Coastal squeeze on temperate reefs: Long-term shifts in salinity, water quality, and oyster-associated communities.

Foundation species, such as mangroves, saltmarshes, kelps, seagrasses, and oysters, thrive within suitable environmental envelopes as narrow ribbons along the land-sea margin. Therefore, these habitat-forming species and resident fauna are sensitive to modified environmental gradients. For oysters, many estuaries impacted by sea-level rise, channelization, and municipal infrastructure are experiencing saltwater intrusion and water-quality degradation that may alter reef distributions, functions, and services. To explore decadal-scale oyster-reef community patterns across a temperate estuary in response to environmental change, we resampled reefs in the Newport River Estuary (NRE) during 2013-2015 that had previously been studied during 1955-1956. We also coalesced historical NRE reef distribution (1880s-2015), salinity (1913-2015), and water-quality-driven shellfish closure boundary (1970s-2015) data to document environmental trends that could influence reef ecology and service delivery. Over the last 60-120 years, the entire NRE has shifted toward higher salinities. Consequently, oyster-reef communities have become less distinct across the estuary, manifest by 20%-27% lower species turnover and decreased faunal richness among NRE reefs in the 2010s relative to the 1950s. During the 2010s, NRE oyster-reef communities tended to cluster around a euhaline, intertidal-reef type more so than during the 1950s. This followed faunal expansions farther up estuary and biological degradation of subtidal reefs as NRE conditions became more marine and favorable for aggressive, reef-destroying taxa. In addition to these biological shifts, the area of suitable bottom on which subtidal reefs persist (contracting due to up-estuary intrusion of marine waters) and support human harvest (driven by water quality, eroding from up-estuary) has decreased by >75% since the natural history of NRE reefs was first explored. This "coastal squeeze" on harvestable subtidal oysters (reduced from a 4.5-km to a 0.75-km envelope along the NRE's main axis) will likely have consequences regarding the economic incentives for future oyster conservation, as well as the suite of services delivered by remaining shellfish reefs (e.g., biodiversity maintenance, seafood supply). More broadly, these findings exemplify how "squeeze" may be a pervasive concern for biogenic habitats along terrestrial or marine ecotones during an era of intense global change.

Oyster reefs as carbon sources and sinks.

Carbon burial is increasingly valued as a service provided by threatened vegetated coastal habitats. Similarly, shellfish reefs contain significant pools of carbon and are globally endangered, yet considerable uncertainty remains regarding shellfish reefs' role as sources (+) or sinks (-) of atmospheric CO2 While CO2 release is a by-product of carbonate shell production (then burial), shellfish also facilitate atmospheric-CO2 drawdown via filtration and rapid biodeposition of carbon-fixing primary producers. We provide a framework to account for the dual burial of inorganic and organic carbon, and demonstrate that decade-old experimental reefs on intertidal sandflats were net sources of CO2 (7.1 ± 1.2 MgC ha-1 yr-1 (µ ± s.e.)) resulting from predominantly carbonate deposition, whereas shallow subtidal reefs (-1.0 ± 0.4 MgC ha-1 yr-1) and saltmarsh-fringing reefs (-1.3 ± 0.4 MgC ha-1 yr-1) were dominated by organic-carbon-rich sediments and functioned as net carbon sinks (on par with vegetated coastal habitats). These landscape-level differences reflect gradients in shellfish growth, survivorship and shell bioerosion. Notably, down-core carbon concentrations in 100- to 4000-year-old reefs mirrored experimental-reef data, suggesting our results are relevant over centennial to millennial scales, although we note that these natural reefs appeared to function as slight carbon sources (0.5 ± 0.3 MgC ha-1 yr-1). Globally, the historical mining of the top metre of shellfish reefs may have reintroduced more than 400 000 000 Mg of organic carbon into estuaries. Importantly, reef formation and destruction do not have reciprocal, counterbalancing impacts on atmospheric CO2 since excavated organic material may be remineralized while shell may experience continued preservation through reburial. Thus, protection of existing reefs could be considered as one component of climate mitigation programmes focused on the coastal zone.

Haslea silbo, A Novel Cosmopolitan Species of Blue Diatoms.

Specimens of a new species of blue diatoms from the genus Haslea Simonsen were discovered in geographically distant sampling sites, first in the Canary Archipelago, then North Carolina, Gulf of Naples, the Croatian South Adriatic Sea, and Turkish coast of the Eastern Mediterranean Sea. An exhaustive characterization of these specimens, using a combined morphological and genomic approach led to the conclusion that they belong to a single new to science cosmopolitan species, Haslea silbo sp. nov. A preliminary characterization of its blue pigment shows similarities to marennine produced by Haslea ostrearia, as evidenced by UV-visible spectrophotometry and Raman spectrometry. Life cycle stages including auxosporulation were also observed, providing data on the cardinal points of this species. For the two most geographically distant populations (North Carolina and East Mediterranean), complete mitochondrial and plastid genomes were sequenced. The mitogenomes of both strains share a rare atp6 pseudogene, but the number, nature, and positions of the group II introns inside its cox1 gene differ between the two populations. There are also two pairs of genes fused in single ORFs. The plastid genomes are characterized by large regions of recombination with plasmid DNA, which are in both cases located between the ycf35 and psbA genes, but whose content differs between the strains. The two sequenced strains hosts three plasmids coding for putative serine recombinase protein whose sequences are compared, and four out of six of these plasmids were highly conserved.

Maximizing oyster-reef growth supports green infrastructure with accelerating sea-level rise.

Within intertidal communities, aerial exposure (emergence during the tidal cycle) generates strong vertical zonation patterns with distinct growth boundaries regulated by physiological and external stressors. Forecasted accelerations in sea-level rise (SLR) will shift the position of these critical boundaries in ways we cannot yet fully predict, but landward migration will be impaired by coastal development, amplifying the importance of foundation species' ability to maintain their position relative to rising sea levels via vertical growth. Here we show the effects of emergence on vertical oyster-reef growth by determining the conditions at which intertidal reefs thrive and the sharp boundaries where reefs fail, which shift with changes in sea level. We found that oyster reef growth is unimodal relative to emergence, with greatest growth rates occurring between 20-40% exposure, and zero-growth boundaries at 10% and 55% exposures. Notably, along the lower growth boundary (10%), increased rates of SLR would outpace reef accretion, thereby reducing the depth range of substrate suitable for reef maintenance and formation, and exacerbating habitat loss along developed shorelines. Our results identify where, within intertidal areas, constructed or natural oyster reefs will persist and function best as green infrastructure to enhance coastal resiliency under conditions of accelerating SLR.

The HMA-LMA dichotomy revisited: an electron microscopical survey of 56 sponge species.

The dichotomy between high microbial abundance (HMA) and low microbial abundance (LMA) sponges has been long recognized. In the present study, 56 sponge species from three geographic regions (greater Caribbean, Mediterranean, Red Sea) were investigated by transmission electron microscopy for the presence of microorganisms in the mesohyl matrix. Additionally, bacterial enumeration by DAPI-counting was performed on a subset of samples. Of the 56 species investigated, 28 were identified as belonging to the HMA and 28 to the LMA category. The sponge orders Agelasida and Verongida consisted exclusively of HMA species, and the Poecilosclerida were composed only of LMA sponges. Other taxa contained both types of microbial associations (e.g., marine Haplosclerida, Homoscleromorpha, Dictyoceratida), and a clear phylogenetic pattern could not be identified. For a few sponge species, an intermediate microbial load was determined, and the microscopy data did not suffice to reliably determine HMA or LMA status. To experimentally determine the HMA or LMA status of a sponge species, we therefore recommend a combination of transmission electron microscopy and 16S rRNA gene sequence data. This study significantly expands previous reports on microbial abundances in sponge tissues and contributes to a better understanding of the HMA-LMA dichotomy in sponge-microbe symbioses.

The pathology of sponge orange band disease affecting the Caribbean barrel sponge Xestospongia muta.

The aim of this study was to examine sponge orange band (SOB) disease affecting the prominent Caribbean sponge Xestospongia muta. Scanning and transmission electron microscopy revealed that SOB is accompanied by the massive destruction of the pinacoderm. Chlorophyll a content and the main secondary metabolites, tetrahydrofurans, characteristic of X. muta, were significantly lower in bleached than in healthy tissues. Denaturing gradient gel electrophoresis using cyanobacteria-specific 16S rRNA gene primers revealed a distinct shift from the Synechococcus/Prochlorococcus clade of sponge symbionts towards several clades of unspecific cyanobacteria, including lineages associated with coral disease (i.e. Leptolyngbya sp.). Underwater infection experiments were conducted by transplanting bleached cores into healthy individuals, but revealed no signs of SOB development. This study provided no evidence for the involvement of a specific microbial pathogen as an etiologic agent of disease; hence, the cause of SOB disease in X. muta remains unidentified.