Chen, C. A. | Search bioRxiv

bioRxiv2021

The prevalence, predominance, and metabolic potentials of Candidatus Prochlorobium terpiosii in the coral-killing sponge, Terpios hoshinota

Terpios hoshinota is a ferocious, space-competing sponge that kills a variety of stony corals by overgrowth. Outbreaks of this species have led to intense coral reef damage and declines in living corals on the square kilometer scale in many geographical locations. Our large-scale 16S rRNA gene survey across three oceans revealed that the core microbiome of T. hoshinota included operational taxonomic units (OTUs) related to Prochloron, Endozoicomonas, Pseudospirillum, SAR116, Magnetospira, and Ruegeria. A Prochloron- related OTU was the most dominant cyanobacterium in T. hoshinota in the western Pacific Ocean, South China Sea, and Indian Ocean. The complete metagenome-assembled genome of the Prochloron-related cyanobacterium and our pigment analysis revealed that this bacterium had phycobiliproteins and phycobilins and lacked chlorophyll b, inconsistent with the iconic definition of Prochloron. Furthermore, the phylogenetic analyses based on 16S rRNA genes and 120 single-copy genes demonstrated that the bacterium was phylogenetically distinct to Prochloron, strongly suggesting that it should be a sister taxon to Prochloron; we therefore proposed this symbiotic cyanobacterium as a novel species under a new genus: Candidatus Paraprochloron terpiosii. With the recovery of the complete genome, we characterized the metabolic potentials of the novel cyanobacterium in carbon and nitrogen cycling and proposed a model for the interaction between Ca. Pp. terpiosi LD05 and T. hoshinota. In addition, comparative genomics analysis revealed that Ca. Paraprochloron and Prochloron showed distinct features in transporter systems and DNA replication. ImportanceThe finding that one species predominates cyanobacteria in T. hoshinota from different geographic locations indicates that this sponge and Ca. Pp. terpiosi LD05 share a tight relationship. This study builds the foundation for T. hoshinotas microbiome and paves a way for understanding the ecosystem, invasion mechanism, and causes of outbreak of this coral-killing sponge. Also, the first Prochloron-related complete genome enables us to study this bacterium with molecular approaches in the future and broadens our knowledge of the evolution of symbiotic cyanobacteria.

ecology↗

bioRxiv2018

Long-term blooming filamentous calcareous alga provides substrate for other coral-competitive macroalgae in the degraded lagoon of Dongsha Atoll (Pratas), Taiwan

BackgroundThe chemically-rich seaweed Galaxaura is not only highly competitive with corals, but also provides substrate for other macroalgae. Its ecology and associated epiphytes remain largely unexplored. To fill this knowledge gap, we herein undertook an ecological assessment to explore the spatial variation, temporal dynamics, and epiphytic macroalgae of G. divaricata on patch reefs in the lagoon of Dongsha Atoll, a shallow coral reef ecosystem in the northern South China Sea, repeatedly impacted by mass coral bleaching events.\n\nMethodsTwelve spatially independent patch reefs in the Dongsha lagoon were first surveyed to assess the benthic composition in April 2016, and then revisited to determine G. divaricata percent cover in September 2017, with one additional Galaxaura-dominated reef (site 9). Four surveys over a period of 17 months were carried out on a degraded patch reef (site 7) to assess the temporal variation in G. divaricata cover. Epiphytic macroalgae associated with G. divaricata were quantified and identified through the aid of DNA barcoding.\n\nResultsPatch reefs in the Dongsha lagoon were degraded, exhibiting relatively low live coral cover (5-43%), but high proportions of macroalgae (13-58%) and other substrates (rubble and dead corals; 23-69%). The distribution of G. divaricata was heterogeneous across the lagoon, with highest abundance (16-41%) in the southeast area. Temporal surveys from site 7 and photo-evidence from site 9 suggested that an overgrowth by G. divaricata was still present to a similar extend after 17 months and 3.5 years. Yet, G. divaricata provides a suitable substrate some allelopathic macroalgae (e.g., Lobophora sp.).\n\nConclusionsOur study demonstrates that an allelopathic seaweed, such as G. divaricata, can overgrow degraded coral reefs for extended periods of time. By providing habitat for harmful macroalgae, a prolonged Galaxaura overgrowth could strengthen negative feedback loops on degraded coral reefs, further decreasing their recovery potential.

ecology↗