Hel­go­land Roads

Spring blooms at Hel­go­land

Blooms of mar­ine al­gae are a global phe­nomenon, in par­tic­u­lar in nu­tri­ent-rich coastal areas dur­ing spring. After winter with in­creas­ing sun­light, di­at­oms and other uni- to pluri­cel­lu­lar mi­croal­gae start to pro­lif­er­ate. Since usu­ally only a few pred­at­ors are present dur­ing these times, the al­gae can pro­lif­er­ate very fast. The res­ult­ing algal blooms can cover such large areas that they are of­ten vis­ible via satel­lite im­agery from space. It is es­tim­ated that mar­ine algal growth is re­spons­ible for about half of the an­nual pho­to­syn­thetic car­bon di­ox­ide fix­a­tion, and thus al­gae play a cru­cial role in global car­bon cyc­ling.

In Ger­many, the is­land Hel­go­land is among the most well-suited sites to study al­gae blooms. Hel­go­land is loc­ated in the Ger­man Bight (south­ern North Sea) and is Ger­many’s only off-shore is­land. To­gether with our col­lab­or­a­tion part­ners from the Uni­versity of Gre­if­swald and the Al­fred We­gener In­sti­tute in Bremer­haven, we have been study­ing spring blooms in­tens­ively since 2009 (Teeling, Fuchs et al., 2009).

While al­gae blooms tend to dif­fer from year to year, there are re­cur­rent pat­terns. At first, nu­tri­ents are aplenty and al­gae can go into al­most un­res­tric­ted growth. After a while, algal growth ceases be­cause nu­tri­ents be­come lim­it­ing, graz­ing by prot­ists (uni­cel­lu­lar an­im­als) in­creases, and also vir­uses at­tack the al­gae. Ul­ti­mately, the blooms col­lapse.

Dur­ing al­gae blooms and in par­tic­u­lar, dur­ing the ter­minal bloom phases, large amounts of algal or­ganic com­pounds are re­leased into the wa­ter and thus be­come avail­able to free-liv­ing mar­ine bac­teria (“bac­terioplank­ton”). Among the ma­jor classes of algal-de­rived sub­strates be­sides pro­teins and lip­ids are poly­sac­char­ides. The reason for this is that al­gae use dif­fer­ently com­posed poly­sac­char­ides for vari­ous pur­poses, i.e. as cell wall con­stitu­ents, as ex­ter­ior slimes (what makes macro-al­gae feel slimy), as sta­bil­iz­ing mat­rix com­pon­ents, or for en­ergy stor­age.

We have ana­lyzed the bac­terial re­sponse to al­gae blooms in great de­tail. In a data-rich study we could show that cer­tain clades of re­spond­ing bac­teria are an­nu­ally re­cur­ring (Teeling, Fuchs et al., 2016). In par­tic­u­lar cer­tain clades of the Bacteroidetes, such as mem­bers of the genus Polaribacter (Avcı et al., 2020), were al­ways present in large num­bers.

Poly­sac­char­ide util­iz­a­tion loci

A sur­vey of col­lect­ive gen­omes (“meta­gen­omes”) of bloom-as­so­ci­ated bac­teria re­vealed a pre­val­ence of genes for the de­com­pos­i­tion of algal poly­sac­char­ides in many of the abund­ant Bacteroidetes clades. These genes code for so-called car­bo­hydrate-act­ive en­zymes (“CAZymes”), and they are usu­ally clustered in poly­sac­char­ide util­iz­a­tion loci (PULs) of some­times only a few and some­times dozens of genes. Typ­ic­ally, a PUL has the genes for the cel­lu­lar ma­chinery to de­com­pose one spe­cific type of poly­sac­char­ide. Since al­gae pro­duce a large num­ber of dif­fer­ent poly­sac­char­ides, no bac­terium has the genes to de­com­pose them all. In­stead, dif­fer­ent bac­teria spe­cial­ize on the de­com­pos­i­tion of dif­fer­ent poly­sac­char­ide types. Thus, the bac­terial de­com­pos­i­tion of algal bio­mass dur­ing and after al­gae blooms is an achieve­ment of an en­tire com­munity of spe­cial­ized bac­teria.

The com­pos­i­tion of PULs can provide hints on the ap­prox­im­ate type of poly­sac­char­ide that a given bac­terium can de­com­pose. In or­der to study PUL di­versity within North Sea Bacteroidetes, we se­quenced 53 spe­cies that were isol­ated from the North Sea in the group of Prof. Dr. Jens Harder (Kappelmann et al., 2016). More re­cently, we ana­lyzed 38 meta­gen­omes that were ob­tained at dif­fer­ent times dur­ing spring phyto­plank­ton blooms in 2010 to 2012 and could show that the bulk of algal poly­sac­char­ides is likely de­com­posed by about a dozen abund­ant and re­cur­rent Bacteroidetes clades (Krüger et al., 2019). Our cur­rent fo­cus is to study these clades via isol­ated rep­res­ent­at­ives and via high time-res­ol­u­tion onsite activ­ity meas­ure­ments (e.g. meta­ge­n­om­ics & meta­tran­scrip­tom­ics).

Fund­ing

This re­search is part of the POMPU (Pro­teo­ge­n­om­ics of Mar­ine Poly­sac­char­ide Util­iz­a­tion) pro­ject fun­ded by the DFG (Deutsche Forschungs­ge­meinsch­aft; pro­ject FOR2406). It be­longs to POMPU subproject B1 that is sup­por­ted by DFG grants to Dr. Hanno Teeling (TE 813/​2-1) and to Prof. Dr. Rudolf I. Amann (AM 73/​9-1).

Cur­rent Pro­ject Mem­bers