The O2, pH and Ca2+ microenvironment of benthic foraminifera in a high CO2 world

Ocean acidification (OA) can have adverse effects on marine calcifiers. Yet, phototrophic marine calcifiers elevate their external oxygen and pH microenvironment in daylight, through the uptake of dissolved inorganic carbon (DIC) by photosynthesis. We studied to which extent pH elevation within their microenvironments in daylight can counteract ambient seawater pH reductions, i.e. OA conditions. We measured the O₂ and pH microenvironment of four photosymbiotic and two symbiont-free benthic tropical foraminiferal species at three different OA treatments (~432, 1141 and 2151 µatm pCO₂). The O₂ concentration difference between the seawater and the test surface (ΔO₂) was taken as a measure for the photosynthetic rate. Our results showed that O₂ and pH levels were significantly higher on photosymbiotic foraminiferal surfaces in light than in dark conditions, and than on surfaces of symbiont-free foraminifera. Rates of photosynthesis at saturated light conditions did not change significantly between OA treatments (except in individuals that exhibited symbiont loss, i.e. bleaching, at elevated pCO₂). The pH at the cell surface decreased during incubations at elevated pCO₂, also during light incubations. Photosynthesis increased the surface pH but this increase was insufficient to compensate for ambient seawater pH decreases. We thus conclude that photosynthesis does only partly protect symbiont bearing foraminifera against OA.

The effects of changing climate on microzooplankton grazing and community structure: drivers, predictions and knowledge gaps

Microzooplankton dominate trophic interactions and biogeochemical processes at the base of pelagic marine food webs and so their responses to a changing ocean environment have potentially large implications for ocean ecosystem functioning. This diverse array of mostly protistan species constitutes an important source of phytoplankton and bacterial mortality, and contributes significantly to the food available to higher trophic levels by packaging minute prey into larger particle sizes that can be consumed by metazooplankton. Microzooplankton are pivotal species in oceanic food webs and nutrient remineralization and so it is essential that we understand the effects that changing climate may have on the biomass, species composition and trophic activities of these assemblages. Yet, our present understanding of this topic is derived from experimental studies of relatively few species subjected to specific environmental variables (e.g. changes in temperature, CO₂, pH) in isolated culture. Most experiments and models employed to predict the effects of climate change have focussed on primary productivity and phytoplankton community structure, with less attention paid to microbial heterotrophy. Here we outline some of the major direct and indirect changes in environmental variables that are anticipated to accompany global climate change, and our present state of knowledge regarding their potential impacts on natural microzooplankton assemblages. We highlight a few specific areas for studies to address glaring omissions in our knowledge regarding how global change influences microzooplankton abundances and activities, and hypothesize that their ecological and biogeochemical roles may become even more prominent due to expected future shifts in marine chemistry and climate.

Effects of ocean acidification on growth, phosphate and nitrate uptake of macroalgae (article in Chinese)

The ocean acidification caused by elevated CO2 concentration can affect the growth, physiology and ecology, and nutrient uptake of macroalgae. In this paper, the growth and nutrient (PO4(3-) and NO3(-)) uptake of three species of macroalgae [Ulva pertusa, Scytosiphon lomentaria and Corallina pilulifera (calcified algae)] at three pHs (8.2, 7.9 and 7.6) were investigated under conditions of monoculture and mixed culture. Under the condition of monoculture, the percents of increased wet weights of U. pertusa, S. lomentaria and C. pilulifera on day 10 were the highest at the pHs of 7.9, 7.6 and 8.2, respectively, when compared with those on day 0, and the relative growth rates of C. pilulifera at pH 7.6 were significantly lower than those at pH 8.2. The data of mixed culture experiments suggested that lower pH was beneficial for the growth of S. lomentaria, while higher pH was beneficial for the growth of C. pilulifera. In both monoculture and mixed culture, the PO4(3-) and NO3(-) concentrations at the three pHs decreased with the time. The PO4(3-) concentrations decreased sharply by 71.9% – 99.0% from day 0 to day 2, and then decreased smoothly. Under the conditions of monoculture, the PO4(3-) uptake rates of U. pertusa, S. lomentaria and C. pilulifera were the highest at pHs of 8.2, 8.2 and 7.6, respectively. The NO3(-) uptake rates of U. pertusa and C. pilulifera in monoculture were the highest at pHs of 8.2 and 7.6, respectively. Under the conditions of mixed culture, the PO4(3-) uptake rates of U. pertusa + S. lomentaria, U. pertusa + C. pilulifera, S. lomentaria + C. pilulifera were the highest at pHs of 7.6, 8.2 and 8.2, respectively. The NO3(-) uptake rates of U. pertusa + C. pilulifera at pH 7.6 were the highest among the three pHs. The lower growth and higher nutrient (PO4(3-) and NO3(-)) uptake rates of monoculture C. pilulifera at pH 7.6 showed that the uptake and assimilation of PO4(3-) and NO3(-) were not coupled. The species composition of algae was changed due to ocean acidification although the dominant species was not change. Therefore, the different responses of growth and nutrient uptake of macroalgae to long-term ocean acidification in natural environment might lead to the change in macroalgal community.

Influence of increasing dissolved inorganic carbon concentrations and decreasing pH on chemolithoautrophic bacteria from oxic-sulfidic interfaces

Increases in the dissolved inorganic carbon (DIC) concentration are  expected to cause a decrease in the pH of ocean waters, a process  known as ocean acidification. In oxygen-deficient zones this will  add to already increased DIC and decreased pH values. It is not  known how this might affect microbial communities and microbially  mediated processes. In this study, the potential effects of ocean  acidification on chemolithoautotrophic prokaryotes of marine  oxic-anoxic transition zones were investigated, using the  chemoautotrophic denitrifying ε-proteobacterium  “Sulfurimonas gotlandica” strain GD1 as a model  organism. This and related taxa use reduced sulfur compounds, e.g.  sulfide and thiosulfate, as electron donors and were previously  shown to be responsible for nitrate removal and sulfide  detoxification in redox zones of the Baltic Sea water column but  occur also in other oxygen-deficient marine systems. Bacterial cell  growth within a broad range of DIC concentrations and pH values was  monitored and substrate utilization was determined. The results  showed that the DIC saturation concentration for growth was already  reached at 800 μM, which is well below in situ DIC  levels. The pH optimum was between 6.6 and 8.0. Within a pH range of  6.6–7.1 there was no significant difference in substrate  utilization; however, at lower pH values cell growth decreased  sharply and cell-specific substrate consumption increased. These  findings suggest that a direct effect of ocean acidification, with  the predicted changes in pH and DIC, on chemolithoautotrophic  bacteria such as “S. gotlandica” str. GD1 is generally  not very probable.

Host-parasite interactions: a litmus test for ocean acidification?

The effects of ocean acidification (OA) on marine species and ecosystems have received significant scientific attention in the past 10 years. However, to date, the effects of OA on hostparasite interactions have been largely ignored. As parasites play a multidimensional role in the regulation of marine population, community, and ecosystem dynamics, this knowledge gap may result in an incomplete understanding of the consequences of OA. In addition, the impact of stressors associated with OA on hostparasite interactions may serve as an indicator of future changes to the biodiversity of marine systems. This opinion article discusses the potential effects of OA on host and parasite species and proposes the use of parasites as bioindicators of OA disturbance.

High CO2 and silicate limitation synergistically increase the toxicity of Pseudo-nitzschia fraudulenta

Anthropogenic CO₂ is progressively acidifying the ocean, but the responses of harmful algal bloom species that produce toxins that can bioaccumulate remain virtually unknown. The neurotoxin domoic acid is produced by the globally-distributed diatom genus Pseudo-nitzschia. This toxin is responsible for amnesic shellfish poisoning, which can result in illness or death in humans and regularly causes mass mortalities of marine mammals and birds. Domoic acid production by Pseudo-nitzschia cells is known to be regulated by nutrient availability, but potential interactions with increasing seawater CO₂ concentrations are poorly understood. Here we present experiments measuring domoic acid production by acclimatized cultures of Pseudo-nitzschia fraudulenta that demonstrate a strong synergism between projected future CO₂ levels (765 ppm) and silicate-limited growth, which greatly increases cellular toxicity relative to growth under modern atmospheric (360 ppm) or pre-industrial (200 ppm) CO₂ conditions. Cellular Si:C ratios decrease with increasing CO₂, in a trend opposite to that seen for domoic acid production. The coastal California upwelling system where this species was isolated currently exhibits rapidly increasing levels of anthropogenic acidification, as well as widespread episodic silicate limitation of diatom growth. Our results suggest that the current ecosystem and human health impacts of toxic Pseudo-nitzschia blooms could be greatly exacerbated by future ocean acidification and ‘carbon fertilization’ of the coastal ocean.

Assembly rules of reef corals are flexible along a steep climatic gradient

Coral reefs, one of the world’s most complex and vulnerable ecosystems, face an uncertain future in coming decades as they continue to respond to anthropogenic climate change, overfishing, pollution, and other human impacts [[1] and [2]]. Traditionally, marine macroecology is based on presence/absence data from taxonomic checklists or geographic ranges, providing a qualitative overview of spatial shifts in species richness that treats rare and common species equally [[3] and [4]]. As a consequence, regional and long-term shifts in relative abundances of individual taxa are poorly understood. Here we apply a more rigorous quantitative approach to examine large-scale spatial variation in the species composition and abundance of corals on midshelf reefs along the length of Australia’s Great Barrier Reef, a biogeographic region where species richness is high and relatively homogeneous [5]. We demonstrate that important functional components of coral assemblages “sample” space differently at 132 sites separated by up to 1740 km, leading to complex latitudinal shifts in patterns of absolute and relative abundance. The flexibility in community composition that we document along latitudinal environmental gradients indicates that climate change is likely to result in a reassortment of coral reef taxa rather than wholesale loss of entire reef ecosystems.