This process is
known as nitrogen fixation, and serves an important ecological function
by channelling environmental nitrogen into the food web. A newly
discovered group of marine nitrogen fixers called diazotrophs work in
symbiosis with other microorganisms called ciliates to fix nitrogen in
the ocean.
The EU-funded 'Evolution of symbiosis between ciliates and
nitrogen-fixing prokaryotes' (SYMNIF) project aimed to investigate the
symbiosis between diazotrophs and their hosts.
Over 100 different samples of ciliates (a type of zooplankton) were
collected from the North Atlantic and tested for the presence of
diazotroph symbionts. Almost half of the ciliates tested harboured a
nitrogen-fixing symbiont.
Tests on five years' worth of cultivated zooplankton showed that the
same diazotroph species was present in many different types of ciliate.
Samples collected in spring and summer showed more symbionts than
during winter, likely due to nitrogen depletion in seawater over
summertime. However, adding nitrogen to growing zooplankton did not
influence the nitrogen fixation rate, showing that the diazotrophs were
not free-living species.
The SYMNIF project shed light on an important group of marine
microorganisms for the first time. Understanding how these symbiotic
microorganisms work together to trap nitrogen will help researchers
better understand global nitrogen flows.