04.05.2012 Algal blooms in the North Sea

To most people, algal blooms are an an­noy­ance, which in­ter­feres with their sum­mer days by the sea. In the coastal zone of tem­per­ate re­gions a spring algal bloom is not a sign of ex­cess­ive nu­tri­ent in­put, but most of all a con­sequence of the more in­tense solar ir­ra­di­ation in spring (Fig­ure 1). Hence, spring algal blooms in these wa­ters are a nat­ural phe­nomenon (Fig­ure 2). When algal blooms end the al­gae die and their rem­nants con­sti­tute an im­port­ant nu­tri­ent sup­ply for the whole eco­sys­tem. This pro­cess is es­sen­tial e.g. for the off­shore abund­ance of fish. But what ex­actly hap­pens if an algal bloom ends? Hanno Teel­ing and Bernhard Fuchs and their col­leagues from the Max Planck In­sti­tute in Bre­men provide a sur­pris­ing and very de­tailed an­swer, along with their coau­thors from the Al­fred We­gener In­sti­tute for Mar­ine and Po­lar Re­search and the Uni­versity of Gre­if­swald. They ex­amined an algal bloom in the North Sea and were able to identify the role that the mi­croor­gan­isms play in the de­grad­a­tion of algal rem­nants and to gain in­sight into the de­grad­a­tion pro­cesses. And they dis­covered that some of these pro­cesses pro­ceed dif­fer­ently than hitherto as­sumed (Fig­ure 3). Their res­ults were just pub­lished in the journal Sci­ence.

For their ana­lyses the sci­ent­ists fil­trated sev­eral hun­dreds of liters of sea­wa­ter on a reg­u­lar basis for al­most a year off the sta­tion ”Ka­beltonne”, a long-term sta­tion of the Bio­lo­gis­che An­stalt Hel­go­land that is part of the Al­fred We­gener In­sti­tute. Hanno Teel­ing from the Max Planck In­sti­tute says: “Pela­gic mi­croor­gan­isms, the so called bac­terioplank­ton, are crit­ical for the break­down of the dead algal bio­mass. Es­pe­cially the dy­namic suc­ces­sion in the bac­terioplank­ton caught our at­ten­tion. Spe­cial­ized bac­terial pop­u­la­tions ac­com­pany dif­fer­ent phases of the algal bloom”. Pro­cesses within the bac­terial pop­u­la­tion con­trol the de­grad­a­tion of the al­gae, as the sci­ent­ists could show.

His col­league Bernhard Fuchs who has been in­vest­ig­at­ing the di­versity and bac­terioplank­ton com­pos­i­tion for many years at the Max Planck In­sti­tute, adds: “For the first time we per­formed a high res­ol­u­tion ana­lysis of the mi­cro­bial com­munity at genus level. We could not only identify the bac­terial groups but also their func­tional tools, the en­zymes, that are in­volved in the break­down of the algal bloom”.

The sci­ent­ists used a novel com­bin­a­tion of tech­niques for their ana­lyses. They de­term­ined the iden­tity of the mi­croor­gan­isms by CARD-FISH, an in situ tech­no­logy that can be ap­plied dir­ectly to en­vir­on­mental samples (Fig­ure 4). Ad­di­tion­ally, they probed the bac­terial pop­u­la­tion dur­ing and after the algal bloom by short se­quences of a phylo­gen­etic marker gene (16S rRNA pyrotag ana­lyses). “By us­ing a com­bin­a­tion of meta­gen­ome and meta­pro­teome ana­lyses we suc­ceeded to de­tect the act­ive key en­zymes in com­plex en­vir­on­mental samples. This al­lows us to in­fer the role of the re­spect­ive bac­terial groups from their meta­bolic func­tion”, ex­plains Thomas Schweder from the Uni­versity of Gre­if­swald. “This was only pos­sibly by the com­puter-con­trolled in­teg­ra­tion of all data. For that task we used bioin­form­at­ics ”, as Frank Oliver Glöck­ner from the Max Planck In­sti­tute states. In the early phase of the algal bloom the sci­ent­ists en­countered a vari­ety of en­zymes for the de­grad­a­tion of com­plex algal car­bo­hydrates such as lam­in­arin. At a later stage trans­port pro­teins for pep­tides, short pro­tein units, and trans­port­ers for the growth lim­it­ing nu­tri­ent phos­phate and simple sugar com­pon­ents dom­in­ated the en­zymatic cock­tail. Note­worthy was the high por­tion of cer­tain trans­port pro­teins, the TonB-de­pend­ent trans­port­ers that can trans­port lar­ger mo­lecules dir­ectly into the in­terior of the cells. This dis­cov­ery may dis­prove the con­ven­tional ac­cept­ance that long-chained mo­lecules need to be broken up into smal­ler com­pon­ents be­fore the cell can take them up. The TonB-trans­porter may en­able the Fla­vobac­teria, one of the dom­in­at­ing bac­terial groups, to couple the as­sim­il­a­tion and de­grad­a­tion and thus to gain a com­pet­it­ive ad­vance to­wards other bac­terial groups. At the end of the bloom the bac­teria in­creas­ingly pro­duced sulfatases that cleave sulfate es­ters from al­gae car­bo­hydrates hard to de­com­pose and thus al­low the com­plete de­grad­a­tion of these sub­stances. Hence, the sci­ent­ists dis­covered a bac­terial pop­u­la­tion in the al­gae bloom that did not only dif­fer in its com­pos­i­tion but also in its func­tion from the bac­terial com­munity found in crys­tal clear, re­mote open wa­ters.