Investigating seed bank potential of crustose coralline algae using DNA metabarcoding

To examine the potential for the autogenic ecosystem engineers, crustose coralline algae (CCA), to serve as seed banks or refugia for life stages of other species, it is critical to develop sampling protocols that reflect the diversity of life present. In this pilot study on two shallow water species of CCA collected from Raoul Island (Kermadec Islands; Rangitāhua) New Zealand, we investigated two preservation methods (ethanol vs. silica gel), sampled inner and outer regions of the crusts, and used DNA metabarcoding and seven genes/gene regions (16S rRNA, 18S rRNA, 23S rRNA, cox1, rbcL, and tufA genes and the ITS rRNA region) to develop a protocol for taxa identification. The results revealed immense diversity, with typically more taxa identified within the inner layers than the outer layers. As highlighted in other metabarcoding studies and in earlier work on rhodoliths (nodose coralline algae), reference databases are incomplete, and to some extent, the use of multiple markers mitigates this issue. Specifically, the 23S rRNA and rbcL genes are currently more suitable for identifying algae, while the cox1 gene fares better at capturing the diversity present inclusive of algae. Further investigation of these autogenic ecosystem engineers that likely act as marine seed banks is needed.     

Ocean warming and acidification have complex interactive effects on the dynamics of a marine fungal disease

Diseases threaten the structure and function of marine ecosystems and are contributing to the global decline of coral reefs. We currently lack an understanding of how climate change stressors, such as ocean acidification (OA) and warming, may simultaneously affect coral reef disease dynamics, particularly diseases threatening key reef-building organisms, for example crustose coralline algae (CCA). Here, we use coralline fungal disease (CFD), a previously described CCA disease from the Pacific, to examine these simultaneous effects using both field observations and experimental manipulations. We identify the associated fungus as belonging to the subphylum Ustilaginomycetes and show linear lesion expansion rates on individual hosts can reach 6.5 mm per day. Further, we demonstrate for the first time, to our knowledge, that ocean-warming events could increase the frequency of CFD outbreaks on coral reefs, but that OA-induced lowering of pH may ameliorate outbreaks by slowing lesion expansion rates on individual hosts. Lowered pH may still reduce overall host survivorship, however, by reducing calcification and facilitating fungal bio-erosion. Such complex, interactive effects between simultaneous extrinsic environmental stressors on disease dynamics are important to consider if we are to accurately predict the response of coral reef communities to future climate change.     

Effects of naturally acidified seawater on seagrass calcareous epibionts

Surface ocean pH is likely to decrease by up to 0.4 units by 2100 due to the uptake of anthropogenic CO2 from the atmosphere. Short-term experiments have revealed that this degree of seawater acidification can alter calcification rates in certain planktonic and benthic organisms, although the effects recorded may be shock responses and the long-term ecological effects are unknown. Here, we show the response of calcareous seagrass epibionts to elevated CO2 partial pressure in aquaria and at a volcanic vent area where seagrass habitat has been exposed to high CO2 levels for decades. Coralline algae were the dominant contributors to calcium carbonate mass on seagrass blades at normal pH but were absent from the system at mean pH 7.7 and were dissolved in aquaria enriched with CO2. In the field, bryozoans were the only calcifiers present on seagrass blades at mean pH 7.7 where the total mass of epiphytic calcium carbonate was 90 per cent lower than that at pH 8.2. These findings suggest that ocean acidification may have dramatic effects on the diversity of seagrass habitats and lead to a shift in the biogeochemical cycling of both carbon and carbonate in coastal ecosystems dominated by seagrass beds.     

Influence of UV radiation on growth of sporelings of three non-geniculate coralline red algae from Southern Iberian Peninsula

The effects of UV radiation (UVR) on growth of sporelings of Melobesia membranacea (Esper) Lamouroux, Lithophyllum incrustans Philippi and Mesophyllum lichenoides (Ellis) Lemoine, were investigated by culturing the algae under different doses of photosynthetically active radiation (PAR) only and PAR + UVR. Under natural conditions, the light fields occurring in the habitats of the three species differ substantially. Whereas M. lichenoides and L. incrustans inhabit sun‐exposed places in the eulittoral and upper part of the sublittoral, M. membranacea grows as an epiphyte in shady crevices in the eulittoral, where irradiance is < 10% of that in sun‐exposed places. The relative growth rate (RGR) of sporelings of these non‐geniculate coralline algae was affected by the UVR. The extent of harmful UVR effects on growth rate showed a similar increase as a function of the logarithm of the dose in the three species, inferred by a similar slope in all the linear regressions for a given action spectrum. The inhibition of growth under the PAR + UVR showed similar features in the two species of non‐geniculate coralline species from sun‐exposed places, that is, similar intercepts and slopes in the linear regressions of RGR as a function of the logarithm of the biologically effective dose.     

Multiple allelopathic activity of the crustose coralline algaLithophyllum yessoenseagainst settlement and germination of seaweed spores

A study was made to investigate possible formation by the crustose coralline algaLithophyllum yessoenseof multiple allelopathic-related substances against the settlement and germination of spores of various seaweeds. Seven different solvents (n-hexane, diethyl ether, acetone, ethyl acetate, acetonitrile, methanol, distilled water) and seawater were used to obtain crude extracts and secretory exudates from the coralline alga. The extracts and the algal conditioned seawater were tested for inhibitory activity against the settlement and germination of spores from 17 species representing 15 genera. Spore settlement of 14 species was inhibited over 90% by one or more extracts of the six organic solvents and conditioned seawater. The germination of spores from 13 species was inhibited by one or more extracts of all seven solvents and conditioned seawater. The species where spore settlement was not significantly affected showed strong inhibition of germination, andvice versa.     

CONCEPTACLE STRUCTURE OF THE PARASITIC CORALLINE RED ALGA CHOREONEMA THURETII (CORALLINALES) AND ITS TAXONOMIC IMPLICATIONS1

The major diagnostic features for erecting the red algal subfamily Choreonematoideae (Corallinales) were a combination of 1) absence of both cell fusions and secondary pit connections, 2) conceptacle roof and wall comprised of a single cell layer, and 3) presence of tetrasporangial pore plugs within a uniporate conceptacle in the monotypic taxon Choreonema thuretii (Bornet) Schmitz. Because this alga is a parasite, the absence of secondary cell connections is most likely an adaptation to a reduced thallus. This study shows that all conceptacles are not composed of a file of cells but rather a single layer of epithallial cells that are underlain by a thick layer of calcified acellular material; both epithallial cells and the calcified layer are produced by peripheral sterile cells. Although the outermost tetrasporangial pore canal is uniporate, there is a calcified acellular multiporate plate recessed just below the rim. The plate is produced by interspersed sterile cells and is continuous with the calcified layer supporting the conceptacle. These unique structures are likely due to parasitism rather than to the ancestral state. Based on these results and a reexamination of published micrographs depicting lenticular cells in Austrolithon intumescens Harvey et Woelkerling, we propose that both subfamily Choreonematoideae and Austrolithoideae are closely allied with subfamily Melobesioideae. This distant relationship to its host (Corallinoideae) plus a combination of unique conceptacle and unusual type of parasitism indicates that C. thuretii is an alloparasite and that it is likely the most ancient red algal parasite studied to date.     

Vertical distribution of coralline algae in the rocky intertidal of northern Chile

Studies on crustose corallines present in the intertidal region at three localities in northern Chile (30°S), show that these algae are well represented in exposed and protected sites, reaching up to 90% cover. Species composition differs between sites with the common occurrence of species or morphological variants of Spongites and a single taxon attributed to Phymatolithon at the most exposed sites, and species of Lithophyllum and Titanoderma at more protected localities. The two Lithophyllum taxa recorded are distinguished by the presence/absence of protuberances, cell size and degree of calcification, while Titanoderma taxa are segregated by the thickness of the thallus, hypothallic and perithallic cell size and shape. Spongites taxa are distinguished on the basis of external morphology and anatomical features such as cell size, degree of calcification and percentage of fusions between cells. The variability of these features within each species is still unknown thus, the taxa remain without specific epithet until further studies. Examination of specific types recorded for nearby regions are also required in order to clarify the taxonomy of the group in these coasts.     

Similar controls on calcification under ocean acidification across unrelated coral reef taxa

Ocean acidification (OA) is a major threat to marine ecosystems, particularly coral reefs which are heavily reliant on calcareous species. OA decreases seawater pH and calcium carbonate saturation state (Ω), and increases the concentration of dissolved inorganic carbon (DIC). Intense scientific effort has attempted to determine the mechanisms via which ocean acidification (OA) influences calcification, led by early hypotheses that calcium carbonate saturation state (Ω) is the main driver. We grew corals and coralline algae for 8–21 weeks, under treatments where the seawater parameters Ω, pH, and DIC were manipulated to examine their differential effects on calcification rates and calcifying fluid chemistry (Ω_cf, pH_cf, and DIC_cf). Here, using long duration experiments, we provide geochemical evidence that differing physiological controls on carbonate chemistry at the site of calcification, rather than seawater Ω, are the main determinants of calcification. We found that changes in seawater pH and DIC rather than Ω had the greatest effects on calcification and calcifying fluid chemistry, though the effects of seawater carbonate chemistry were limited. Our results demonstrate the capacity of organisms from taxa with vastly different calcification mechanisms to regulate their internal chemistry under extreme chemical conditions. These findings provide an explanation for the resistance of some species to OA, while also demonstrating how changes in seawater DIC and pH under OA influence calcification of key coral reef taxa.     

High CO2 decreases the long‐term resilience of the free‐living coralline algae Phymatolithon lusitanicum

Mäerl/rhodolith beds are protected habitats that may be affected by ocean acidification (OA), but it is still unclear how the availability of CO₂ will affect the metabolism of these organisms. Some of the inconsistencies found among OA experimental studies may be related to experimental exposure time and synergetic effects with other stressors. Here, we investigated the long‐term (up to 20 months) effects of OA on the production and calcification of the most common mäerl species of southern Portugal, Phymatolithon lusitanicum. Both the photosynthetic and calcification rates increased with CO₂ after the first 11 months of the experiment, whereas respiration slightly decreased with CO₂. After 20 months, the pattern was reversed. Acidified algae showed lower photosynthetic and calcification rates, as well as lower accumulated growth than control algae, suggesting that a metabolic threshold was exceeded. Our results indicate that long‐term exposure to high CO₂ will decrease the resilience of Phymatolithon lusitanicum. Our results also show that shallow communities of these rhodoliths may be particularly at risk, while deeper rhodolith beds may become ocean acidification refuges for this biological community.     

Phylogenetic relationships of corallinaceae (Corallinales,Rhodophyta): taxonomic implications for reef‐building corallines

A new, more complete, five‐marker (SSU, LSU, psbA, COI, 23S) molecular phylogeny of the family Corallinaceae, order Corallinales, shows a paraphyletic grouping of seven well‐supported monophyletic clades. The taxonomic implications included the amendment of two subfamilies, Neogoniolithoideae and Metagoniolithoideae, and the rejection of Porolithoideae as an independent subfamily. Metagoniolithoideae contained Harveylithon gen. nov., with H. rupestre comb. nov. as the generitype, and H. canariense stat. nov., H. munitum comb. nov., and H. samoënse comb. nov. Spongites and Pneophyllum belonged to separate clades. The subfamily Neogoniolithoideae included the generitype of Spongites, S. fruticulosus, for which an epitype was designated. Pneophyllum requires reassesment. The generitype of Hydrolithon, H. reinboldii, was a younger heterotypic synonym of H. boergesenii. The evolutionary novelty of the subfamilies Hydrolithoideae, Metagoniolithoideae, and Lithophylloideae was the development of tetra/bisporangial conceptacle roofs by filaments surrounding and interspersed among the sporangial initials.