Red coralline algae are the deepest living macroalgae, capable of creating spatially complex reefs from the intertidal to 100+ m depth with global ecological and biogeochemical significance. How these algae maintain photosynthetic function under increasingly limiting light intensity and spectral availability is key to explaining their large depth distribution. Here, we investigated the photo- and chromatic acclimation and morphological change of free-living red coralline algae towards mesophotic depths in the Fernando do Noronha archipelago, Brazil. From 13 to 86 m depth, thalli tended to become smaller and less complex. We observed a dominance of the photo-acclimatory response, characterized by an increase in photosynthetic efficiency and a decrease in maximum electron transport rate. Chromatic acclimation was generally stable across the euphotic-mesophotic transition with no clear depth trend. Taxonomic comparisons suggest these photosynthetic strategies are conserved to at least the Order level. Light saturation necessitated the use of photoprotection to 65 m depth, while optimal light levels were met at 86 m. Changes to the light environment (e.g. reduced water clarity) due to human activities therefore places these mesophotic algae at risk of light limitation, necessitating the importance of maintaining good water quality for the conservation and protection of mesophotic habitats.

The euphotic-mesophotic transition is characterized by dramatic changes in environmental conditions, which can significantly alter the functioning of ecosystem engineers and the structure of their associated communities. However, the drivers of biodiversity change across the euphotic-mesophotic transition remain unclear. Here, we investigated the mechanisms affecting the biodiversity-supporting potential of free-living red coralline algae-globally important habitat creators-towards mesophotic depths. Across a 73 m depth gradient, we observed a general decline in macrofaunal biodiversity (fauna abundance, taxon richness and alpha diversity), but an increase in beta-diversity (i.e. variation between assemblages) at the deepest site (86 m depth, where light levels were less than 1% surface irradiance). We identified a gradient in abundance decline rather than distinct ecological shifts, driven by a complex interaction between declining light availability, declining size of the coralline algal host individuals and a changing host taxonomy. However, despite abundance declines, high between-assemblage variability at deeper depths allowed biodiversity-supporting potential to be maintained, highlighting their importance as coastal refugia.

Background Despite a global prevalence of photosynthetic organisms in the ocean’s mesophotic zone (30–200+ m depth), the mechanisms that enable photosynthesis to proceed in this low light environment are poorly defined. Red coralline algae are the deepest known marine benthic macroalgae — here we investigated the light harvesting mechanism and mesophotic acclimatory response of the red coralline alga Lithothamnion glaciale . Results Following initial absorption by phycourobilin and phycoerythrobilin in phycoerythrin, energy was transferred from the phycobilisome to photosystems I and II within 120 ps. This enabled delivery of 94% of excitations to reaction centres. Low light intensity, and to a lesser extent a mesophotic spectrum, caused significant acclimatory change in chromophores and biliproteins, including a 10% increase in phycoerythrin light harvesting capacity and a 20% reduction in chlorophyll- a concentration and photon requirements for photosystems I and II. The rate of energy transfer remained consistent across experimental treatments, indicating an acclimatory response that maintains energy transfer. Conclusions Our results demonstrate that responsive light harvesting by phycobilisomes and photosystem functional acclimation are key to red algal success in the mesophotic zone.

A species’ ability to spread is in part governed by the extent to which recipient habitats either resist, tolerate or promote the species’ recruitment. In disturbed marine environments, there is a general trend for the loss of algal canopies, thought to resist invasion, toward algal turf or rock barrens habitat. This study tested whether the spread of the range-expanding native macroalga Caulerpa filiformis was resisted by algal canopies but facilitated by algal turf or barrens habitats. Large-scale field surveys generally supported the predicted recruitment patterns, with C. filiformis recruits being most abundant on turf (but not barrens) and absent under kelp canopies. However, a discrepancy existed between different structural forms of turf, with the positive association holding true only for geniculate corallines, not filamentous turf. Secondly, a laboratory experiment tested whether the physical structure and/or the sediment-trapping properties of coralline turf influenced the recruitment success of C. filiformis . Whilst the structural complexity of turf aided overall recruitment performance (i.e., increased rhizoid production, attachment speed, success, and strength), a positive influence of turf-derived sediment on recruits’ growth was less obvious, at least over 10 days. The high morphological plasticity of C. filiformis propagules resulted in possible benefits of faster or stronger attachment of more developed propagules being only temporary, and that recruitment may be regulated in accordance with habitat preference. Finally, a field experiment confirmed the observed positive role of turf and the negative influence of algal canopies in the short-term, however, adverse environmental conditions in the longer-term resulted in the loss of most fragments. In conclusion, this study demonstrated the importance of both positive and negative species interactions for the recruitment success of a native alga, suggesting that a shift from kelp to turf algae can initiate further community change.

Understanding how species' traits can shape winners and losers of environmental change can help resolve drivers of current community composition patterns and predict future drivers. Sedimentation is one of the main environmental stressors shaping coastal marine communities and tolerance of high sedimentation rates (e.g. via morphological variation) may allow for competitive dominance. In New South Wales, Australia, the abundance and range of the native green macroalga Caulerpa filiformis have increased over recent decades, apparently associated with sediment disturbance. We used field measurements to test hypotheses about morphological variability in C. filiformis in relation to local- and large-scale environmental variation in water depth, sediment cover and latitude. Using a lab experiment, we tested hypotheses about survival and morphological change under different sedimentation regimes. In the field, C. filiformis fronds were more elongated and less branched when a sediment veneer is present and when water depth increased (i.e. reduced light). At larger spatial scales, frond length and width decreased with increased latitude, but latitude was less important in explaining the variation C. filiformis' length than were depth or sedimentation. Our lab experiment showed a high tolerance to sedimentation, aided by increased investment in vertical growth. This study shows that rapid morphological plasticity is a likely key attribute of the spreading native macroalga C. filiformis. We argue that having a broad environmental tolerance is key to define a species success under environmental change.

Propagule pressure is acknowledged as a key determinant of invasion success. Nonetheless, the role of morphological or physiological attributes of propagules (i.e. their quality) in regulating invader establishment has been little explored. In particular, no study has investigated how the presence of propagules differing in quality within an inoculum influences establishment across heterogeneous landscapes. We experimentally tested the hypothesis that the quality (+Fronds+Rhizoids; +Fronds−Rhizoids; −Fronds+Rhizoids) and the diversity (1, 2 and 3 fragment types) of vegetative fragments of the seaweed Caulerpa taxifolia determine their establishment success across seascapes consisting of bare sediments and patches of the seagrass Zostera muelleri exposed to different disturbance intensities (control, seagrass canopy clipping and total removal). After 6 weeks, seaweed biomass, stolon and frond length, frond and rhizoid number were generally greater in unvegetated habitats (bare sediments and total seagrass removal) than full or reduced seagrass canopies. The type and the diversity of types of fragments inoculated had significant effects on the final biomass and morphological features of C. taxifolia only in vegetated habitats. In control plots, inocula of fragments retaining both fronds and rhizoids achieved higher biomass, developed longer stolons and more fronds. In canopy clipping plots, mixed inocula of +Fronds+Rhizoids and −Fronds+Rhizoids fragments had the greatest biomass and stolon length. Synthesis. Assessing how propagules differing in quality perform in different habitats might be not sufficient to draw a comprehensive picture of invasion risk, as their establishment can be modulated by both negative and positive interactions among them. Propagule composition should be, therefore, considered as a further dimension of propagule pressure. Our results also suggest that the relevance of specific propagule traits for invader establishment decreases from intact to degraded habitats. Considering propagule size in terms of amount of competent propagules, rather than an absolute measure, would refine our ability of predicting invasion risk across habitats differing in biotic or abiotic conditions.

There are many examples of native macrophytes becoming locally dominant and spreading outside their traditional distributions, but the causes and impacts are often not understood. In New South Wales, Australia, the green alga Caulerpa filiformis is undergoing a range expansion and has transitioned from a subdominant to a dominant alga on several rocky shores around the Sydney coastline. Here we investigated relationships between established patches of C. filiformis, the habitat it occupies and associated algal communities at multiple subtidal sites over the green alga's 700 km range. We tested the following predictions: 1) C. filiformis cover differs among substrata, being greatest on turf-forming algae; 2) C. filiformis cover is positively related to environmental variables linked to increased sedimentation (e.g. reduced reef width, surface slope, increased rugosity and distance from shore); 3) occurrence of C. filiformis is associated with a change in macrophyte community structure and a reduction of macrophyte richness; 4) intact native algal canopies inhibit C. filiformis spread, but turf-forming algae and bare sand are susceptible to invasion. Substratum associations were highly consistent among sites, but contrary to our prediction, C. filiformis was most commonly associated with rock or rock + sand substratum and less frequently associated with turf-forming algae substratum. C. filiformis cover was negatively correlated with reef width, which explained most of the variation observed, although local scale variables distance from shore, reef slope, and water depth were also correlated with C. filiformis cover. Algal diversity and community composition typically differed in the presence of C. filiformis, often with a reduction of algal abundances, in particular Sargassum spp., although results varied among substrata and sites. However, monitoring of borders suggested that C. filiformis does not invade and outcompete undisturbed adjacent canopy-forming algae over a 12 month period. Our results suggest that disturbance processes (possibly linked to sedimentation) acting at the site and quadrat scale are likely important determinants of C. filiformis cover and spread, and hence its potential ecological impacts.

Background Zostera marina (also known as eelgrass) is a foundation species in coastal and marine ecosystems worldwide and is a model for studies of seagrasses (a paraphyletic group in the order Alismatales) that include all the known fully submerged marine angiosperms. In recent years, there has been a growing appreciation of the potential importance of the microbial communities (i.e., microbiomes) associated with various plant species. Here we report a study of variation in Z. marina microbiomes from a field site in Bodega Bay, CA. Methods We characterized and then compared the microbial communities of root, leaf and sediment samples (using 16S ribosomal RNA gene PCR and sequencing) and associated environmental parameters from the inside, edge and outside of a single subtidal Z. marina patch. Multiple comparative approaches were used to examine associations between microbiome features (e.g., diversity, taxonomic composition) and environmental parameters and to compare sample types and sites. Results Microbial communities differed significantly between sample types (root, leaf and sediment) and in sediments from different sites (inside, edge, outside). Carbon:Nitrogen ratio and eelgrass density were both significantly correlated to sediment community composition. Enrichment of certain taxonomic groups in each sample type was detected and analyzed in regard to possible functional implications (especially regarding sulfur metabolism). Discussion Our results are mostly consistent with prior work on seagrass associated microbiomes with a few differences and additional findings. From a functional point of view, the most significant finding is that many of the taxa that differ significantly between sample types and sites are closely related to ones commonly associated with various aspects of sulfur and nitrogen metabolism. Though not a traditional model organism, we believe that Z. marina can become a model for studies of marine plant-microbiome interactions.

Caulerpa filiformis is a green seaweed found in New South Wales (NSW, Australia), South Africa, Mozambique and Peru. It has been suggested that the abundance of the species has increased in NSW over recent decades. Extensive aerial and diver surveys identified a 500-km northerly extension to the range of C. filiformis in NSW (to 28°21′S) compared with previous records. The alga has a disjunct distribution with small isolated populations around rocky headlands in far northern NSW, but then no apparent populations for 350km southwards. The far northern populations could be the result of recent human-mediated transport (a species introduction), or were simply not detected previously. The increased distribution around the previous northerly limit is likely a natural range expansion. The distribution of C. filiformis in NSW and globally seems confined to a temperature range of ~16–23°C. We found no relationship between abundance of C. filiformis and human population or oceanic chlorophyll-a (a surrogate for nutrient availability). We demonstrate that C. filiformis is predominately subtidal, being found along sections of coastline where there is a mixture of rocky reefs and beaches. It is argued that sand movement may have facilitated increases in abundance of C. filiformis.

Reconstructions suggest a massive decline of nearly 1400 ha of kelp forest in North Western Spain in 2007. In line with global rising temperatures, we hypothesized that Sea Surface Temperature (SST) surpassed a lethal threshold for kelp. We examined whether changes in SST correlated to the proposed decline in kelp forest. All investigated SST characteristics suggested to affect kelp abundance increased significantly during the past thirty years, reaching extreme values during the last decade. In addition over the past two decades, the landscape formerly dominated by both cold and warm temperate canopy forming and understory species changed to one dominated by warm temperate understory species, resulting in a loss of vertical community structure. Fisheries landing data of kelp associated species was used to support the suggested change in kelp abundance. Subsequent recovery of the kelp appears to be occurring in deeper waters.