Publication Date:
2019-01-16
Description:
Benthic cyanobacterial mats (BCMs) have
increased in abundance on coral reefs worldwide. However,
their species diversity and role in nitrogen fixation are
poorly understood. We assessed the cyanobacterial diversity
of BCMs at four coral reef sites in Curac¸ao, Southern
Caribbean. In addition, nitrogen fixation rates of six common
mats were measured. Microscopic examinations
showed 22 cyanobacterial species, all from the order Oscillatoriales.
Species diversity was similar among sites
despite differences in overall BCM abundance. Dominant
mats were primarily composed of Hydrocoleum glutinosum,
Oscillatoria bonnemaisonii or Lyngbya majuscula.
However, some mats exhibited highly variable species
composition despite consistent macroscopic appearance.
16S rRNA-based phylogeny revealed similar species as
those identified by microscopy, with additional sequences
of unicellular (Xenococcus and Chroococcidiopsis) and
heterocystous (Rivularia and Calothrix) cyanobacteria.
Vice versa, morphotypes of Tychonema, Schizothrix and
Dichothrix were found by microscopy only. The detection
of similar species at the same sites in a study conducted
40 years ago indicates that changes in environmental
conditions over these years may have favored indigenous
species to bloom, rather than facilitated the introduction
and proliferation of invasive species. Nitrogen fixation
rates of mats were 3–10 times higher in the light than in the
dark. The highest areal nitrogen fixation rate
(169.1 mg N m-2 d-1) was recorded in the cyanobacterial
patch dominated by O. bonnemaisonii. A scale-up of
nitrogen fixation at a site with 26% BCM cover at 7 m
depth yielded an aerial rate of 13 mg N m-2 reef d-1,
which exceeds rates reported in open ocean blooms of
Trichodesmium in the Caribbean. Our results suggest that
the Caribbean basin is not only a hotspot for planktonic
nitrogen fixation, but also for benthic nitrogen fixation.
Because BCMs fix vast amounts of nitrogen, their proliferation
will strongly alter the nitrogen budget of coral
reefs.
Repository Name:
EPIC Alfred Wegener Institut
Type:
Article
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isiRev
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