Physiological responses of scleractinian corals in marginal habitat

Polapa FS, Werorilangi S, Ali SM, Jompa J. 2021. Physiological responses of scleractinian corals in marginal habitat. Biodiversitas 22: 4011-4018. This study aims to analyze physiological differences in corals in marginal habitats. Under different conditions, the production/respiration (P/R) ratio and photobiology of various coral genera were compared. Samples were taken from three coral reef zones representing typical reef habitats and from the mangrove ecosystem as a marginal habitat. Surveys revealed two coral genera surviving in extreme conditions (marginal habitat). The P/R ratio measurements indicated that corals living in the mangrove ecosystem tend to be heterotrophic. This was supported by observations of colonies with tentacles extended from the corallites. Furthermore, Porites living in the mangrove habitat consume more O2 directly than saving it for other purposes, such as growth. The genus Dipsastraea exhibited elevated zooxanthellae density in the mangrove ecosystem, whereas Porites exhibited similar densities in both ecosystems.

Reef environments shape microbial partners in a highly connected coral population

ABSTRACTCorals from more thermally variable environments often fare better under thermal stress compared to those from less thermally variable environments, an important finding given that ocean warming threatens corals worldwide. Evidence is mounting that thermal tolerance can be attributed to the coral itself, as well as microbial communities present within the holobiont (coral host and its associated microorganisms). However, few studies have characterized how thermally variable environments structure multiple holobiont members in situ. Here, using 2b-RAD sequencing of the coral and metabarcoding of algal (ITS2) and bacterial (16S) communities, we show evidence that reef zones (locales differing in proximity to shore, physical characteristics, and environmental variability) structure algal and bacterial communities at different scales within a highly connected coral population (Acropora hyacinthus) in French Polynesia. Fore reef (more stable) algal communities were on average more diverse than the back reef (more variable), suggesting that variability constrains algal diversity. In contrast, microbial communities were structured on smaller scales with site-specific indicator species and enriched functions across reef zones. Our results illuminate how associations with unique microbial communities can depend on spatial scale across highly dispersive coral populations, which may have fitness consequences in thermally divergent regions and rapidly changing oceans.

Boring worms (Sipuncula and Annelida: Polychaeta): their early impact on Eastern Tropical Pacific coral reefs

The form, condition, and survival of coral reefs depends on the balance between construction and destruction. Natural processes such as bioerosion can cause this balance to lean towards destruction, threatening these ecosystems. Polychaetes and sipunculids are members of the boring community; however, knowledge of their identity and role in the bioerosive process and their capacity to remove calcium carbonate (CaCO3) in the coral reefs of the Eastern Tropical Pacific (ETP) is scarce. To tackle this problem, 5 experimental units of Pocillopora spp. branches were deployed in 4 reef zones (back-reef, reef-flat, reef-front, reef-slope) at 2 reefs (La Azufrada, Playa Blanca) for 2 time periods (P1: 6 mo, P2: 9 mo; n = 80) in Gorgona National Natural Park, Colombia. All worms (polychaetes and sipunculids) were identified and net removal and bioerosion rate were determined. In total, 137 worms were found: 64.2% in La Azufrada and 35.8% in Playa Blanca. There were no significant effects of reef, reef-zone, or duration of exposure (6 vs. 9 mo) for either net removal of CaCO3 or bioerosion rate. Irrespective of reef or exposure duration, average net removal was 0.022 and 0.027 g during P1 and P2, and 0.032 and 0.018 g at La Azufrada and Playa Blanca, respectively. Average bioerosion rate, also irrespective of reef or exposure duration, was 2.553 and 2.011 g kg-1 yr-1 for P1 and P2, and 2.839 and 1.807 g kg-1 yr-1 at La Azufrada and Playa Blanca, respectively. The trend between periods was opposite for net removal and bioerosion rate, which indicates a decelerating impact of worms on the coral substrate as time passes. We suggest that, regardless of the small size of the boring worms, their role in CaCO3 removal is very important. The information provided here—species involved and amounts removed—is key in understanding the bioerosion process in ETP coral reefs.

Functional consequences of the long-term decline of reef-building corals in the Caribbean: evidence of across-reef functional convergence

Functional integrity on coral reefs is strongly dependent upon coral cover and coral carbonate production rate being sufficient to maintain three-dimensional reef structures. Increasing environmental and anthropogenic pressures in recent decades have reduced the cover of key reef-building species, producing a shift towards the relative dominance of more stress-tolerant taxa and leading to a reduction in the physical functional integrity. Understanding how changes in coral community composition influence the potential of reefs to maintain their physical reef functioning is a priority for their conservation and management. Here, we evaluate how coral communities have changed in the northern sector of the Mexican Caribbean between 1985 and 2016, and the implications for the maintenance of physical reef functions in the back- and fore-reef zones. We used the cover of coral species to explore changes in four morpho-functional groups, coral community composition, coral community calcification, the reef functional index and the reef carbonate budget. Over a period of 31 years, ecological homogenization occurred between the two reef zones mostly due to a reduction in the cover of framework-building branching ( Acropora spp.) and foliose-digitiform ( Porites porites and Agaricia tenuifolia ) coral species in the back-reef, and a relative increase in non-framework species in the fore-reef ( Agaricia agaricites and Porites astreoides ). This resulted in a significant decrease in the physical functionality of the back-reef zone. At present, both reef zones have negative carbonate budgets, and thus limited capacity to sustain reef accretion, compromising the existing reef structure and its future capacity to provide habitat and environmental services.

Eukaryotic plankton community stability across reef environments in Bocas del Toro Archipelago (Panamá)

ABSTRACTVariation in light and temperature can influence the genetic diversity and structure of marine plankton communities. While open ocean plankton communities receive much scientific attention, little is known about how environmental variation affects tropical coral reef plankton communities. Here, we characterize eukaryotic plankton communities on coral reefs across the Bocas del Toro Archipelago in Panamá. Temperature loggers were deployed for one year and mid-day light levels were measured to quantify environmental differences across reef zones at four inner and four outer reef sites: Inner: Punta Donato, Smithsonian Tropical Research Institute (STRI) Point, Cristobal, Punta Laurel and Outer: Drago Mar, Bastimentos North, Bastimentos South, and Popa Island. Triplicate vertical plankton tows were collected mid-day and high-throughput 18S ribosomal DNA metabarcoding was leveraged to investigate the relationship between eukaryotic plankton community structure and reef zones. Plankton communities from STRI Point were additionally characterized in the morning (∼08:00), mid-day (∼12:00), and evening (∼16:00) to quantify diel variation within a single site. We found that inshore reefs experienced higher average seawater temperatures, while offshore sites offered higher light levels, presumably associated with reduced water turbidity on reefs further from shore. However, these significant reef zone-specific environmental differences did not correlate with overall plankton community differences or changes in plankton genetic diversity. Instead, we found that time of day within a site and diel vertical migration played structuring roles within these plankton communities, and therefore conclude that the time of community sampling is an important consideration for future studies. Overall, plankton communities in the Bocas del Toro Archipelago appear relatively well mixed across space; however, follow-up studies focusing on more intensive sampling efforts across space and time coupled with techniques that can detect more subtle genetic differences between and within communities will more fully capture plankton dynamics in this region.