Budget of Primary Production and Dinitrogen Fixation in a Highly Seasonal Red Sea Coral Reef
Ecosystems
Budget of Primary Production and Dinitrogen Fixation in a Highly Seasonal Red Sea Coral Reef
Ulisse Cardini 1 2
Vanessa N. Bednarz 2
Nanne van Hoytema 2
Alessio Rovere 0
Malik S. Naumann 2
Mamoon M. D. Al-Rshaidat 4 5
Christian Wild 2 3
0 Sea Level and Coastal Changes Group, Leibniz Center for Tropical Marine Ecology (ZMT) and MARUM - Center for Marine Environmental Sciences and Faculty of Geosciences, University of Bremen , 28334 Bremen , Germany
1 Division of Microbial Ecology, Department of Microbiology and Ecosystem Science, Research Network Chemistry meets Microbiology, University of Vienna , Althanstrasse 14, 1090 Vienna , Austria
2 Coral Reef Ecology Group, Leibniz Center for Tropical Marine Ecology (ZMT) , Fahrenheitstr. 6, 28359 Bremen , Germany
3 Faculty of Biology and Chemistry (FB 2), University of Bremen , 28359 Bremen , Germany
4 Department of Marine Biology, The University of Jordan - Aqaba Branch , Aqaba 77110 , Jordan
5 Laboratory for Molecular Microbial Ecology (LaMME), Marine Science Station , Aqaba 77110 , Jordan
Biological dinitrogen (N2) fixation (diazotrophy, BNF) relieves marine primary producers of nitrogen (N) limitation in a large part of the world oceans. N concentrations are particularly low in tropical regions where coral reefs are located, and N is therefore a key limiting nutrient for these productive ecosystems. In this context, the importance of diazotrophy for reef productivity is still not resolved, with studies up to now lacking organismal and seasonal resolution. Here, we present a budget of gross primary production (GPP) and BNF for a highly seasonal Red Sea fringing reef, based on ecophysiological and benthic cover measurements combined with geospatial analyses. Benthic GPP varied from 215 to 262 mmol C m-2 reef d-1, with hard corals making the largest contribution (41-76%). Diazotrophy was
Diazotrophy; Photosynthesis; Productivity; Nutrient budget; Biogeochemical cycling; Gulf of Aqaba
-
omnipresent in space and time, and benthic BNF
varied from 0.16 to 0.92 mmol N m-2 reef d-1.
Planktonic GPP and BNF rates were respectively
approximately 60- and 20-fold lower than those of
the benthos, emphasizing the importance of the
benthic compartment in reef biogeochemical
cycling. BNF showed higher sensitivity to seasonality
than GPP, implying greater climatic control on reef
BNF. Up to about 20% of net reef primary
production could be supported by BNF during summer,
suggesting a strong biogeochemical coupling
between diazotrophy and the reef carbon cycle.
INTRODUCTION
Nitrogen (N) is a fundamental component of all
living organisms. In particular, N is needed by
primary producers in capturing energy through
photosynthesis and building biomass, leading to a tight
coupling of the N and carbon (C) cycles
(Gruber
and Galloway 2008)
and dictating constraints to the
flexibility of the ecosystem C:N stoichiometry
(Geider and La Roche 2002)
. In unperturbed and
oligotrophic marine systems, primary productivity
is often limited by bioavailable forms of N, which
are scarce due to low atmospheric inputs and N loss
pathways
(Vitousek and Howarth 1991)
. This
limitation is of particular significance in coral reef
ecosystems, as these are among the ecosystems
displaying the highest rates of gross primary
production (GPP), yet experiencing very low ambient
concentrations of dissolved nutrients. Here,
biological dinitrogen (N2) fixation (diazotrophy, BNF)
is thought to play an important role in replenishing
the N pool, maintaining the ecosystem productivity
and its biological storage of C
(D’Elia and Wiebe
1990; Capone 1996; O’Neil and Capone 2008)
.
Since the early work by Wiebe and others (1975)
on BNF in algal reef flats of the Marshall Islands,
many studies have found high rates of BNF
associated with several benthic substrates, ranging from
sediments and cyanobacterial mats to macroalgae
and scleractinian corals
(for reviews, see O’Neil and
Capone 2008; Cardini and others 2014)
. Moreover,
after first evidence of a diazotroph–coral association
by Shashar and others (1994a) and the subsequent
discovery of diazotrophic cyanobacteria in the
tissue of scleractinian corals of the genus Montastraea
(Lesser and others 2004)
, there has been emergent
recognition of the potential contribution of
N2fixing symbioses in coral reefs
(Fiore and others
2010; Cardini and others 2014)
. Although reef
sediments and cyanobacterial mats show high rates
of BNF
(O’Neil and Capone 1989; Capone and
others 1992; Shashar and others 1994b;
CharpyRoubaud and others 2001; Bednarz and others
2015b)
, N2-fixing coral symbioses may also be
responsible for significant inputs of N on the
ecosystem level, particularly in reef habitats with
high live coral coverage.
BNF in coral reef habitats is highly variable
(O’Neil and Capone 2008)
and potentially affected
by global and local anthropogenic disturbances
(Cardini and others 2014). Therefore, it is
increasingly important to quantify B (...truncated)