Cell wall organic matrix composition and biomineralization across reef-building coralline algae under global change

Species composition and abundance of crustose coralline algae and formation of Xinfeng algal reef in Northwestern Taiwan

The composition and abundance of crustose coralline algae (CCA) have been documented in the Xinfeng algal reef (XAR). Eight CCA species were identified, including four in Phymatolithon (P. margoundulatum, P. taiwanense, P. variabile, and Phymatolithon sp.) and four only identified to the genus level (Chamberlainium sp. and three Dawsoniolithon spp.). Chamberlainium sp. was the most abundant species, while the three species of Dawsoniolithon were rarely observed. The CCA coverage varied from 18% ± 15% to 27% ± 21% in spring seasons of 2021-2024, but there were no significant differences. Based on carbon-14 dating, the CCA sediments in the XAR were estimated to be 5850-5990 years old. These CCA sediments were primarily composed of two paleo-CCA species, resembling modern Chamberlainium sp. and Dawsoniolithon sp. 1. However, the modern species of Phymatolithon, which are widespread and abundant in the algal reefs of northwestern Taiwan, were not observed in the CCA sediments. It is unclear whether their absence was due to fragile thalli after their death or their lack of occurrence, and this requires further investigation. The paleoenvironmental data and biogenic sediment composition analysis suggests that the XAR started to develop ~6000 years ago when the sea surface temperature began to decrease and seawater turbidity increased due to rainfall. Additional CCA sediment samples from this algal reef are needed to fully understand the historical changes in the paleo-environments in the XAR.

New branched Porolithon species (Corallinales, Rhodophyta) from the Great Barrier Reef, Coral Sea, and Lord Howe Island

Porolithon is one of the most ecologically important genera of tropical and subtropical crustose (non-geniculate) coralline algae growing abundantly along the shallow margins of coral reefs and functioning to cement reef frameworks. Thalli of branched, fruticose Porolithon specimens from the Indo-Pacific Ocean traditionally have been called P. gardineri, while massive, columnar forms have been called P. craspedium. Sequence comparisons of the rbcL gene both from type specimens of P. gardineri and P. craspedium and from field-collected specimens demonstrate that neither species is present in east Australia and instead resolve into four unique genetic lineages. Porolithon howensis sp. nov. forms columnar protuberances and loosely attached margins and occurs predominantly at Lord Howe Island; P. lobulatum sp. nov. has fruticose to clavate forms and free margins that are lobed and occurs in the Coral Sea and on the Great Barrier Reef (GBR); P. parvulum sp. nov. has short (<2 cm), unbranched protuberances and attached margins and is restricted to the central and southern GBR; and P. pinnaculum sp. nov. has a mountain-like, columnar morphology and occurs on oceanic Coral Sea reefs. A rbcL gene sequence of the isotype of P. castellum demonstrates it is a different species from other columnar species. In addition to the diagnostic rbcL and psbA marker sequences, the four new species may be distinguished by a combination of features including thallus growth form, margin shape (attached or unattached), and medullary system (coaxial or plumose). Porolithon species, because of their ecological importance and sensitivity to ocean acidification, need urgent documentation of their taxonomic diversity.

Hierarchical settlement behaviours of coral larvae to common coralline algae

Natural regeneration of degraded reefs relies on the recruitment of larvae to restore populations. Intervention strategies are being developed to enhance this process through aquaculture production of coral larvae and their deployment as spat. Larval settlement relies on cues associated with crustose coralline algae (CCA) that are known to induce attachment and metamorphosis. To understand processes underpinning recruitment, we tested larval settlement responses of 15 coral species, to 15 species of CCA from the Great Barrier Reef (GBR). CCA in the family Lithophyllaceae were overall the best inducer across most coral species, with Titanoderma cf. tessellatum being the most effective species that induced at least 50% settlement in 14 of the coral species (mean 81%). Taxonomic level associations were found, with species of Porolithon inducing high settlement in the genus Acropora; while a previously understudied CCA, Sporolithon sp., was a strong inducer for the Lobophyllidae. Habitat-specific associations were detected, with CCA collected from similar light environment as the coral inducing higher levels of settlement. This study revealed the intimate relationships between coral larvae and CCA and provides optimal coral-algal species pairings that could be utilized to increase the success of larval settlement to generate healthy spat for reef restoration.