Antifouling coatings can reduce algal growth while preserving coral settlement

Abstract

In the early stages after larval settlement, coral spat can be rapidly overgrown and outcompeted by algae, reducing overall survival for coral reef replenishment and supply for restoration programs. Here we investigated three antifouling (AF) coatings for their ability to inhibit algal fouling on coral settlement plugs, a commonly-used restoration substrate. Plugs were either fully or partially coated with the AF coatings and incubated in mesocosm systems with partial recirculation for 37 days to track fouling succession. In addition, settlement of Acropora tenuis larvae was measured to determine whether AF coatings were a settlement deterrent. Uncoated control plugs became heavily fouled, yielding only 4-8% bare substrate on upper surfaces after 37 days. During this period, an encapsulated dichlorooctylisothiazolinone (DCOIT)-coating was most effective in reducing fouling, yielding 61-63% bare substrate. Antiadhesive and cerium dioxide (CeO_2-x) nanoparticle (NP) coatings were less effective, yielding 11-17% and 2% bare substrate, respectively. Average settlement of A. tenuis larvae on the three types of AF-coated plugs did not statistically differ from settlement on uncoated controls. However, settlement on the NP-coating was generally the highest and was significantly higher than settlement found on the antiadhesive- and DCOIT-coating. Furthermore, on plugs only partially-covered with AF coatings, larval settlement on coated NP- areas was significantly higher than settlement on coated antiadhesive- and DCOIT-areas. These results demonstrate that AF coatings can reduce fouling intensity on biologically-relevant timescales while preserving robust levels of coral settlement. This represents an important step towards reducing fine-scale competition with benthic fouling organisms in coral breeding and propagation.

Figure 1

Uncoated Control (CO; a), partially-coated (PC; d), and fully-coated (FC; g) plugs, cropped images of CO (b), PC (e), and FC (h) DCOIT plugs (after 37 days in the aquaria), and the segmentation of fouling classes on the same CO (c), PC (f), and FC (i) DCOIT plugs, using machine learning-based image classification with the Trainable Weka Segmentation (TWS) in ImageJ. Each fouling class was segmented using a specific color (c,f,i): red (CCA), violet (brown algae), green (green algae), yellow (bare substrate), blue (background).

Figure 2

Fully-coated (FC) plugs (Nanoparticles, Antiadhesive, DCOIT) and uncoated Control plugs (n = 45 plugs per treatment) were incubated in semi-recirculating seawater systems prior to coral settlement trials to examine the mean coverage (%) of fouling classes (CCA, Green/brown algae, Bare substrate) over time (after 9, 23, 37 days). Note that bare substrate coverage at experimental start (day 0) was 100% (Supplementary Table S1).

Figure 3

Partially-coated (PC) plugs (Nanoparticles, Antiadhesive, DCOIT) with coated and uncoated areas (n = 45 plugs per treatment) were incubated in semi-recirculating seawater systems prior to coral settlement trials to examine the mean coverage (%) of fouling classes (CCA, Green/brown algae, Bare substrate) over time (after 9, 23,37 days). Note that bare substrate coverage at experimental start (day 0) was 100% (Supplementary Table S5).

Figure 4

Pre-conditioned fully-coated (FC) plugs (Nanoparticles, Antiadhesive, DCOIT) and uncoated Control plugs (n = 30 plugs per treatment) were tested for mean coral larval settlement (settlers per cm²) in individual glass jars (one plug with 15 larvae per jar). Error bars represent SEM. Asterisks indicate statistically significant differences based on pairwise post-hoc tests with least-squares means (Supplementary Tables S9 and S11; *p < 0.05).

Figure 5

Pre-conditioned partially-coated (PC) plugs (Nanoparticles, Antiadhesive, DCOIT) with coated and uncoated areas (n = 30 plugs per treatment) were tested for mean coral larval settlement (settlers per cm²) in individual glass jars (one plug with 15 larvae per jar). Error bars represent SEM. Asterisks indicate statistically significant differences based on pairwise post-hoc tests with estimated marginal means (Supplementary Tables S12 and S14; *p < 0.05, **p < 0.01, ***p < 0.001). Images (right) show coral settlers (circled in magenta) on a PC DCOIT plug.