White Rot Disease Occurs on Wild Individuals of the Economically Important Red Seaweed Halymenia durvillei (Rhodophyta)

The red seaweed Halymenia durvillei Bory de Saint-Vincent (Halymeniales, Rhodophyta), characterized by its upright, highly branched, bushy, cartilaginous and slimy thallus, is commonly found attached to hard substrates in the intertidal to shallow subtidal zones of tropical reefs, including in the Philippines [1,2]. An economically important species, H. durvillei can be consumed as food and has been prospected for its commercially important derivatives, such as carrageenan and pigments r-phycoerythrin and r-phycocyanin [3,4].

During the course of our research for development work on H. durvillei, we reported the occurrence and described the progression of a white rot disease on open sea-outplanted crops [4]. At that time, we thought that such disease occurred only on transplanted laboratory-cultured H. durvillei, noting that these were relatively more vulnerable considering that they were initially reared in a sterile environment. While these in vitro cultured H. durvillei have been acclimated in a hatchery facility, they were still grown in a controlled environment.

In a research expedition conducted along the coast of Palauig Bay, Zambales, from 3 to 5 May 2024, I observed indications of the natural occurrence of white rot disease in two H. durvillei individuals found on the reef flat (15°24′10.8″ N, 119°53′53.988″ E). One of these H. durvillei individuals (MSI30702; Figure 1A,B) has a red-orange–brown-colored thallus that was attached to a rocky substrate via a discoid holdfast; the other individual is represented by a branch portion (MSI30703; Figure 1C) with deep red coloration. Both individuals were collected at 5–6 m depth, with temperatures of 29–30 °C, and showed indications of white rot disease (Figure 1, arrowheads). Voucher specimens are deposited at the Gregorio T. Velasquez Phycological Herbarium (MSI) of the Marine Science Institute, University of the Philippines, Diliman, Quezon City, Philippines.

In both specimens, I observed the same characteristic semi-circular discoloration and disintegration of branches (Figure 1, arrowheads) that we observed in our previous report [4]. In the case of MSI30702, several branches have margins that have various levels of deformation resulting from tissue degeneration (Figure 1A,B, arrowhead), although some show indications of healing and recovery (Figure 1B, double arrowhead). As for MSI30703, this drifting branch portion is very likely a consequence of the disease as its basal portion also showed tissue discoloration and disintegration, like its other branch portions (Figure 1C, arrowhead). The causative agent(s) of white rot disease in H. durvillei have remained unknown since our first report in 2016, as work on tropical seaweed diseases has largely focused on the commercially important carrageenan-producing red seaweeds Kappaphycus and Eucheuma [5].

Red seaweeds are known to produce a wide array of biochemicals that function as defenses against viruses, bacteria, and fungi [6]. It is therefore curious why such disease is observed in both laboratory-reared and wild H. durvillei. Aside from knowing the identities of the causative agent(s) of white rot disease and the mechanism of their infection on H. durvillei, it is also worth investigating the value of allowing such pathogenic action for the biology of H. durvillei, especially its reproduction. As with other red seaweeds, H. durvillei produces non-motile reproductive structures [7], and propagation and settlement (and consequently, distribution) are therefore dictated by water movement characteristics. However, H. durvillei has also been shown to grow vegetatively under land-based culture conditions [1]. Taking all of these into consideration, allowing white rot disease to progress and break (fertile) branches can be a good strategy for H. durvillei to maximize its reproductive capacity by ‘delivering’ propagules to new colonizable areas, ensuring the proliferation and, consequently, the survival of the species. Nevertheless, it is apparent that further research is needed to answer questions related to my observations and propositions, especially considering commercial importance and the need to conserve wild populations of H. durvillei in anticipation of the demand for its derivatives.