Functional significance of dinitrogen fixation in sustaining coral productivity under oligotrophic conditions.
Functional traits define species by their ecological role in the ecosystem. Animals themselves are host-microbe ecosystems (holobionts), and the application of ecophysiological approaches can help to understand their functioning. In hard coral holobionts, communities of dinitrogen (N2)-fixing prokaryotes (diazotrophs) may contribute a functional trait by providing bioavailable nitrogen (N) that could sustain coral productivity under oligotrophic conditions. This study quantified N2 fixation by diazotrophs associated with four genera of hermatypic corals on a northern Red Sea fringing reef exposed to high seasonality. We found N2 fixation activity to be 5- to 10-fold higher in summer, when inorganic nutrient concentrations were lowest and water temperature and light availability highest. Concurrently, coral gross primary productivity remained stable despite lower Symbiodinium densities and tissue chlorophyll a contents. In contrast, chlorophyll a content per Symbiodinium cell increased from spring to summer, suggesting that algal cells overcame limitation of N, an essential element for chlorophyll synthesis. In fact, N2 fixation was positively correlated with coral productivity in summer, when its contribution was estimated to meet 11% of the Symbiodinium N requirements. These results provide evidence of an important functional role of diazotrophs in sustaining coral productivity when alternative external N sources are scarce.
Coral Reef Ecology Group (CORE), Leibniz Center for Tropical Marine Ecology (ZMT), Fahrenheitstrasse 6, 28359 Bremen, Germany ulisse.cardini@gmail.com.
Ulisse
http://orcid.org/0000-0002-0816-6158
U
Cardini
Coral Reef Ecology Group (CORE), Leibniz Center for Tropical Marine Ecology (ZMT), Fahrenheitstrasse 6, 28359 Bremen, Germany.
Vanessa N
VN
Bednarz
Coral Reef Ecology Group (CORE), Leibniz Center for Tropical Marine Ecology (ZMT), Fahrenheitstrasse 6, 28359 Bremen, Germany.
Malik S
MS
Naumann
Coral Reef Ecology Group (CORE), Leibniz Center for Tropical Marine Ecology (ZMT), Fahrenheitstrasse 6, 28359 Bremen, Germany.
Nanne
N
van Hoytema
Coral Reef Ecology Group (CORE), Leibniz Center for Tropical Marine Ecology (ZMT), Fahrenheitstrasse 6, 28359 Bremen, Germany.
Laura
L
Rix
Department of Ecology, Environment and Plant Sciences, Stockholm University, 10691 Stockholm, Sweden.
Rachel A
RA
Foster
Laboratory for Molecular Microbial Ecology (LaMME), Marine Science Station, Aqaba 77110, Jordan Department of Marine Biology, The University of Jordan-Aqaba Branch, Aqaba 77110, Jordan.
Mamoon M D
MM
Al-Rshaidat
Coral Reef Ecology Group (CORE), Leibniz Center for Tropical Marine Ecology (ZMT), Fahrenheitstrasse 6, 28359 Bremen, Germany Faculty of Biology and Chemistry (FB 2), University of Bremen, 28359 Bremen, Germany.
Christian
C
Wild
D000818:Animals; D037421:Anthozoa / Q000378:metabolism / Q000382:microbiology*; D002734:Chlorophyll / Q000032:analysis; D000077194:Chlorophyll A; D057894:Coral Reefs; D004141:Dinoflagellida / Q000378:metabolism*; D019156:Indian Ocean; D008027:Light; D009584:Nitrogen / Q000378:metabolism; D009586:Nitrogen Fixation; D010788:Photosynthesis / Q000502:physiology; D012621:Seasons; D013559:Symbiosis; D013696:Temperature
D002734:Chlorophyll; D009584:Nitrogen; D000077194:Chlorophyll A
Symbiodinium; diazotrophs; hermatypic corals; nitrogen fixation; organic matter release; photosynthesis