Seaweed fails to prevent ocean acidification impact on foraminifera along a shallow‐water CO2 gradient

Ocean acidification causes biodiversity loss, alters ecosystems, and may impact food security, as shells of small organisms dissolve easily in corrosive waters. There is a suggestion that photosynthetic organisms could mitigate ocean acidification on a local scale, through seagrass protection or seaweed cultivation, as net ecosystem organic production raises the saturation state of calcium carbonate making seawater less corrosive. Here, we used a natural gradient in calcium carbonate saturation, caused by shallow‐water CO₂ seeps in the Mediterranean Sea, to assess whether seaweed that is resistant to acidification (Padina pavonica) could prevent adverse effects of acidification on epiphytic foraminifera. We found a reduction in the number of species of foraminifera as calcium carbonate saturation state fell and that the assemblage shifted from one dominated by calcareous species at reference sites (pH ~8.19) to one dominated by agglutinated foraminifera at elevated levels of CO₂ (pH ~7.71). It is expected that ocean acidification will result in changes in foraminiferal assemblage composition and agglutinated forms may become more prevalent. Although Padina did not prevent adverse effects of ocean acidification, high biomass stands of seagrass or seaweed farms might be more successful in protecting epiphytic foraminifera.     

Calcifying coral abundance near low-pH springs: implications for future ocean acidification

Rising atmospheric CO₂ and its equilibration with surface ocean seawater is lowering both the pH and carbonate saturation state (Ω) of the oceans. Numerous calcifying organisms, including reef-building corals, may be severely impacted by declining aragonite and calcite saturation, but the fate of coral reef ecosystems in response to ocean acidification remains largely unexplored. Naturally low saturation (Ω ~ 0.5) low pH (6.70–7.30) groundwater has been discharging for millennia at localized submarine springs (called “ojos”) at Puerto Morelos, México near the Mesoamerican Reef. This ecosystem provides insights into potential long term responses of coral ecosystems to low saturation conditions. In-situ chemical and biological data indicate that both coral species richness and coral colony size decline with increasing proximity to low-saturation, low-pH waters at the ojo centers. Only three scleractinian coral species (Porites astreoides, Porites divaricata, and Siderastrea radians) occur in undersaturated waters at all ojos examined. Because these three species are rarely major contributors to Caribbean reef framework, these data may indicate that today’s more complex frame-building species may be replaced by smaller, possibly patchy, colonies of only a few species along the Mesoamerican Barrier Reef. The growth of these scleractinian coral species at undersaturated conditions illustrates that the response to ocean acidification is likely to vary across species and environments; thus, our data emphasize the need to better understand the mechanisms of calcification to more accurately predict future impacts of ocean acidification.     

Effects of variations in carbonate chemistry on the calcification rates of Madracis auretenra (= Madracis mirabilis sensu Wells, 1973): bicarbonate concentrations best predict calcification rates

Physiological data and models of coral calcification indicate that corals utilize a combination of seawater bicarbonate and (mainly) respiratory CO₂ for calcification, not seawater carbonate. However, a number of investigators are attributing observed negative effects of experimental seawater acidification by CO₂ or hydrochloric acid additions to a reduction in seawater carbonate ion concentration and thus aragonite saturation state. Thus, there is a discrepancy between the physiological and geochemical views of coral biomineralization. Furthermore, not all calcifying organisms respond negatively to decreased pH or saturation state. Together, these discrepancies suggest that other physiological mechanisms, such as a direct effect of reduced pH on calcium or bicarbonate ion transport and/or variable ability to regulate internal pH, are responsible for the variability in reported experimental effects of acidification on calcification. To distinguish the effects of pH, carbonate concentration and bicarbonate concentration on coral calcification, incubations were performed with the coral Madracis auretenra (= Madracis mirabilis sensu Wells, 1973) in modified seawater chemistries. Carbonate parameters were manipulated to isolate the effects of each parameter more effectively than in previous studies, with a total of six different chemistries. Among treatment differences were highly significant. The corals responded strongly to variation in bicarbonate concentration, but not consistently to carbonate concentration, aragonite saturation state or pH. Corals calcified at normal or elevated rates under low pH (7.6–7.8) when the seawater bicarbonate concentrations were above 1800 μm . Conversely, corals incubated at normal pH had low calcification rates if the bicarbonate concentration was lowered. These results demonstrate that coral responses to ocean acidification are more diverse than currently thought, and question the reliability of using carbonate concentration or aragonite saturation state as the sole predictor of the effects of ocean acidification on coral calcification.     

Ocean acidification accelerates net calcium carbonate loss in a coral rubble community

Coral rubble communities are an important yet often overlooked component of a healthy reef ecosystem. The organisms inhabiting reef rubble are primarily bioeroders that contribute to the breakdown and dissolution of carbonate material. While the effects of ocean acidification on calcifying communities have been well studied, there are few studies investigating the response of bioeroding communities to future changes in pH and calcium carbonate saturation state. Using a flow-through pH-stat system, coral rubble pieces with a naturally occurring suite of organisms, along with bleached control rubble pieces, were subjected to three different levels of acidification over an 8-week period. Rates of net carbonate loss in bleached control rubble doubled in the acidification treatments (0.02 vs. 0.04% CaCO₃ d^?1 in ambient vs. moderate and high acidification), and living rubble communities experienced significantly increased rates of net carbonate loss from ambient to high acidification conditions (0.06 vs. 0.10% CaCO₃ d^?1, respectively). Although more experimentation is necessary to understand the long-term response and succession of coral rubble communities under projected conditions, these results suggest that rates of carbonate loss will increase in coral rubble as pH and calcium carbonate saturation states are reduced. This study demonstrates a need to thoroughly investigate the contribution of coral rubble to the overall carbonate budget, reef resilience, recovery, and function under future conditions.     

Ocean acidification at high latitudes: potential effects on functioning of the Antarctic bivalve Laternula elliptica

Ocean acidification is a well recognised threat to marine ecosystems. High latitude regions are predicted to be particularly affected due to cold waters and naturally low carbonate saturation levels. This is of concern for organisms utilising calcium carbonate (CaCO₃) to generate shells or skeletons. Studies of potential effects of future levels of pCO₂ on high latitude calcifiers are at present limited, and there is little understanding of their potential to acclimate to these changes. We describe a laboratory experiment to compare physiological and metabolic responses of a key benthic bivalve, Laternula elliptica, at pCO₂ levels of their natural environment (430 µatm, pH 7.99; based on field measurements) with those predicted for 2100 (735 µatm, pH 7.78) and glacial levels (187 µatm, pH 8.32). Adult L. elliptica basal metabolism (oxygen consumption rates) and heat shock protein HSP70 gene expression levels increased in response both to lowering and elevation of pH. Expression of chitin synthase (CHS), a key enzyme involved in synthesis of bivalve shells, was significantly up-regulated in individuals at pH 7.78, indicating L. elliptica were working harder to calcify in seawater undersaturated in aragonite (Ω_Ar = 0.71), the CaCO₃ polymorph of which their shells are comprised. The different response variables were influenced by pH in differing ways, highlighting the importance of assessing a variety of factors to determine the likely impact of pH change. In combination, the results indicate a negative effect of ocean acidification on whole-organism functioning of L. elliptica over relatively short terms (weeks-months) that may be energetically difficult to maintain over longer time periods. Importantly, however, the observed changes in L. elliptica CHS gene expression provides evidence for biological control over the shell formation process, which may enable some degree of adaptation or acclimation to future ocean acidification scenarios.     

Ocean Acidification Has Multiple Modes of Action on Bivalve Larvae

Ocean acidification (OA) is altering the chemistry of the world’s oceans at rates unparalleled in the past roughly 1 million years. Understanding the impacts of this rapid change in baseline carbonate chemistry on marine organisms needs a precise, mechanistic understanding of physiological responses to carbonate chemistry. Recent experimental work has shown shell development and growth in some bivalve larvae, have direct sensitivities to calcium carbonate saturation state that is not modulated through organismal acid-base chemistry. To understand different modes of action of OA on bivalve larvae, we experimentally tested how pH, P_CO2, and saturation state independently affect shell growth and development, respiration rate, and initiation of feeding in Mytilus californianus embryos and larvae. We found, as documented in other bivalve larvae, that shell development and growth were affected by aragonite saturation state, and not by pH or P_CO2. Respiration rate was elevated under very low pH (~7.4) with no change between pH of ~ 8.3 to ~7.8. Initiation of feeding appeared to be most sensitive to P_CO2, and possibly minor response to pH under elevated P_CO2. Although different components of physiology responded to different carbonate system variables, the inability to normally develop a shell due to lower saturation state precludes pH or P_CO2 effects later in the life history. However, saturation state effects during early shell development will carry-over to later stages, where pH or P_CO2 effects can compound OA effects on bivalve larvae. Our findings suggest OA may be a multi-stressor unto itself. Shell development and growth of the native mussel, M. californianus, was indistinguishable from the Mediterranean mussel, Mytilus galloprovincialis, collected from the southern U.S. Pacific coast, an area not subjected to seasonal upwelling. The concordance in responses suggests a fundamental OA bottleneck during development of the first shell material affected only by saturation state.     

Bio-deposition of a calcium carbonate layer on degraded limestone by Bacillus species

To obtain a restoring and protective calcite layer on degraded limestone, five different strains of the Bacillus sphaericus group and one strain of Bacillus lentus were tested for their ureolytic driven calcium carbonate precipitation. Although all the Bacillus strains were capable of depositing calcium carbonate, differences occurred in the amount of precipitated calcium carbonate on agar plate colonies. Seven parameters involved in the process were examined: calcite deposition on limestone cubes, pH increase, urea degrading capacity, extracellular polymeric substances (EPS)-production, biofilm formation, ζ-potential and deposition of dense crystal layers. The strain selection for optimal deposition of a dense CaCO₃ layer on limestone, was based on decrease in water absorption rate by treated limestone. Not all of the bacterial strains were effective in the restoration of deteriorated Euville limestone. The best calcite precipitating strains were characterised by high ureolytic efficiency, homogeneous calcite deposition on limestone cubes and a very negative ζ-potential.     

Indirect effects of fish predation on calcite supersaturation, precipitation and turbidity in a shallow prairie lake

SUMMARY.

• 1 We tracked calcite saturation and seston composition during 1987 and 1988 in a shallow, hardwater prairie lake, 1 year before and 1 year after a lake-wide fish removal. We also measured the contribution of calcite to turbidity during 1988.
• 2 In both years calcite saturation increased rapidly after all ice had thawed and peaked during mid-late summer with the mineral saturation index of calcite (SI) sometimes exceeding 30.
• 3 Removal of calcite from lake seston by gentle acidification of unfiltered water samples showed suspended calcite to be an important source of lurbidity in this lake.
• 4 The lake-wide fish removal produced detectable changes in the annual cycle of calcite saturation and precipitation. Increased grazing by Daphnia galeata and Daphnia pulex apparently reduced calcite saturation during early summer by suppressing the phytoplankton, and lowering the demand for CO₂.
• 5 Lower calcite precipitation, as well as direct removal of calcite by Daphnia grazing, probably contributed to the improvement in water transparency observed after the fish kill.

     

A nonlinear calcification response to CO2-induced ocean acidification by the coral Oculina arbuscula

Anthropogenic elevation of atmospheric pCO₂ is predicted to cause the pH of surface seawater to decline by 0.3–0.4 units by 2100 AD, causing a 50% reduction in seawater [CO₃ ²⁻] and undersaturation with respect to aragonite in high-latitude surface waters. We investigated the impact of CO₂-induced ocean acidification on the temperate scleractinian coral Oculina arbuscula by rearing colonies for 60 days in experimental seawaters bubbled with air-CO₂ gas mixtures of 409, 606, 903, and 2,856 ppm pCO₂, yielding average aragonite saturation states (Ω_A) of 2.6, 2.3, 1.6, and 0.8. Measurement of calcification (via buoyant weighing) and linear extension (relative to a ¹³⁷Ba/¹³⁸Ba spike) revealed that skeletal accretion was only minimally impaired by reductions in Ω_A from 2.6 to 1.6, although major reductions were observed at 0.8 (undersaturation). Notably, the corals continued accreting new skeletal material even in undersaturated conditions, although at reduced rates. Correlation between rates of linear extension and calcification suggests that reduced calcification under Ω_A = 0.8 resulted from reduced aragonite accretion, rather than from localized dissolution. Accretion of pure aragonite under each Ω_A discounts the possibility that these corals will begin producing calcite, a less soluble form of CaCO₃, as the oceans acidify. The corals’ nonlinear response to reduced Ω_A and their ability to accrete new skeletal material in undersaturated conditions suggest that they strongly control the biomineralization process. However, our data suggest that a threshold seawater [CO₃ ²⁻] exists, below which calcification within this species (and possibly others) becomes impaired. Indeed, the strong negative response of O. arbuscula to Ω_A = 0.8 indicates that their response to future pCO₂-induced ocean acidification could be both abrupt and severe once the critical Ω_A is reached.     

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.     

Enhanced Calcite Dissolution in the Presence of the Aerobic Methanotroph Methylosinus trichosporium

Microbial aerobic methane oxidation (MOx) is intrinsically coupled to the production of carbon dioxide, favoring carbonate dissolution. Recently, microbial organic polymers were shown to be able to induce carbonate dissolution. To discriminate between different mechanisms causing calcite dissolution, experiments were conducted in the presence of solid calcite with (1) actively growing cells (2) starving cells, and (3) dead cells of the methanotrophic bacterium Methylosinus trichosporium under brackish conditions (salinity 10) near calcite saturation (saturation state (Ω) 1.76 to 2.22). Total alkalinity and the amount of dissolved calcium markedly increased in all experiments containing M. trichosporium cells. After initial system equilibration, similar calcite dissolution rates, ranging between 20.16 (dead cells) and 25.68 μmol L^?1 d^?1 (actively growing cells), were observed. Although concentrations of transparent exopolymer particles declined with time in the presence of actively growing and starving cells, they increased in experiments with dead cells. Scanning electron microscopy images of calcite crystals revealed visible surface corrosion after exposure to live and dead M. trichosporium cells. The results of this study indicate a strong potential for microbial MOx to affect calcite stability negatively, facilitating calcite dissolution. In addition to CO₂ production by methanotrophically active cells, we suggest that the release of acidic or Ca²⁺-chelating organic carbon compounds from dead cells could also enhance calcite dissolution.     

Impacts of Ocean Acidification on Sediment Processes in Shallow Waters of the Arctic Ocean

Despite the important roles of shallow-water sediments in global biogeochemical cycling, the effects of ocean acidification on sedimentary processes have received relatively little attention. As high-latitude cold waters can absorb more CO₂ and usually have a lower buffering capacity than warmer waters, acidification rates in these areas are faster than those in sub-tropical regions. The present study investigates the effects of ocean acidification on sediment composition, processes and sediment-water fluxes in an Arctic coastal system. Undisturbed sediment cores, exempt of large dwelling organisms, were collected, incubated for a period of 14 days, and subject to a gradient of pCO₂ covering the range of values projected for the end of the century. On five occasions during the experimental period, the sediment cores were isolated for flux measurements (oxygen, alkalinity, dissolved inorganic carbon, ammonium, nitrate, nitrite, phosphate and silicate). At the end of the experimental period, denitrification rates were measured and sediment samples were taken at several depth intervals for solid-phase analyses. Most of the parameters and processes (i.e. mineralization, denitrification) investigated showed no relationship with the overlying seawater pH, suggesting that ocean acidification will have limited impacts on the microbial activity and associated sediment-water fluxes on Arctic shelves, in the absence of active bio-irrigating organisms. Only following a pH decrease of 1 pH unit, not foreseen in the coming 300 years, significant enhancements of calcium carbonate dissolution and anammox rates were observed. Longer-term experiments on different sediment types are still required to confirm the limited impact of ocean acidification on shallow Arctic sediment processes as observed in this study.     

Coral reefs modify their seawater carbon chemistry – case study from a barrier reef (Moorea,French Polynesia)

Changes in the carbonate chemistry of coral reef waters are driven by carbon fluxes from two sources: concentrations of CO₂ in the atmospheric and source water, and the primary production/respiration and calcification/dissolution of the benthic community. Recent model analyses have shown that, depending on the composition of the reef community, the air‐sea flux of CO₂ driven by benthic community processes can exceed that due to increases in atmospheric CO₂ (ocean acidification). We field test this model and examine the role of three key members of benthic reef communities in modifying the chemistry of the ocean source water: corals, macroalgae, and sand. Building on data from previous carbon flux studies along a reef‐flat transect in Moorea (French Polynesia), we illustrate that the drawdown of total dissolved inorganic carbon (C_T) due to photosynthesis and calcification of reef communities can exceed the draw down of total alkalinity (A_T) due to calcification of corals and calcifying algae, leading to a net increase in aragonite saturation state (Ω_a). We use the model to test how changes in atmospheric CO₂ forcing and benthic community structure affect the overall calcification rates on the reef flat. Results show that between the preindustrial period and 1992, ocean acidification caused reef flat calcification rates to decline by an estimated 15%, but loss of coral cover caused calcification rates to decline by at least three times that amount. The results also show that the upstream–downstream patterns of carbonate chemistry were affected by the spatial patterns of benthic community structure. Changes in the ratio of photosynthesis to calcification can thus partially compensate for ocean acidification, at least on shallow reef flats. With no change in benthic community structure, however, ocean acidification depressed net calcification of the reef flat consistent with findings of previous studies.     

Multiple phases of mg‐calcite in crustose coralline algae suggest caution for temperature proxy and ocean acidification assessment: lessons from the ultrastructure and biomineralization in Phymatolithon (Rhodophyta,Corallinales)1

Magnesium content, strongly correlated with temperature, has been developed as a climate archive for the late Holocene without considering anatomical controls on Mg content. In this paper, we explore the ultrastructure and cellular scale Mg‐content variations within four species of North Atlantic crust‐forming Phymatolithon. The cell wall has radial grains of Mg‐calcite, whereas the interfilament (middle lamella) has grains aligned parallel to the filament axis. The proportion of interfilament and cell wall carbonate varies by tissue and species. Three distinct primary phases of Mg‐calcite were identified: interfilament Mg‐calcite (mean 8.9 mol% MgCO₃), perithallial cell walls Mg‐calcite (mean 13.4 mol% MgCO₃), and hypothallium Mg‐calcite (mean 17.1 mol% MgCO₃). Magnesium content for the bulk crust, an average of all phases present, showed a strongly correlated (R² = 0.975) increase of 0.31 mol% MgCO₃ per °C. Of concern for climate reconstructions is the potential for false warming signals from undetected postgrazing wound repair carbonate that is substantially enriched in Mg, unrelated to temperature. Within a single crust, Mg‐content of component carbonates ranged from 8 to 20 mol% MgCO₃, representing theoretical thermodynamic stabilities from aragonite‐equivalent to unstable higher‐Mg‐calcite. It is unlikely that existing current predictions of ocean acidification impact on coralline algae, based on saturation states calculated using average Mg contents, provide an environmentally relevant estimate.     

Characterization of the exoskeleton of the Antarctic king crab Paralomis birsteini

Ocean acidification is projected to inhibit the biogenic production of calcium carbonate skeletons in marine organisms. Antarctic waters represent a natural environment in which to examine the long‐term effects of carbonate undersaturation on calcification in marine predators. King crabs (Decapoda: Anomura: Lithodidae), which currently inhabit the undersaturated environment of the continental slope off Antarctica, are potential invasives on the Antarctic shelf as oceanic temperatures rise. Here, we describe the chemical, physical, and mechanical properties of the exoskeleton of the deep‐water Antarctic lithodid Paralomis birsteini and compare our measurements with two decapod species from shallow water at lower latitudes, Callinectes sapidus (Brachyura: Portunidae) and Cancer borealis (Brachyura: Cancridae). In Paralomis birsteini, crabs deposit proportionally more calcium carbonate in their predatory chelae than their protective carapaces, compared with the other two crab species. When exoskeleton thickness and microhardness were compared between the chelae and carapace, the magnitude of the difference between these body regions was significantly greater in P. birsteini than in the other species tested. Hence, there appeared to be a greater disparity in P. birsteini in overall investment in calcium carbonate structures among regions of the exoskeleton. The imperatives of prey consumption and predator avoidance may be influencing the deposition of calcium to different parts of the exoskeleton in lithodids living in an environment undersaturated with respect to calcium carbonate.     

Acclimation to ocean acidification during long‐term CO2 exposure in the cold‐water coral Lophelia pertusa

Ocean acidity has increased by 30% since preindustrial times due to the uptake of anthropogenic CO₂ and is projected to rise by another 120% before 2100 if CO₂ emissions continue at current rates. Ocean acidification is expected to have wide‐ranging impacts on marine life, including reduced growth and net erosion of coral reefs. Our present understanding of the impacts of ocean acidification on marine life, however, relies heavily on results from short‐term CO₂ perturbation studies. Here, we present results from the first long‐term CO₂ perturbation study on the dominant reef‐building cold‐water coral Lophelia pertusa and relate them to results from a short‐term study to compare the effect of exposure time on the coral's responses. Short‐term (1 week) high CO₂ exposure resulted in a decline of calcification by 26–29% for a pH decrease of 0.1 units and net dissolution of calcium carbonate. In contrast, L. pertusa was capable to acclimate to acidified conditions in long‐term (6 months) incubations, leading to even slightly enhanced rates of calcification. Net growth is sustained even in waters sub‐saturated with respect to aragonite. Acclimation to seawater acidification did not cause a measurable increase in metabolic rates. This is the first evidence of successful acclimation in a coral species to ocean acidification, emphasizing the general need for long‐term incubations in ocean acidification research. To conclude on the sensitivity of cold‐water coral reefs to future ocean acidification further ecophysiological studies are necessary which should also encompass the role of food availability and rising temperatures.     

Irukandji jellyfish polyps exhibit tolerance to interacting climate change stressors

Increasing ocean temperatures and strengthening boundary currents have caused the poleward migration of many marine species. Cubozoan jellyfish known to cause Irukandji syndrome have historically been confined to tropical waters but may be expanding into subtropical regions. Here, we examine the interactive effects of warming and acidification on the population dynamics of polyps of an Irukandji jellyfish, Alatina nr mordens, and the formation of statoliths in newly metamorphosed medusae, to determine if this jellyfish could tolerate future conditions predicted for southeast Queensland (SEQ), Australia. Two experiments, examining the orthogonal factors of temperature and pH, were undertaken. Experiment 1 mimicked the current, ca. 2050 and ca. 2100 summer temperature and pH conditions predicted for SEQ using A1F1 scenarios (temperature: 25, 27, 29 °C; pH: 7.9, 7.8, 7.6) and Experiment 2 mimicked current and future winter conditions (18 and 22 °C, pH 7.9, 7.8, 7.6). All polyps in Experiment 1 survived and budded. Fewer polyps budded in the lower pH treatments; however, patterns varied slightly among temperature treatments. Statoliths at pH 7.6 were 24% narrower than those at pH 7.8 and 7.9. Most polyps survived the winter conditions mimicked by Experiment 2 but only polyps in the 22 °C, pH 7.9 treatment increased significantly. The current absence of A. nr mordens medusae in SEQ, despite the polyps' ability to tolerate the current temperature and pH conditions, suggests that ecological, rather than abiotic factors currently limit their distribution. Observations that budding was lower under low pH treatments suggest that rates of asexual reproduction will likely be much slower in the future. We consider that A. nr mordens polyps are likely to tolerate future conditions but are unlikely to thrive in the long term. However, if polyps can overcome potential ecological boundaries and acidification proceeds slowly A. nr mordens could expand polewards in the short term.     

Altered sediment biota and lagoon habitat carbonate dynamics due to sea cucumber bioturbation in a high‐pCO2 environment

The effects of global change on biological systems and functioning are already measurable, but how ecological interactions are being altered is poorly understood. Ecosystem resilience is strengthened by ecological functionality, which depends on trophic interactions between key species and resilience generated through biogenic buffering. Climate‐driven alterations to coral reef metabolism, structural complexity and biodiversity are well documented, but the feedbacks between ocean change and trophic interactions of non‐coral invertebrates are understudied. Sea cucumbers, some of the largest benthic inhabitants of tropical lagoon systems, can influence diel changes in reef carbonate dynamics. Whether they have the potential to exacerbate or buffer ocean acidification over diel cycles depends on their relative production of total alkalinity (A_T) through the dissolution of ingested calcium carbonate (CaCO₃) sediments and release of dissolved inorganic carbon (C_T) through respiration and trophic interactions. In this study, the potential for the sea cucumber, Stichopus herrmanni, a bêche‐de‐mer (fished) species listed as vulnerable to extinction, to buffer the impacts of ocean acidification on reef carbonate chemistry was investigated in lagoon sediment mesocosms across diel cycles. Stichopus herrmanni directly reduced the abundance of meiofauna and benthic primary producers through its deposit‐feeding activity under present‐day and near‐future pCO₂. These changes in benthic community structure, as well as A_T (sediment dissolution) and C_T (respiration) production by S. herrmanni, played a significant role in modifying seawater carbonate dynamics night and day. This previously unappreciated role of tropical sea cucumbers, in support of ecosystem resilience in the face of global change, is an important consideration with respect to the bêche‐de‐mer trade to ensure sea cucumber populations are sustained in a future ocean.     

Decline in growth of foraminifer Marginopora rossi under eutrophication and ocean acidification scenarios

The combination of global and local stressors is leading to a decline in coral reef health globally. In the case of eutrophication, increased concentrations of dissolved inorganic nitrogen (DIN) and phosphorus (DIP) are largely attributed to local land use changes. From the global perspective, increased atmospheric CO₂ levels are not only contributing to global warming but also ocean acidification (OA). Both eutrophication and OA have serious implications for calcium carbonate production and dissolution among calcifying organisms. In particular, benthic foraminifera precipitate the most soluble form of mineral calcium carbonate (high‐Mg calcite), potentially making them more sensitive to dissolution. In this study, a manipulative orthogonal two‐factor experiment was conducted to test the effects of dissolved inorganic nutrients and OA on the growth, respiration and photophysiology of the large photosymbiont‐bearing benthic foraminifer, Marginopora rossi. This study found the growth rate of M. rossi was inhibited by the interaction of eutrophication and acidification. The relationship between M. rossi and its photosymbionts became destabilized due to the photosymbiont's release from nutrient limitation in the nitrate‐enriched treatment, as shown by an increase in zooxanthellae cells per host surface area. Foraminifers from the OA treatments had an increased amount of Chl a per cell, suggesting a greater potential to harvest light energy, however, there was no net benefit to the foraminifer growth. Overall, this study demonstrates that the impacts of OA and eutrophication are dose dependent and interactive. This research indicates an OA threshold at pH 7.6, alone or in combination with eutrophication, will lead to a decline in M. rossi calcification. The decline in foraminifera calcification associated with pollution and OA will have broad ecological implications across their ubiquitous range and suggests that without mitigation it could have serious implications for the future of coral reefs.     

Biologically induced mineralization in the tree Milicia excelsa (Moraceae): its causes and consequences to the environment

Iroko trees (Milicia excelsa) in Ivory Coast and Cameroon are unusual because of their highly biomineralized tissues, which can virtually transform the trunk into stone. Oxalic acid (C₂O₄H₂) and metal‐oxalate play important roles in their ecosystems. In this study, the various forms of oxalate and carbonate mineralization reactions are investigated by using scanning electron microscopy and X‐ray diffraction. Calcium oxalate monohydrate is associated with stem, bark and root tissues, whereas calcium oxalate dihydrate is found with wood rot fungi in soils, as well as in decaying wood. Laboratory cultures show that many soil bacteria are able to oxidize calcium oxalate rapidly, resulting in an increase in solution pH. In terms of M. excelsa, these transformations lead to the precipitation of calcium carbonate, not only within the wood tissue, but also within the litter and soil. We calculate that c. 500 kg of inorganic carbon is accumulated inside an 80‐year‐old tree, and c. 1000 kg is associated with its surrounding soil. Crucially, the fixation of atmospheric CO₂ during tree photosynthesis, and its ultimate transformation into calcite, potentially represents a long‐term carbon sink, because inorganic carbon has a longer residence time than organic carbon. Considering that calcium oxalate biosynthesis is widespread in the plant and fungal kingdoms, the biomineralization displayed by M. excelsa may be an extremely common phenomena.