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.
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.
"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.
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."
Karen Krüger, Meghan
Chafee, T. Ben Francis, Tijana Glavina del Rio, Dörte Becher, Thomas Schweder,
Rudolf I. Amann, Hanno Teeling. In marine Bacteroidetes the bulk of glycan
degradation during algae blooms is mediated by few clades using a restricted
set of genes. The ISME Journal, 2019; DOI: 10.1038/s41396-019-0476-y
Posted by Dr. Tim Sandle,
Pharmaceutical Microbiology
