Halogenated compound secreted by marine bacteria halts larval urchin development

Marine bacteria are ubiquitous, and yet their ecological functions have not been fully characterized. The globally distributed, biofilm-forming Pseudoalteromonas genus produces a variety of potentially bioactive halogenated organic products. One such secreted compound, 2,3,4,5-tetrabromopyrrole (TBP), has been found to be biocidal for several taxa of phytoplankton while stimulating the settlement and metamorphosis of coral at nanomolar doses. Here, we tested whether the presence of TBP affects early development in sea urchins that are not in direct contact with a benthic microbial film. Lytechinus variegatus embryos were exposed to varying TBP concentrations for different time durations over the first 48 h post-fertilization. Concentrations as low as 500 nM markedly reduced larval survivorship and retarded development. These deleterious effects became more pronounced as the concentration and duration of exposure to TBP were increased. Impairments in development when exposed to TBP appeared to be reversible, provided low exposure concentrations (< 500 nM – 1000 nM) for limited exposure durations (< 1–4 h). Preliminary immunofluorescence staining showed spindle defects in dividing embryos when exposed to TBP at high concentrations, which may have contributed to mortality and impaired growth. While TBP and other bacterial compounds like it may serve as settlement cues for corals, their cytotoxicity to single-cell algae and larval urchins could hint at the dangers of the benthos to some developing invertebrate embryos. If true, such compounds, along with the bacterial taxa that produce them, likely play underappreciated roles in the ecology, and potentially the evolution, of planktonic development in macroinvertebrates.