Evidence of an inflammatory-like response in non-normally pigmented tissues of two scleractinian corals

Abstract

Increasing evidence of links between climate change, anthropogenic stress and coral disease underscores the importance of understanding the mechanisms by which reef-building corals resist infection and recover from injury. Cellular inflammation and melanin-producing signalling pathway are two mechanisms employed by invertebrates to remove foreign organisms such as pathogens, but they have not been recorded previously in scleractinian corals. This study demonstrates the presence of the phenoloxidase (PO) activating melanin pathway in two species of coral, Acropora millepora and a massive species of Porites, which both develop local pigmentation in response to interactions with a variety of organisms. L-DOPA (3-(3,4-dihydroxyphenyl)-Lalanine) substrate-based enzyme activation assays demonstrated PO activity in healthy tissues of both species and upregulation in pigmented tissues of A. millepora. Histological staining conclusively identified the presence of melanin in Porites tissues. These results demonstrate that the PO pathway is active in both coral species. Moreover, the upregulation of PO activity in areas of non-normal pigmentation in A. millepora and increased melanin production in pigmented Porites tissues suggest the presence of a generalized defence response to localized stress. Interspecific differences in the usage of pathways involved in innate immunity may underlie the comparative success of massive Porites sp. as long-lived stress tolerators.

Locally accelerated growth is part of the innate immune response and repair mechanisms in reef-building corals as detected by green fluorescent protein (GFP)-like pigments

“…Elevated PO activity levels were previously found in nonnormally pigmented tissue of A. millepora and were linked to a potential immune response of corals (Palmer et al 2008). Here, we show that a significantly increased PO activity can be found in growing colony parts such as margins or branch tips in both M. foliosa and A. pulchra.…”

“…A previous report noted elevated levels of PO activity in strongly pigmented coral tissue (Palmer et al 2008); therefore, we tested whether elevated PO levels might be also indicative of growing tissue. Comparing the pigmented growth zones of M. foliosa to the established tissue of the inner colony areas, we found a significant increase in PO activity in growing tissue (Fig.…”

“…Hence, it is used as growth marker in biomedical research to characterize quickly proliferating tissue, for instance, in tumours (Leonardi et al 1992). As increased phenoloxidase (PO) activity was reported previously for pigmented coral tissue (Palmer et al 2008), we assessed whether also PO activity might be indicative of growth zones. In our study, we focussed on four coral species to define the effects of colony morphology on the pigment and growth responses: Montipora foliosa (plate-like), Acropora pulchra/polystoma (branching) and Porites lobata (massive).…”

“…Finally, the non-normal purple and red pigmentation has been associated with potential immune responses of Acropora millepora and Porites sp. (Palmer et al 2008).…”

Dynamics of seasonal outbreaks of black band disease in an assemblage of Montipora species at Pelorus Island (Great Barrier Reef, Australia)

“…Although the source of pathogens in recurrent cases is unclear, the following hypotheses may contribute. BBD may compromise a coral's immune responses, which may include amoebocytes (Hildemann et al 1977), melanin deposition (Palmer et al 2008) and antibacterial chemicals (Koh 1997;Gochfeld et al 2006;Ritchie 2006), increasing its susceptibility to recurring summer infections. Secondly, although undetectable in the field, pathogens may remain on or within apparently healthy colonies and act as winter reservoirs, contributing to reappearance of BBD signs in the following summer.…”