Macroalgae Decrease Growth and Alter Microbial Community Structure of the Reef-Building Coral, Porites astreoides

Abstract

With the continued and unprecedented decline of coral reefs worldwide, evaluating the factors that contribute to coral demise is of critical importance. As coral cover declines, macroalgae are becoming more common on tropical reefs. Interactions between these macroalgae and corals may alter the coral microbiome, which is thought to play an important role in colony health and survival. Together, such changes in benthic macroalgae and in the coral microbiome may result in a feedback mechanism that contributes to additional coral cover loss. To determine if macroalgae alter the coral microbiome, we conducted a field-based experiment in which the coral Porites astreoides was placed in competition with five species of macroalgae. Macroalgal contact increased variance in the coral-associated microbial community, and two algal species significantly altered microbial community composition. All macroalgae caused the disappearance of a γ-proteobacterium previously hypothesized to be an important mutualist of P. astreoides. Macroalgal contact also triggered: 1) increases or 2) decreases in microbial taxa already present in corals, 3) establishment of new taxa to the coral microbiome, and 4) vectoring and growth of microbial taxa from the macroalgae to the coral. Furthermore, macroalgal competition decreased coral growth rates by an average of 36.8%. Overall, this study found that competition between corals and certain species of macroalgae leads to an altered coral microbiome, providing a potential mechanism by which macroalgae-coral interactions reduce coral health and lead to coral loss on impacted reefs.

Dominance of Endozoicomonas bacteria throughout coral bleaching and mortality suggests structural inflexibility of the Pocillopora verrucosa microbiome

“…Importantly, we did not observe an intrusion and propagation of putative opportunistic or pathogenic bacteria from the surrounding seawater, which harbored bacterial communities that were highly distinct from bacteria associated with P. verrucosa (Figure 4). While the absence of bacterial community changes in coral holobionts counters previous work (Vega Thurber et al., 2009, 2012; Ziegler et al., 2016), recent work from the Red Sea reports on similarly stable bacterial communities in P. verrucosa across sites subject to differential anthropogenic impact (sewage, municipal waste water, and sediment input). Notably, this bacterial community “stability” in P. verrucosa as reported by Ziegler et al.…”

Responses of Coral-Associated Bacterial Communities to Local and Global Stressors

“…Macroalgae are hypothesized to outcompete corals via a variety of mechanisms including alterations to the microbiome (Smith et al, 2006;Morrow et al, 2012b), faster growth rates, shading, allelopathic interactions (Rasher and Hay, 2010), and abrasion and preventing coral recruitment (Jompa and McCook, 2003). For example, macroalgal contact with the coral Porites astreoides caused multiple changes in the coral microbiome including increased dispersion (i.e., beta diversity), disappearance of a potentially mutualistic Gammaproteobacteria, changes in abundance for taxa already present, establishment of new taxa, and growth of algae-associated microbes within the coral (Vega Thurber et al, 2012). Macroalgal contact has also been shown to shift the coral microbiome to become more similar to the macroalgal microbiome (Morrow et al, 2013).…”

“…Moreover, algae may act as reservoirs for coral pathogens (Sweet et al, 2013) and thus enhance disease events. Algae harbor distinctly different microbial communities than corals (Barott et al, 2011;Vega Thurber et al, 2012) and produce more carbon exudate that can stimulate heterotrophic microbial growth in reef waters (Haas et al, 2011). Algae also produce dissolved organic matter (DOM) that is enriched in dissolved neutral sugars (DNS).…”

“…Finally, coral-associated bacteria may support coral resistance to algal growth simulated by overfishing given that resident bacteria defend the coral from microbial invasions (Rypien et al, 2010;Shnit-Orland et al, 2012;Welsh et al, 2016), thus potentially providing protection from algae altering the coral microbiome (Vega Thurber et al, 2012). There are clearly multiple mechanisms that influence these microbial roles and, as demonstrated above, a conflicting base of evidence.…”

Farming behaviour of reef fishes increases the prevalence of coral disease associated microbes and black band disease

“…Moreover, marine benthic microbes are known to be important drivers of environmental change, so an understanding of benthic microbial community structure on coral reefs has the potential to provide forewarning of macro-ecological change [4]. Patterns of microbial communities on coral reefs are strongly influenced by the presence, physiological activity and ecological interactions of corals and algae [5,6]; however, it is largely unknown how fishes mediate these links between microbes and the macrobenthos. There is strong evidence that territorial grazers, particularly territorial damselfishes, play a key role in benthic dynamics on coral reefs [7][8][9].…”