The success of scleractinian corals in nutrient-poor waters surrounding coral reefs is due to their symbiosis with unicellular dinoflagellates belonging to the Symbiodiniaceae family [
1]. These symbionts translocate a large fraction of photosynthetically fixed carbon (C) and amino acids to the host for its nutritional needs [
2]. Coral reefs are experiencing increasingly frequent and devastating bleaching events [
3]. During bleaching, corals expel millions of their symbionts and/or suffer a loss of photosynthetic pigments, depriving the host from its main nutrition source [
4,
5]. This process leads to coral death and to the decline of coral reefs if bleached corals are not able to rapidly recover their symbionts [
3]. Bleaching susceptibility varies among species, depths, and locations (e.g., [
6,
7,
8,
9]), and is also influenced by coral morphology [
10,
11], physiological responses of both animal and symbionts [
12,
13], and symbiont types hosted by corals (e.g., [
14,
15,
16]).
Another mechanism used to resist and recover from bleaching consists in exploiting food sources other than autotrophy. Indeed, corals are voracious predators that can feed on a wide range of prey ranging from pico-nanoplanktonic cells (with a size < 20 µm; [
17]) to macrozooplankton [
18,
19]. Multiple laboratory experiments have shown that heterotrophy increases skeletal and tissue growth [
17,
20,
21,
22,
23], allows corals to build up energy reserves [
24,
25], increases fertility [
26], and reduces sensitivity to acidification [
27,
28,
29]. Coral ability to switch from an autotrophic to a heterotrophic diet by increasing their feeding rates on zooplankton [
22,
30,
31], making them more resistant to bleaching [
30,
32,
33,
34], has been much less studied. Only two studies have shown that corals increase their consumption of pico-nanoplankton during heat stress [
35,
36]. Among these small preys, dinitrogen (N
2)-fixing prokaryotes (subsequently referred to as planktonic diazotrophs, hereafter called ‘PD’) have received little attention. Many coral reef ecosystems are characterized by high planktonic diazotroph abundance and activity; they are very widespread in the Western South Pacific (e.g., New Caledonia, Papua New Guinea, and the Australian Great Barrier Reef) [
37,
38,
39,
40] but also in Hawaii, in the Caribbean, and the Red Sea [
41,
42,
43]. In New Caledonia, planktonic diazotrophs support ~80% of the primary production of other phytoplankton in summer [
44,
45]. The lagoon is dominated by picoplankton (non-diazotrophic, i.e.,
Synechococcus and
Prochlorococcus) but the diazotrophs, even if they are less abundant, are much larger (nanoplankton and microplankton size) and thus represent an important part of the total biomass [
46]. Planktonic diazotrophs reduce atmospheric N
2 into bioavailable ammonium (NH
4+), and release part of the recently fixed nitrogen (Diazotroph-Derived Nitrogen, DDN) in seawater, providing sufficient nitrogen stocks for the development of the planktonic food web in oligotrophic waters [
47]. These planktonic diazotroph cells are thus, by definition, very rich in nitrogen [
48,
49]. Their ingestion by scleractinian corals was first demonstrated by [
50] using the
15N
2 labelling method. Direct consumption of planktonic diazotrophs would provide 0.76 ± 0.15 μg N cm
−2 h
−1 to corals, which corresponds to six times the daily N supply by non-diazotrophic pico- and nanoplankton [
17]. Bleached colonies of
S. pistillata incorporate more PD than healthy colonies [
36], but the consequences on coral metabolism are not known. By providing an alternative source of bioavailable N and C, this increased incorporation of PD may have a profound influence on coral bleaching recovery.
To improve our understanding of the physiological response of corals to heat stress in anticipation of more frequent and severe coral bleaching events, we investigated if and how nutrition on PD is involved in the resistance to high temperature of the coral species S. pistillata. We evaluated key physiological measurements (photosynthetic efficiency, growth rates) and tissue parameters (chlorophyll, protein concentrations, and symbiont densities) of colonies of S. pistillata, fed or not with PD during heat stress.