Intergenerational effects of macroalgae on a reef coral: major declines in larval survival but subtle changes in microbiomes

Abstract

Tropical reefs are shifting from coral to macroalgal dominance, with macroalgae suppressing coral recovery, potentially via effects on coral microbiomes. Understanding how macroalgae affect corals and their microbiomes requires comparing algae- versus coral-dominated reefs without confounding aspects of time and geography. We compared survival, settlement, and post-settlement survival of larvae, as well as the microbiomes of larvae and adults, of the Pacific coral Pocillopora damicornis between an Marine Protected Area (MPA) dominated by corals versus an adjacent fished area dominated by macroalgae. Microbiome composition in adult coral, larval coral, and seawater did not differ between the MPA and fished area. However, microbiomes of adult coral were more variable in the fished area and Vibrionaceae bacteria, including strains most closely related to the pathogen Vibrio shilonii, were significantly enriched, but rare, in adult and larval coral from the fished area. Larvae from the macroalgae-dominated area exhibited higher pre-settlement mortality and reduced settlement compared to those from the coral-dominated area. Juveniles planted into a coral-dominated area survived better than those placed into a fished area dominated by macroalgae. Differential survival depended on whether macroalgae were immediately adjacent to juvenile coral rather than on traits of the areas per se. Contrary to our expectations, coral microbiomes were relatively uniform at the community level despite dramatic differences in macroalgal cover between the MPA (~2% cover) and fished (~90%) area. Reducing macroalgae may elicit declines in rare but potentially harmful microbes in coral and their larvae, as well as positive intergenerational effects on offspring survival.

Establishment of Coral-Bacteria Symbioses Reveal Changes in the Core Bacterial Community With Host Ontogeny

“…The presence of diazotroph bacteria across early life stages highlights the possible functional importance of nitrogen fixing bacteria for the coral holobiont (Lema et al, 2014), due to their ability to provide additional source of fixed nitrogen in oligotrophic waters (Lesser et al, 2004, 2007; Olson et al, 2009; Lema et al, 2012, 2014). Early life stages were all characterized by the ubiquitous presence of members of the family Xanthomonadaceae which are known to promote plant growth and suppress pathogenic symbionts (Li et al, 2015) and establish symbiotic relationships with a wide range of organisms such as beetle larvae (Geib et al, 2009; Rizzi et al, 2013) as well as coral (Polson, 2007; Ceh et al, 2012; Godwin et al, 2012; Beatty et al, 2018).…”

Comparison of microbiomes of cold-water corals Primnoa pacifica and Primnoa resedaeformis, with possible link between microbiome composition and host genotype

“…In addition to appearing as core members of the P. resedaeformis microbiome, Vibrionaceae bacteria were recently identified as core members of the microbiome of stony coral Cladocora caespitosa 81 , and appear in the microbiomes of corals worldwide 19 , 21 , 22 , 35 – 37 , 41 , 42 , 45 , 54 , 63 – 65 , 71 , 76 , 82 – 85 . Though Vibrios are often associated with coral disease, their widespread association with healthy corals suggests an important function in the normal functioning of the coral holobiont.…”

Can Herbivore Management Increase the Persistence of Indo-Pacific Coral Reefs?

“…Macroalgae continued to dominate on fished reefs but had become scarce within MPAs (Bonaldo et al, 2017). Multiple lines of evidence indicate that the reduction of the direct and indirect impacts of macroalgae was key to coral recovery within the MPAs, including demonstrated negative effects of macroalgae on coral recruitment, growth, and mortality (Bonaldo and Hay, 2014;Clements and Hay, 2015;Beatty et al, 2018;Clements et al, 2018). The obvious driver of differences in algal assemblages between these MPAs and the nearby fished areas was that herbivorous fish biomass and grazing rates had become 3-6 times higher, and herbivore diversity 2-3 times higher inside the MPAs than in fished area (Rasher et al, 2013;Bonito et al, 2014;Bonaldo et al, 2017).…”