Coralline algae are globally distributed benthic primary producers that secrete calcium carbonate skeletons. In the context of ocean acidification, they have received much recent attention due to the potential vulnerability of their high-Mg calcite skeletons and their many important ecological roles. Herein, we summarize what is known about coralline algal ecology and physiology, providing context to understand their responses to global climate change. We review the impacts of these changes, including ocean acidification, rising temperatures, and pollution, on coralline algal growth and calcification. We also assess the ongoing use of coralline algae as marine climate proxies via calibration of skeletal morphology and geochemistry to environmental conditions. Finally, we indicate critical gaps in our understanding of coralline algal calcification and physiology and highlight key areas for future research. These include analytical areas that recently have become more accessible, such as resolving phylogenetic relationships at all taxonomic ranks, elucidating the genes regulating algal photosynthesis and calcification, and calibrating skeletal geochemical metrics, as well as research directions that are broadly applicable to global change ecology, such as the importance of community-scale and long-term experiments in stress response.

/pdf/coralline-algae-rhodophyta-in-a-changing-world-integrating-5trom5h76v.pdf

Coralline algae (Rhodophyta) in a changing world: integrating ecological, physiological, and geochemical responses to global change.

The coralline algal genus Clathromorphum is a dominant calcifier in the rocky Subarctic biogeographic region, stretching through the lower Arctic from the Labrador Sea to the Bering Sea. Although commonly 2–10 cm in thickness, Clathromorphum can reach a thickness of up to 50 cm while forming an annually layered structure that can reach currently documented ages of up to 850 years. Geochemical and growth information archived in annual growth bands of Clathromorphum sp. has been used to provide long time series of past environmental conditions in regions that are poorly understood major drivers of Northern Hemisphere climate. However, information on Clathromorphum calcification, growth, and ecology that would allow interpretation of these records has previously been quite limited. Here we relate extensive field and laboratory data on the biology, physiology, and ecology of species of this genus and their controlling environmental parameters. We show that Clathromorphum has evolved a unique mode of double calcification, with high-magnesium calcite crystals, that enhances long life and leads to a multielement climate archive. Growth rates are controlled by temperature, and carbonate density is controlled by light, determined by both latitude and sea ice cover, whereas carbonate buildup and ultimate thickness are determined by local geomorphology and faunal interactions. Reproduction is complexly linked to vegetative anatomy. Precise paleoenvironmental information can be retrieved from Clathromorphum because of its unique cytological and anatomical structures, described and modeled for the first time in this volume.

/pdf/the-coralline-genus-clathromorphum-foslie-emend-adey-2zxyeemwun.pdf

The Coralline Genus Clathromorphum Foslie emend. Adey: Biological, Physiological, and Ecological Factors Controlling Carbonate Production in an Arctic-Subarctic Climate Archive

Northern Hemisphere sea ice has been declining sharply over the past decades and 2012 exhibited the lowest Arctic summer sea-ice cover in historic times. Whereas ongoing changes are closely monitored through satellite observations, we have only limited data of past Arctic sea-ice cover derived from short historical records, indirect terrestrial proxies, and low-resolution marine sediment cores. A multicentury time series from extremely long-lived annual increment-forming crustose coralline algal buildups now provides the first high-resolution in situ marine proxy for sea-ice cover. Growth and Mg/Ca ratios of these Arctic-wide occurring calcified algae are sensitive to changes in both temperature and solar radiation. Growth sharply declines with increasing sea-ice blockage of light from the benthic algal habitat. The 646-y multisite record from the Canadian Arctic indicates that during the Little Ice Age, sea ice was extensive but highly variable on subdecadal time scales and coincided with an expansion of ice-dependent Thule/Labrador Inuit sea mammal hunters in the region. The past 150 y instead have been characterized by sea ice exhibiting multidecadal variability with a long-term decline distinctly steeper than at any time since the 14th century.

https://www.pnas.org/content/pnas/110/49/19737.full.pdf

Arctic sea-ice decline archived by multicentury annual-resolution record from crustose coralline algal proxy

Polar coralline red algae (Corallinales, Rhodophyta) that form rhodoliths have received little attention concerning their potential as ecosystem engineers and carbonate factories; although, recent findings revealed that they are much more widespread in polar waters than previously thought. The present study deals with the northernmost rhodolith communities currently known, discovered in 2006 at 80 degrees 31'N in Nordkappbukta (North Cape Bay) at Nordaustlandet, Svalbard. These perennial coralline algae must be adapted to extreme seasonality in terms of light regime (c. 4 months winter darkness), sea ice coverage, nutrient supply, turbidity of the water column, temperature and salinity. The rhodolith communities and their environment were investigated using multibeam swath bathymetry, CTD measurements, recordings of the photosynthetic active radiation (PAR) and determination of the water chemistry, seabed imaging and targeted sampling by means of the manned submersible JAGO as well as benthic collections with a dredge. The coralline flora was composed mainly of Lithothamnion glaciale, with a lesser amount of Phymatolithon tenue. Based on their distribution and development at different depth levels, a facies model was developed. Rhodoliths occurred between 30 and 51 m, while coralline algae attached to cobbles were present as deep as 78 m. Measurements of the PAR indicated their adaptation to extreme low light levels. Ambient waters were always saturated with reference to calcite and aragonite for the whole area. The rhodolith-associated macrobenthic fauna samples yielded 59 species, only one of which was typically Arctic, and the concomitant appearance of corallines and grazers kept the corallines free from epiphytes and coequally provided feeding grounds for the grazers. Overall, L. glaciate and P. tenue appeared to be well adapted to the extreme environment of the Arctic.

Rhodolith beds (Corallinales, Rhodophyta) and their physical and biological environment at 80°31′N in Nordkappbukta (Nordaustlandet, Svalbard Archipelago, Norway)

No records exist to evaluate long-term pH dynamics in high-latitude oceans, which have the greatest probability of rapid acidification from anthropogenic CO2 emissions. We reconstructed both seasonal variability and anthropogenic change in seawater pH and temperature by using laser ablation high-resolution 2D images of stable boron isotopes (δ11B) on a long-lived coralline alga that grew continuously through the 20th century. Analyses focused on four multiannual growth segments. We show a long-term decline of 0.08 ± 0.01 pH units between the end of the 19th and 20th century, which is consistent with atmospheric CO2 records. Additionally, a strong seasonal cycle (∼0.22 pH units) is observed and interpreted as episodic annual pH increases caused by the consumption of CO2 during strong algal (kelp) growth in spring and summer. The rate of acidification intensifies from –0.006 ± 0.007 pH units per decade (between 1920s and 1960s) to –0.019 ± 0.009 pH units per decade (between 1960s and 1990s), and the episodic pH increases show a continuous shift to earlier times of the year throughout the centennial record. This is indicative of ecosystem shifts in shallow water algal productivity in this high-latitude habitat resulting from warming and acidification.

/pdf/century-scale-trends-and-seasonality-in-ph-and-temperature-8gxpgsidrb.pdf

Century-scale trends and seasonality in pH and temperature for shallow zones of the Bering Sea

Bering Sea climate and ecosystem dynamics have recently undergone major changes that have affected seasonal sea ice distribution and marine life, including commercially important salmon fisheries. Unfortunately, long-term Bering Sea dynamics are poorly understood, largely because of an absence of high-resolution marine proxy archives. Here we present the first record compiled from annual growth-increment widths of long-lived coralline algae collected in shallow-water habitats spanning the entire Aleutian Islands. While algal growth in the Aleutians exhibits a variable relationship with regional temperatures, it is strongly driven by changes in solar radiation reaching the seafloor. Therefore, it provides an exceptional archive of long-term light dynamics, which in the Bering Sea is attributed to changes in strength of the Aleutian Low (AL), the dominant climate pattern of the subarctic North Pacific. The AL is positively related to Bering Sea cloudiness and wind strength, which in turn fosters upper-ocean mixing. Mixing raises surface-water nutrient concentrations and stimulates plankton production, which is positively linked to Alaskan salmon abundance. Enhanced clouds and plankton production increase shading on the shallow seafloor and reduce algal growth. Light-driven algal growth rates track proxy-derived salmon abundance from 1782 onward, but are poorly related to temperature-dominated Pacific Decadal Oscillation (PDO) variability prior to the twentieth century. The algal record suggests that the present-day relationship of AL and PDO varied historically and that salmon stocks have been more closely related to AL strength via its effect on plankton abundance rather than PDO-related temperatures.

225 years of Bering Sea climate and ecosystem dynamics revealed by coralline algal growth-increment widths

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225 years of Bering Sea climate and ecosystem dynamics revealed by coralline algal growth-increment widths