Coral reefs are highly endangered ecosystems. The identification and quantification of potential stress factors are essential to protect them. UV filters from sunscreens that are introduced to coral reef areas are considered as one of these stressors and their impact on corals needs to be further investigated. Even though UV filters are functionally similar, their structural features are very diverse. Their impact on limnic organisms have also been shown to be highly variable ranging from no or low to high toxicity. It is therefore to be expected that their effect on corals also differs significantly and that each compound has to be evaluated individually. The demand for conclusive benchmarks and guidelines from policy makers and the public over the past years shows the necessity for an objective literature review on the effects of various UV filters on scleractinian corals. Here, we review the present literature, summarize the data on the different UV filters and discuss the different approaches, advantages and limitations of the studies. However, the methods used in the latter studies vary greatly. They differ in many aspects such as species and life stage used, field and laboratory approaches, with exposure times ranging from hours to weeks. Some studies include analytics and measure the actual test concentration, others only provide nominal concentrations. The lack of standardized methods renders comparisons between studies futile. Additionally, most UV filters have only been investigated in a single or a few studies of different quality. Reliable thresholds are therefore impossible to draw on the basis of currently available studies. Nevertheless, certain UV filters repeatedly showed comparable toxicity in both freshwater and marine species tested. Yet, existing differences in results from coral tests emphasize the need for a standardized testing method comparable to those established for other aquatic organisms in order to allow for a more conclusive assessment. In this review, we describe what a scientifically sound testing proposal should include in order to obtain reliable and reproducible data, which ultimately should result in an internationally organized standardized ring test trial. Such standardized toxicity tests would enable validation of coral toxicity data related to UV filters, but also testing of other types of compounds that are known to be introduced and effect coral reefs, thus helping to identify significant stressors and enabling objective policy decisions.

Microorganisms have been reported to induce settlement in various marine invertebrate larvae but their specificity of inductive capacities for the settlement of coral larvae remains poorly understood. In this study, we isolated 56 microbial strains from the crustose coralline alga (CCA) Hydrolithon reinboldii using five different media either with or without additional antibiotics and/or spiked CCA extract. We tested the isolates for their potential to induce settlement behavior in larvae of the brooding scleractinian coral Leptastrea purpurea. From these 56 CCA-associated microbial strains, we identified six bacterial classes and 18 families. The culturable bacterial community associated with H. reinboldii was dominated by Gammaproteobacteria, Actinobacteria, and Alphaproteobacteria while the Illumina MiSeq analysis showed that the culture-independent bacterial community was dominated by Gammaproteobacteria, Alphaproteobacteria, and Flavobacteria. Furthermore, we found no correlation between inductive settlement capacities and phylogenetic relationships. Instead, settlement behavior of L. purpurea larvae was induced by specific isolated species. Strains #1792 (Pseudovibrio denitrificans), #1678 (Acinetobacter pittii), #1633 (Pseudoalteromonas phenolica), #1772 (Marine bacterium LMG1), #1721 (Microbulbifer variabilis), and #1783 (Pseudoalteromonas rubra) induced settlement behavior in coral larvae at mostly high and significant levels (≥ 40%) but the remaining isolates strongly varied in their activity. Multispecies biofilms consisting of four strains (#1792, #1678, #1633, and #1721) were observed to synergistically increase settlement behavior levels (> 90%); however, the addition of #1772 to the multispecies biofilms negatively affected coral larvae and resulted in a total loss of inducing activity. The findings provide new insights into the role of bacteria in the settlement process of scleractinian corals and may help to identify the true nature of bacteria-derived morphogenic cues.

Settlement of pelagic coral larvae is commonly induced by chemical cues that originate from biofilms and coralline algae. These natural settlement cues initiate signal pathways leading to attachment and metamorphosis of the coral larva. In order to investigate the settlement process and its natural inducers, it is necessary to gain a better understanding of these signal pathways. At present, the pathways and neurotransmitters involved in this signal transduction are still widely unknown. In this study, we exposed larvae of the brooding coral Leptastrea purpurea to five neuroactive compounds known to be present in cnidarians, and K⁺ Ions. All compounds were applied at different dilutions and settlement behavior of the larvae was documented over 48 h. Dopamine, glutamic acid and epinephrine significantly induced settlement in the coral larvae. The highest observed metamorphosis response was 54% in 10⁻⁵ M dopamine. Serotonin, L-DOPA and K⁺ ions did not have an influence on settlement behavior in our experiments. Exposing larvae to settlement-inducing neurotransmitters and thus bypassing the initial induction could be utilized in coral aquaculture. The active neurotransmitters should be used to further study the settlement process in L. purpurea in greater detail. Their role and relevance should also be assessed for other coral species as they may represent or reveal a universal inducer for coral settlement.

Coral reefs have been declining globally at a historically unprecedented rate. Ultraviolet (UV) filters used in sunscreens may contribute to this decline at local scales, which has already led to bans on various organic UV filters in some regions. However, the underlying studies for these bans demonstrated significant flaws in the experimental design due to a lack of validated and standardized testing methods for corals. This study aimed to investigate options for the development of a standard acute toxicity test for the larval stage of scleractinian corals. Planula larvae of two brooding (Leptastrea purpurea and Tubastraea faulkneri) and two spawning (Acropora digitifera and A. millepora) species were exposed to the organic UV filter benzophenone-3 (BP3) for 48 h under static conditions. We observed interspecific variations in toxicity, with A. digitifera being the most sensitive (LC₅₀ = 0.75 µg L⁻¹) and T. faulkneri the least sensitive (LC₅₀ = 2951.24 µg L⁻¹) species. Inhibition of settlement was found to be a useful endpoint leading to an EC₅₀ of 1.84 µg L⁻¹ in L. purpurea larvae. Although the analytical challenges of measuring lipophilic substances in small volume test setups remain, the here applied test design and selected endpoints are suitable for further validation and subsequent standardization.

Organic solvents are often used in aquatic toxicity tests to facilitate the testing of hydrophobic or poorly water-soluble substances such as ultraviolet (UV) filters, pesticides, or polycyclic aromatic hydrocarbons (PAHs). Knowledge of intrinsic effects (i.e., measured as standardized and non-standardized endpoints) of such carrier solvents in non-standardized organisms (i.e., corals), is critical to regulatory processes. Therefore, we exposed the reef-building coral Montipora digitata to the most commonly used carrier solvents ethanol, methanol, dimethyl sulfoxide, and dimethylformamide in the range of 10–100 µL L⁻¹ for 16 days. The effects on mortality, photobiological, morphological, and oxidative stress markers were evaluated. In our study, all solvents resulted in significant morphological and/or oxidative stress responses, but not in mortality. Moreover, ethanol led to a rapid increase in turbidity, thus questioning its suitability as a carrier solvent in aquatic studies in general. Based on our observations, we could rank the solvent effects as follows: dimethylformamide < dimethyl sulfoxide ≈ methanol ≤ ethanol, with dimethylformamide showing the least and ethanol the most pronounced effects. We conclude that the use of solvents in toxicity studies with corals, particularly by examining non-standardized (e.g., morphological, physiological) endpoints, should be taken with caution and requires further elaboration.