Seaweed-coral competition in the field: effects on coral growth, photosynthesis and microbiomes require direct contact

Abstract

A number of tropical reefs have transitioned from coral to macroalgal dominance, but the role of macroalgal competition in coral decline is debated. There is a need to understand the relative roles of direct coral-algal effects versus indirect, microbially mediated effects shaping these interactions, as well as the relevant scales at which interactions operate under natural field, as opposed to laboratory, conditions. We conducted a manipulative field experiment investigating how direct contact versus close proximity (approx. 1.5 cm) with macroalgae (Galaxaura rugosa, Sargassum polycystum) impacted the growth, photosynthetic efficiency, and prokaryotic microbiome of the common Indo-Pacific coral Acropora millepora. Both coral growth and photosynthetic efficiency were suppressed when in direct contact with algae or their inert mimics--but not when in close proximity to corals without direct contact. Coral microbiomes were largely unaltered in composition, variability, or diversity regardless of treatment, although a few uncommon taxa differed in abundance among treatments. Negative impacts of macroalgae were contact dependent, accounted for by physical structure alone and had minimal effects on coral microbiomes. The spatial constraints of these interactions have important implications for understanding and predicting benthic community dynamics as reefs degrade.

Keywords: Fiji; coral reef; coral-algal competition; macroalgae.

Figure 1.

Experimental design and outcomes. (a) Coral replicates showing the bottle neck/cap and rope methods used for planting corals and applying the algal treatments: control coral lacking exposure to macroalgae or algal mimics, direct contact with four thalli of Galaxaura rugosa, close proximity to four thalli of Galaxaura, direct contact with four Galaxaura mimics, close proximity to four Galaxaura mimics, direct contact with four Sargassum polycystum thalli, close proximity to four Sargassum thalli, direct contact with four Sargassum mimics, and indirect exposure to four Sargassum mimics. (b) Coral growth (mean ± s.e.) at 33 days, and (c) ‘overall’ and (d) ‘worst-case’ photosynthetic efficiency (ϕ_PSII) at 32 days of control corals and those in direct contact or close proximity with Galaxaura rugosa or Galaxaura mimics. (e) Coral growth (mean ± s.e.) at 33 days, and (f) ‘overall’ and (g) ‘worst-case’ photosynthetic efficiency (ϕ_PSII) at 32 days of control corals and those in direct contact or close proximity with Sargassum polycystum or Sargassum mimics. Letters indicate significant groupings (p < 0.05) via Tukey contrasts. Dots show values for individual data points. (Online version in colour.)

Figure 2.

(a) Microbiome composition (β-diversity) and variability (β-dispersion) and (b,c) diversity of samples from control corals and those in direct contact or close proximity with Galaxaura rugosa or Galaxaura mimics. (d) Microbiome composition (β-diversity) and variability (β-dispersion) and (e,f) diversity of samples from control corals and those in direct contact or close proximity with Sargassum polycystum or Sargassum mimics. p-values from analyses of microbiome composition (PERMANOVA), variability (PERMDISP), and diversity (LME) are listed. (Online version in colour.)