Beneficial and detrimental fungi within the culturable mycobiome of the Red Sea coral Stylophora pistilatta

Abstract

The presence of fungi in the coral microbiome is increasingly recognized, yet their potential impact on the holobiont's health, particularly under stress conditions, remains underexplored. To address this gap, we isolated over 200 strains (predominantly Ascomycota) from the common scleractinian Red Sea coral, Stylophora pistillata. Using conidia from a rare (Stachybotrys chlorohalonata) and a common (Cladosporium halotolerans) fungal symbiont, we investigated their effects on coral fragments maintained at ambient (25°C) and elevated (33°C) sea temperatures. Inoculation with S. chlorohalonata resulted in significant tissue loss, across both water temperature treatments. Conversely, inoculation with C. halotolerans did not result in visible effects at ambient temperature, but mitigated tissue loss at elevated temperature. This protective effect was accompanied by reduced expression of stress-induced peroxiredoxin-6 and Rad51 host genes, yet not that of Hsp70. Additionally, potential algal symbiont photosynthetic efficiency was higher by over 25% in the elevated temperature treatment, concurrent with higher bacterial diversity, including a marked reduction (>3-fold) in the proliferation of Vibrionaceae in the C. halotolerans-treated coral nubbins. These findings reveal the contrasting impacts of fungal symbionts on coral health, highlighting the dual roles of the mycobiome in influencing holobiont resilience under environmental stress.

Keywords: Cladosporium; Stachybotrys; Stylophora; coral mycobiome; marine fungi; ocean warming.

Figure 1

Relative abundance of cultured members of fungal families, grouped by phylum, isolated from Stylophora pistillata based on internal transcribed spacer sequences. The break in the x-axis between 80% and 84% was introduced to improve visualization of the less abundant taxa.

Figure 2

Effect of Cladosporium halotolerans and Stachybotrys chlorohalonata on Stylophora pistillata under thermal stress: (a) Nubbins of S. pistillata were inoculated with conidia of S. chlorohalonata or C. halotolerans (top row; for details, see experimental design fig. S1). Maintaining the nubbins at 25°C or 33°C resulted in adverse effects, ranging from tissue loss in nubbins at the high temperature and in the S. chlorohalonata-inoculated coral (regardless of temperature) to reduced tissue loss in nubbins inoculated with C. halotolerans (middle row). C. halotolerans conidia adhered and germinated on the coral surface (bottom row). Scanning electron micrograph (SEM) images of S. pistillata inoculated with C. halotolerans. The left panel (×60 magnification) shows an overview of the coral surface with fungal colonization, while the right panel (×2000 magnification) highlights conidia germination. SEM images were captured using a JEOL microscope (Vacc = 2.00 kV, detector = LED, WD = 10.0 mm). Scale bars: left panel = 100 μm, right panel = 10 μm. (b) Relative expression of three stress-related genes in S. pistillata as affected by temperature and inoculation with C. halotolerans. Statistical analysis was performed at each time point using two-way ANOVA, Tukey HSD post hoc test (* ≤ .05, ** ≤ .01 *** ≤ .001, n = 4 for peroxiredoxin-6 and RAD51, n = 3 for HSP70). Error bars represent standard deviation. (c) Mean maximum quantum yield of PSII (Fv/Fm) in S. pistillata coral fragments as affected by temperature and inoculation with C. halotolerans. Statistical analysis was performed at each time point using two-way ANOVA, Tukey HSD post hoc test (* < 0.05, ** < 0.01, n = 5). Comparisons represent differences between treatments within each temperature. Error bars represent standard deviation. (d) Principal coordinate analysis of microbial community composition based on Bray–Curtis distance matrices, visualizing differences of bacterial community structure between treatments and time points. The ordination illustrates the clustering of samples according to their microbiome profiles under the different temperature/fungal inoculation regimes. Results are based on n = 3 or n = 4 biological replicates per treatment. (e) Relative abundance of microbial communities at the order level across treatments. Orders representing <1.5% of the total abundance, or those without taxonomic classification at the order level, are grouped as “other.” Excluding the 0 h control, all samples were collected after 96 h.