in­dic­ates it feeds on the cor­res­pond­ing algal sugar.

Low PUL di­versity

“Con­trary to what we ex­pec­ted, the di­versity of im­port­ant PULs was re­l­at­ively low”, says Krüger. Only five ma­jor poly­sac­char­ide classes were be­ing reg­u­larly tar­geted by mul­tiple spe­cies of bac­teria, namely beta-glu­cans (such as lam­in­arin, the main di­atom stor­age com­pound), al­pha-glu­cans (such as starch and gly­co­gen, also algal and bac­terial stor­age com­pounds), man­nans and xy­lans (typ­ic­ally algal cell wall com­pon­ents), and al­gin­ates (mostly known as slimy stuff pro­duced by brown mac­roal­gae). Of these five sub­strates, only two (al­pha- and beta-glu­cans) make up the ma­jor­ity of sub­strates avail­able to the bac­teria dur­ing a phyto­plank­ton bloom. This im­plies that the most im­port­ant poly­sac­char­ide sub­strates re­leased by dy­ing al­gae are made up of a fairly small set of ba­sic com­pon­ents.

“Given what we know of algal and bac­terial spe­cies di­versity, and the enorm­ous po­ten­tial com­plex­ity of poly­sac­char­ides, it came as no small sur­prise to see such a lim­ited spec­trum of PULs, and in only a re­l­at­ively small num­ber bac­terial clades”, co-au­thor Ben Fran­cis from the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy sums up in an accompanying comment. “This was es­pe­cially un­ex­pec­ted be­cause pre­vi­ous stud­ies sug­ges­ted something dif­fer­ent. An analysis of more than 50 bacterial isolates – i.e. bac­teria that can be cul­tured in the lab – that our work­ing group car­ried out in the same sampling re­gion re­vealed a much broader di­versity of PULs”, he adds.

 

 

A microbial system with unexpectedly low complexity?

Ac­com­pa­ny­ing com­ment by Thomas Ben Fran­cis (Nature Mi­cro­bi­o­logy Com­munity)

Tem­poral suc­ces­sion of poly­sac­char­ide de­grad­a­tion

Dur­ing the course of the algal bloom, the sci­ent­ists ob­served a dis­tinct pat­tern: In early bloom stages, fewer and sim­pler poly­sac­char­ides dom­in­ated, while more com­plex poly­sac­char­ides be­came avail­able as the bloom pro­gressed. This might be caused by two factors, Fran­cis ex­plains: “First, bac­teria will in gen­eral prefer eas­ily de­grad­able sub­strates such as simple stor­age glycans over bio­chem­ic­ally more de­mand­ing ones. Second, more com­plex poly­sac­char­ides be­come in­creas­ingly avail­able over a blooms’ course, when more and more al­gae die.”

This study provides un­pre­ced­en­ted in­sights into the dy­nam­ics of a phyto­plank­ton bloom and its prot­ag­on­ists. A fun­da­mental un­der­stand­ing of the bulk of glycan-me­di­ated car­bon flow dur­ing phyto­plank­ton bloom events is now within reach. “Next, we want to dig deeper into pro­cesses un­der­ly­ing the ob­served dy­nam­ics”, says Krüger. “Moreover, it will be in­ter­est­ing to in­vest­ig­ate poly­sac­char­ide de­grad­a­tion in hab­it­ats with other car­bon sources, such as the Arc­tic Seas or the sed­i­ment.”

Ori­ginal pub­lic­a­tion:

Par­ti­cip­at­ing in­sti­tu­tions:

Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy, 28359 Bre­men, Ger­many

DOE Joint Gen­ome In­sti­tute, Wal­nut Creek, CA 94598, USA

Uni­versity of Gre­if­swald, 17489 Gre­if­swald, Ger­many

Please dir­ect your quer­ies to:

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Department of Molecular Ecology

Prof. Dr. Rudolf Amann

MPI for Marine Microbiology
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Phone: 

+49 421 2028-9300

Prof. Dr. Rudolf Amann

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Dr. Fanni Aspetsberger

MPI for Marine Microbiology
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D-28359 Bremen
Germany

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+49 421 2028-9470

Dr. Fanni Aspetsberger
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