Limited effects of environmentally-relevant concentrations in seawater of dibutyl phthalate, dimethyl phthalate, bisphenol A, and 4-nonylphenol on the reproductive products of coral-reef organisms

Highlights

• Plastic additives (PAs) selectively affected coral-reef organisms.

• High laboratory concentrations of PAs negatively affected gametes and larvae.

• At environmentally-relevant concentrations, only 4-NP reduced planulae settlement.

• Negative effects of PAs can selectively disrupt coral-reef community structure.

• Effects of PAs on brooding organisms require determining at within-tissue levels.

Abstract

Plastic additives (PAs) are chemical compounds incorporated into the plastic during the manufacturing process. Phthalate acid esters, bisphenols, and nonylphenols are all PAs found in marine environments and associated with endocrine-disrupting processes. However, our knowledge regarding the impact of endocrine-disrupting PAs on coral-reef organisms is limited. As reef population structure is directly linked to reproduction and larval settlement processes, interference with hormonal systems can impact coral-reef community structure, particularly if the effects of PAs differ among species. In the current study we exposed the reproductive products of four tropical coral-reef invertebrates to environmentally-relevant concentrations of four prevalent PAs in seawater: dibutyl phthalate (DBP), dimethyl phthalate, (DMP), 4-nonylphenol (4-NP), and bisphenol A (BPA), as well as to 10³ higher laboratory concentrations of these PAs. Our results revealed that apart from the significant negative effect of the 1 μg/L of 4-NP on the settlement of the soft coral Rhytisma fulvum, none of the other tested materials demonstrated a significant effect on the exposed organisms at environmentally-relevant concentrations in seawater. The 4-NP high laboratory concentration (1000 μg/L), however, had significant negative effects on all the examined species. The high laboratory BPA concentration (1000 μg/L) significantly reduced fertilization success in the solitary ascidian Herdmania momus, up to its complete failure to reproduce. Moreover, the high laboratory DMP concentration (100 μg/L) had a significant negative effect on planulae settlement of the stony coral Stylophora pistillata. Our findings demonstrate the negative and selective effects of PAs on the development and reproduction of coral-reef organisms; and, specifically, the significant effect found following exposure to 4-NP. Consequently, if we aim to fully understand the impact of these contaminants on this endangered ecosystem, we suggest that the actual concentrations within the living organism tissues should be tested in order to produce relevant risk assessments for brooding-coral species.

Introduction

Plastic debris has now polluted nearly every habitat on earth, dispersing thousands of types of synthetic chemicals into the natural environment (Cózar et al., 2017; Hahladakis et al., 2018; Miyagi et al., 2018; Napper et al., 2020). Plastic materials are not pure substances, as their polymer building blocks are combined with chemical additives (e.g. antioxidants, plasticizers, and flame retardants) to improve product properties. The majority of these plastic additives (PAs) are not covalently bound to the polymer matrix but, rather, linked by weak secondary bonds. Thus, they are more likely to migrate from the plastic material into the surrounding environment (Hahladakis et al., 2018). Some PAs have a moderate-to-high log octanol-water partition coefficient (log K_ow) indicating their low affinity for water, but are partially soluble in biological fluids and have the potential to bio-concentrate in living organisms (USEPA, 2022). Bisphenols, nonylphenols, and phthalate esters are widely used as PAs despite their endocrine-disrupting properties (Groh et al., 2019; Darbre, 2020). Endocrine-disrupting compounds (EDCs) alter the hormonal and homeostatic systems responsible for reproduction, and developmental processes (Diamanti-Kandarakis et al., 2009).

Coral reefs provide a vast variety of biodiversity and ecosystem goods and services that benefit society, from food security and coastal protection to novel drug sources (Cinner, 2014; ICRI, 2004). Climate change, ocean acidification, and ongoing anthropogenic stressors put these exceptional ecosystems at existential risk (Hughes et al., 2017). As about 80% of marine plastic debris originates from waste generated by coastal populations (Jambeck et al., 2015), biota-debris interactions are more likely to occur in coastal marine habitats (Galloway et al., 2017), and even more so in those featuring high biological activity, such as fringing reefs. Lamb et al. (2018) surveyed 159 coral-reefs in the Asia-Pacific region and found that corals in contact with plastic debris are up to twenty times more susceptible to pathogens. De Carvalho-Souza et al. (2018), based on field and literature investigations, estimated that at least 418 reef species had encountered anthropogenic marine debris.

Reef population structure is directly linked to the reproduction, offspring development, and settlement of coral-reef organisms. Plastic-associated EDCs have been shown to affect the development of early life stages in a variety of marine invertebrates, with large differences among phyla (Gandara e Silva et al., 2016; Messinetti et al., 2019; Oehlmann et al., 2009). The particular life stage at which the organism is exposed to the EDC is of great importance. A developing organism in its larval stage or during metamorphosis undergoes various sensitive processes that can be affected by EDCs, some of which may only become visible upon maturity (Diamanti-Kandarakis et al., 2009; Darbre, 2020).

Corals and other coral-reef invertebrates can be either broadcast-spawners or brooders (Harrison, 2011; Shlesinger & Loya, 1985). Broadcast-spawners release eggs and sperm into the water column, where external fertilization and embryogenesis take place; whereas in brooders, fertilization and embryonic development take place internally, and the fully developed larvae are released into the water column. Consequently, the particular reproduction strategy may be a key factor in a species' vulnerability to EDCs, in particular if these EDCs accumulate within the parents' tissues. To date, no published experiments exist regarding the possible effects of common plastic-associated EDCs on the fecundity and development of tropical coral-reef invertebrates.

Here we studied four types of PAs detected in marine environments and considered to be environmental pollutants of emerging concern (Hermabessiere et al., 2017). (1) Dibutyl phthalate (DBP) and (2) dimethyl phthalate (DMP) are both phthalate acid esters and can be found in consumer products such as paints, adhesives, and cosmetics, however, 80% of all produced phthalates are used to soften plastic, making them ubiquitous contaminants in natural environments (Net et al., 2015). Isa et al. (2022) measured the bioconcentration of phthalate esters in soft corals and found that DBP and DMP were the most represented in soft-coral tissues. (3) 4-nonylphenol (4-NP) is used for the production of non-ionic surfactants in detergents and cleaning products, and is also a common antioxidant and stabilizer used in rubber and plastic food packaging (Fernandes et al., 2008). (4) Bisphenol A (BPA) appears to be both an estrogen receptor agonist and an androgen receptor antagonist and is a major monomer and additive of epoxy resins and polycarbonate (Flint et al., 2012; Groh et al., 2019).

While the effects of these PAs have been investigated in humans, commercial species, and in model organisms (Flint et al., 2012; Mankidy et al., 2013; Groh et al., 2019), their endocrinal and systemic effects on marine wildlife are less well known, and reports on their environmental concentrations in tropical seas featuring coral-reefs are scarce (Kawahata et al., 2004; Malem et al., 2019; Montano et al., 2020; Ranjbar Jafarabadi et al., 2021a; Saliu et al., 2019).

The present study examined the effects of exposure to the above-mentioned PAs on the early life stages of four tropical coral-reef invertebrates common in the Red Sea (Table 1): (1) the soft coral Rhytisma fulvum (Cnidaria, Alcyonacea) is an octocoral with a wide distribution in the Red Sea and the Indian Ocean. It is a gonochoric surface-brooder, with the oocytes being fertilized within the coral tissues, and the developing planulae then brooded on the surface of the female colony (Benayahu & Loya, 1983). (2) The stony coral Stylophora pistillata (Cnidaria, Scleractinia) is a branching coral with small polyps and a high surface/volume ratio, and is an hermaphroditic internal-brooder highly abundant in the reefs of the Red Sea (Rinkevich & Loya, 1979). (3) The calcifying hydrocoral, Millepora dichotoma (Cnidaria, Hydrozoa), is highly abundant and serves as an important reef-builder in the northern Red Sea. It is a gonochoric broadcast-spawner, and its life cycle features either a male or female planktonic medusa stage. The gametes are shed into the open water shortly after the medusa spawns (Shlesinger & Loya, 2018). (4) Herdmania momus (Urochordata, Ascidiacea) is a solitary ascidian, a sessile marine filter-feeder, known for its ability to thrive in both polluted and clean habitats in tropical environments around the world. It is an hermaphroditic broadcast-spawner, and as an invertebrate chordate its larval stage shares characteristics with vertebrates, making it an ideal model system for evo-devo studies (Gallo & Tosti, 2015).

The selected organisms play an important role in tropical coral-reef ecology, and any interference in their reproduction could initiate a cascade of deleterious changes in the community structure of the reef.