Labyrinthula merlionensis sp. nov.: a novel labyrinthulid infecting marine diatoms
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Abstract
Labyrinthulomycetes are a class of fungus-like heterotrophic protists from the Stramenopiles lineage, recognized for their ecological role as decomposers and contributors to nutrient cycling. They colonize various substrates, from seaweed to terrestrial environments, utilizing ectoplasmic networks for nutrient absorption. This study characterized a novel Labyrinthula strain associated with the marine diatom Biddulphia . Phylogenetic analysis of the full-length 18S rRNA gene positioned this strain as a new species, Labyrinthula merlionensis sp. nov. Scanning electron and light microscopy observations revealed bi-flagellated zoospores and spindle-shaped vegetative cells with ectoplasmic networks. Time-series observations of the interactions between L. merlionensis and Biddulphia were categorised into different phases: establishment, infection, and aggregation. Scanning electron and confocal microscopy observations during the infection phase established the use of ectoplasmic nets to target the marginal ridge regions between diatoms, and the detection of labyrinthulid cells within diatom frustules. These findings enhance the understanding of the diversity, morphology, and ecological roles of Labyrinthulomycetes, particularly their intra- and extra-cellular interactions with diatom hosts.
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Co-culture temporal dynamics
The time series is described as starting from "day of culture medium change," but the medium renewal process itself isn't described beyond "renewed every 4 to 5 weeks," leaving some important questions about how to interpret the temporal dynamics presented in Figure 4.
What does medium renewal involve practically? If it involves centrifugation, the differential sedimentation properties of chain-forming Biddulphia versus small spindle-shaped L. merlionensis cells would substantially reshape the population ratio at day 0, and zoospores — being small and motile — would likely be lost in the supernatant. If it's a simple partial medium exchange with minimal disturbance, the population structure carries over largely intact. These two scenarios lead to very different interpretations of the day 4 zoospore …
Co-culture temporal dynamics
The time series is described as starting from "day of culture medium change," but the medium renewal process itself isn't described beyond "renewed every 4 to 5 weeks," leaving some important questions about how to interpret the temporal dynamics presented in Figure 4.
What does medium renewal involve practically? If it involves centrifugation, the differential sedimentation properties of chain-forming Biddulphia versus small spindle-shaped L. merlionensis cells would substantially reshape the population ratio at day 0, and zoospores — being small and motile — would likely be lost in the supernatant. If it's a simple partial medium exchange with minimal disturbance, the population structure carries over largely intact. These two scenarios lead to very different interpretations of the day 4 zoospore observations. Similarly, since the ectoplasmic net is an extracellular structure, the degree of physical disturbance during renewal matters — gentle exchange leaves existing networks intact and ongoing, whereas vigorous pipetting or centrifugation could fragment networks and synchronize cells artificially, making the "establishment phase" at day 4 look like a natural infection initiation when it may actually reflect synchronized recovery from mechanical disruption.
It's also not clear from the methods whether all experiments were performed on the original continuously passaged field isolate, or whether the co-culture was reconstituted using the separately maintained diatom isolate described later in the methods. If the former, both organisms have been co-passaged since 2020 and may have adapted to each other under lab conditions in ways that don't reflect natural infection dynamics. If the latter, at what ratio were the partners combined, and what was the physiological state of each at day 0? The methods also note that L. merlionensis was never successfully isolated in pure culture, which means there's no defined inoculum — but having a separately maintained naive diatom culture raises the question of whether a controlled infection experiment was attempted, which would have provided a cleaner starting point for the temporal framework and directly strengthened the parasitism interpretation in the conclusion.
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