CO2-Induced Ocean Acidification Alters the Burrowing Behavior of Manila Clam Ruditapes philippinarum by Reversing GABAA Receptor Function

Biological burrowing behavior is an important driver shaping ecosystems that is being threatened by CO₂-induced ocean acidification; however, the effects of ocean acidification on burrowing behavior and its neurological mechanism remain unclear. This study showed that elevated pCO₂ significantly affected the burrowing behaviors of the Manila clam Ruditapes philippinarum, such as increased foot contraction, burrowing time, and intrabottom movement and decreased burrowing depth. Delving deeper into the mechanism, exposure to elevated pCO₂ significantly decreased extracellular pH and increased [HCO₃^-]. Moreover, an indicator GABA_A receptor, a neuroinhibitor for movement, was found to be closely associated with behavioral changes. In situ hybridization confirmed that the GABA_A receptor was widely distributed in ganglia and foot muscles, and elevated pCO₂ significantly increased the mRNA level and GABA concentration. However, the increase in GABA_A receptor and its ligand did not suppress the foot movement, but rather sent "excitatory" signals for foot contraction. The destabilization of acid-base homeostasis was demonstrated to induce an increase in the reversal potential for GABA_A receptor and an alteration in GABA_A receptor function under elevated pCO₂. This study revealed that elevated pCO₂ affects the burrowing behavior of Manila clams by altering GABA_A receptor function from inhibitory to excitatory.

Rapid evolution fuels transcriptional plasticity to ocean acidification

Ocean acidification (OA) is postulated to affect the physiology, behavior, and life-history of marine species, but potential for acclimation or adaptation to elevated pCO₂ in wild populations remains largely untested. We measured brain transcriptomes of six coral reef fish species at a natural volcanic CO₂  seep and an adjacent control reef in Papua New Guinea. We show that elevated pCO₂ induced common molecular responses related to circadian rhythm and immune system but different magnitudes of molecular response across the six species. Notably, elevated transcriptional plasticity was associated with core circadian genes affecting the regulation of intracellular pH and neural activity in Acanthochromis polyacanthus. Gene expression patterns were reversible in this species as evidenced upon reduction of CO₂ following a natural storm-event. Compared with other species, Ac. polyacanthus has a more rapid evolutionary rate and more positively selected genes in key functions under the influence of elevated CO₂ , thus fueling increased transcriptional plasticity. Our study reveals the basis to variable gene expression changes across species, with some species possessing evolved molecular toolkits to cope with future OA.

Insignificant Response of Bacterioplankton Community to Elevated pCO2 During a Short-Term Microcosm Experiment in a Subtropical Eutrophic Coastal Ecosystem

Ocean acidification, as one of the major consequences of global climate change, markedly affects multiple ecosystem functions in disparate marine environments from coastal habitats to the deep ocean. Evaluation of the responses of marine microbial community to the increasing partial pressure of CO₂ (pCO₂) is crucial to explore the microbe-driven biogeochemical processes in the future ocean. In this study, a microcosm incubation of eutrophic coastal water from Xiamen Bay under elevated pCO₂ (about 1,000 μatm) and control (ambient air, about 380-410 μatm) conditions was conducted to investigate the effect of ocean acidification on the natural bacterioplankton community. During the 5-day incubation period, the chlorophyll a concentration and bacterioplankton abundance were not significantly affected by increased pCO₂. Hierarchical clustering and non-metric multidimensional scaling analysis based on Bray-Curtis similarity among the bacterioplankton community derived from the 16S rRNA genes revealed an inconspicuous impact of elevated pCO₂ on the bacterial community. During the incubation period, Proteobacteria, Bacteroidetes, Actinobacteria, Cyanobacteria, and Epsilonbacteraeota were predominant in all microcosms. Despite the distinct temporal variation in the composition of the bacterioplankton community during the experimental period, statistical analyses showed that no significant difference was found on bacterioplankton taxa between elevated pCO₂ and control, indicating that the bacterioplankton at the population-level were also insensitive to elevated pCO₂. Our results therefore suggest that the bacterioplankton communities in the fluctuating and eutrophic coastal ecosystems appear to be adaptable to the short-term elevated pCO₂.

Additive impacts of deoxygenation and acidification threaten marine biota

Deoxygenation in coastal and open-ocean ecosystems rarely exists in isolation but occurs concomitantly with acidification. Here, we first combine meta-data of experimental assessments from across the globe to investigate the potential interactive impacts of deoxygenation and acidification on a broad range of marine taxa. We then characterize the differing degrees of deoxygenation and acidification tested in our dataset using a ratio between the partial pressure of oxygen and carbon dioxide (pO₂ /pCO₂ ) to assess how biological processes change under an extensive, yet diverse range of pO₂ and pCO₂ conditions. The dataset comprised 375 experimental comparisons and revealed predominantly additive but variable effects (91.7%, additive; 6.0%, synergistic; and 2.3%, antagonistic) of the dual stressors, yielding negative impacts across almost all responses examined. Our data indicate that the pO₂ /pCO₂ -ratio offers a simplified metric to characterize the extremity of the concurrent stressors and shows that more severe impacts occurred when ratios represented more extreme deoxygenation and acidification conditions. Importantly, our analysis highlights the need to assess the concurrent impacts of deoxygenation and acidification on marine taxa and that assessments considering the impact of O₂ depletion alone will likely underestimate the impacts of deoxygenation events and their ecosystem-wide consequences.

Elevated pCO2 Level Affects the Extracellular Polymer Metabolism of Phaeodactylum tricornutum

Extracellular polymeric substances (EPS) play an important role in diatom physiology and carbon biogeochemical cycling in marine ecosystems. Both the composition and yield of EPS in diatom cells can vary with environmental changes. However, information on intracellular pathways and controls of both biochemical and genetic of EPS is limited. Further, how such changes would affect their critical ecological roles in marine systems is also unclear. Here, we evaluated the physiological characteristics, EPS yields, EPS compositions, and gene expression levels of Phaeodactylum tricornutum under elevated pCO2 levels. Genes and pathways related to EPS metabolism in P. tricornutum were identified. Carbohydrate yields in different EPS fractions increased with elevated pCO2 exposure. Although the proportions of monosaccharide sugars among total sugars did not change, higher abundances of uronic acid were observed under high pCO2 conditions, suggesting the alterations of EPS composition. Elevated pCO2 increased PSII light energy conversion efficiency and carbon sequestration efficiency. The up-regulation of most genes involved in carbon fixation pathways led to increased growth and EPS release. RNA-Seq analysis revealed a number of genes and divergent alleles related to EPS production that were up-regulated by elevated pCO2 levels. Nucleotide diphosphate (NDP)-sugar activation and accelerated glycosylation could be responsible for more EPS responding to environmental signals. Further, NDP-sugar transporters exhibited increased expression levels, suggesting roles in EPS over-production. Overall, these results provide critical data for understanding the mechanisms of EPS production in diatoms and evaluating the metabolic plasticity of these organisms in response to environmental changes.

Photosynthetic Responses of Turf-forming Red Macroalgae to High CO2 Conditions

Seaweeds are important components of near-shore ecosystems as primary producers, foundation species, and biogeochemical engineers. Seaweed communities are likely to alter under predicted climate change scenarios. We tested the physiological responses of three perennial, turf-building, intertidal rhodophytes, Mastocarpus stellatus, Osmundea pinnatifida, and the calcified Ellisolandia elongata, to elevated pCO₂ over 6 weeks. Responses varied between these three species. E. elongata was strongly affected by high pCO₂ , whereas non-calcified species were not. Elevated pCO₂ did not induce consistent responses of photosynthesis and respiration across these three species. While baseline photophysiology differed significantly between species, we found few clear effects of elevated pCO₂ on this aspect of macroalgal physiology. We found effects of within-species variation in elevated pCO₂ response in M. stellatus, but not in the other species. Overall, our data confirm the sensitivity of calcified macroalgae to elevated pCO₂ , but we found no evidence suggesting that elevated pCO₂ conditions will have a strong positive or negative impact on photosynthetic parameters in non-calcified macroalgae.

Symbiodinium mitigate the combined effects of hypoxia and acidification on a noncalcifying cnidarian

Anthropogenic nutrient inputs enhance microbial respiration within many coastal ecosystems, driving concurrent hypoxia and acidification. During photosynthesis, Symbiodinium spp., the microalgal endosymbionts of cnidarians and other marine phyla, produce O₂ and assimilate CO₂ and thus potentially mitigate the exposure of the host to these stresses. However, such a role for Symbiodinium remains untested for noncalcifying cnidarians. We therefore contrasted the fitness of symbiotic and aposymbiotic polyps of a model host jellyfish (Cassiopea sp.) under reduced O₂ (~2.09 mg/L) and pH (~ 7.63) scenarios in a full-factorial experiment. Host fitness was characterized as asexual reproduction and their ability to regulate internal pH and Symbiodinium performance characterized by maximum photochemical efficiency, chla content and cell density. Acidification alone resulted in 58% more asexual reproduction of symbiotic polyps than aposymbiotic polyps (and enhanced Symbiodinium cell density) suggesting Cassiopea sp. fitness was enhanced by CO₂ -stimulated Symbiodinium photosynthetic activity. Indeed, greater CO₂ drawdown (elevated pH) was observed within host tissues of symbiotic polyps under acidification regardless of O₂ conditions. Hypoxia alone produced 22% fewer polyps than ambient conditions regardless of acidification and symbiont status, suggesting Symbiodinium photosynthetic activity did not mitigate its effects. Combined hypoxia and acidification, however, produced similar numbers of symbiotic polyps compared with aposymbiotic kept under ambient conditions, demonstrating that the presence of Symbiodinium was key for mitigating the combined effects of hypoxia and acidification on asexual reproduction. We hypothesize that this mitigation occurred because of reduced photorespiration under elevated CO₂ conditions where increased net O₂ production ameliorates oxygen debt. We show that Symbiodinium play an important role in facilitating enhanced fitness of Cassiopea sp. polyps, and perhaps also other noncalcifying cnidarian hosts, to the ubiquitous effects of ocean acidification. Importantly we highlight that symbiotic, noncalcifying cnidarians may be particularly advantaged in productive coastal waters that are subject to simultaneous hypoxia and acidification.

Parental exposure to elevated pCO2 influences the reproductive success of copepods

Substantial variations are reported for egg production and hatching rates of copepods exposed to elevated carbon dioxide concentrations (pCO₂). One possible explanation, as found in other marine taxa, is that prior parental exposure to elevated pCO₂ (and/or decreased pH) affects reproductive performance. Previous studies have adopted two distinct approaches, either (1) expose male and female copepoda to the test pCO₂/pH scenarios, or (2) solely expose egg-laying females to the tests. Although the former approach is more realistic, the majority of studies have used the latter approach. Here, we investigated the variation in egg production and hatching success of Acartia tonsa between these two experimental designs, across five different pCO₂ concentrations (385-6000 µatm pCO₂). In addition, to determine the effect of pCO₂ on the hatching success with no prior parental exposure, eggs produced and fertilized under ambient conditions were also exposed to these pCO₂ scenarios. Significant variations were found between experimental designs, with approach (1) resulting in higher impacts; here >20% difference was seen in hatching success between experiments at 1000 µatm pCO₂ scenarios (2100 year scenario), and >85% at 6000 µatm pCO₂. This study highlights the potential to misrepresent the reproductive response of a species to elevated pCO₂ dependent on parental exposure.