Michael Y. Roleda

BEFORE OCEAN ACIDIFICATION: CALCIFIER CHEMISTRY LESSONS(1).

Ocean Acidification (OA) has been an important research topic for a decade. Scientists have focused on how the predicted 56% decline in the seawater carbonate ion () concentration will dramatically impair the ability of calcifiers, ranging from coccolithophores to shellfish, to form calcium carbonate (CaCO3) structures, and the implications of the reduced carbonate saturation state (Ω) for increased dissolution of such structures. However, many published OA studies have overlooked a fundamental issue: most calcifying organisms do not rely on carbonate from seawater to calcify; they use either bicarbonate () or metabolically‐produced CO2. The ability of important primary (corals, coralline seaweeds, and coccolithophores) and secondary (mollusks) producers to modify their local carbonate chemistry suggests that the primary threat to them from OA is by dissolution rather than impaired calcification. Here, we draw on calcification research from an era before OA and combine it with recent studies that question the source of the carbonate ion, to provide new insights into how OA might affect calcifying organisms. Organismal modification of local carbonate chemistry may enable some calcifiers to successfully form calcareous structures despite OA.

Short- and long-term conditioning of a temperate marine diatom community to acidification and warming

Ocean acidification and greenhouse warming will interactively influence competitive success of key phytoplankton groups such as diatoms, but how long-term responses to global change will affect community structure is unknown. We incubated a mixed natural diatom community from coastal New Zealand waters in a short-term (two-week) incubation experiment using a factorial matrix of warming and/or elevated pCO2 and measured effects on community structure. We then isolated the dominant diatoms in clonal cultures and conditioned them for 1 year under the same temperature and pCO2 conditions from which they were isolated, in order to allow for extended selection or acclimation by these abiotic environmental change factors in the absence of interspecific interactions. These conditioned isolates were then recombined into ‘artificial’ communities modelled after the original natural assemblage and allowed to compete under conditions identical to those in the short-term natural community experiment. In general, the resulting structure of both the unconditioned natural community and conditioned ‘artificial’ community experiments was similar, despite differences such as the loss of two species in the latter. pCO2 and temperature had both individual and interactive effects on community structure, but temperature was more influential, as warming significantly reduced species richness. In this case, our short-term manipulative experiment with a mixed natural assemblage spanning weeks served as a reasonable proxy to predict the effects of global change forcing on diatom community structure after the component species were conditioned in isolation over an extended timescale. Future studies will be required to assess whether or not this is also the case for other types of algal communities from other marine regimes.

Effects of ocean acidification on the photosynthetic performance, carbonic anhydrase activity and growth of the giant kelp Macrocystis pyrifera

Under ocean acidification (OA), the 200xa0% increase in CO2(aq) and the reduction of pH by 0.3–0.4 units are predicted to affect the carbon physiology and growth of macroalgae. Here we examined how the physiology of the giant kelp Macrocystis pyrifera is affected by elevated pCO2/low pH. Growth and photosynthetic rates, external and internal carbonic anhydrase (CA) activity, HCO3− versus CO2 use were determined over a 7-day incubation at ambient pCO2 400xa0µatm/pH 8.00 and a future OA treatment of pCO2 1200 µatm/pH 7.59. Neither the photosynthetic nor growth rates were changed by elevated CO2 supply in the OA treatment. These results were explained by the greater use of HCO3− compared to CO2 as an inorganic carbon (Ci) source to support photosynthesis. Macrocystis is a mixed HCO3− and CO2 user that exhibits two effective mechanisms for HCO3− utilization; as predicted for species that possess carbon-concentrating mechanisms (CCMs), photosynthesis was not substantially affected by elevated pCO2. The internal CA activity was also unaffected by OA, and it remained high and active throughout the experiment; this suggests that HCO3− uptake via an anion exchange protein was not affected by OA. Our results suggest that photosynthetic Ci uptake and growth of Macrocystis will not be affected by elevated pCO2/low pH predicted for the future, but the combined effects with other environmental factors like temperature and nutrient availability could change the physiological response of Macrocystis to OA. Therefore, further studies will be important to elucidate how this species might respond to the global environmental change predicted for the ocean.

Saturating light and not increased carbon dioxide under ocean acidification drives photosynthesis and growth in Ulva rigida (Chlorophyta).

Carbon physiology of a genetically identified Ulva rigida was investigated under different CO2(aq) and light levels. The study was designed to answer whether (1) light or exogenous inorganic carbon (Ci) pool is driving growth; and (2) elevated CO2(aq) concentration under ocean acidification (OA) will downregulate CAext-mediated dehydration and alter the stable carbon isotope (δ13C) signatures toward more CO2 use to support higher growth rate. At pHT 9.0 where CO2(aq) is <1 μmol L−1, inhibition of the known use mechanisms, that is, direct uptake through the AE port and CAext-mediated dehydration decreased net photosynthesis (NPS) by only 56–83%, leaving the carbon uptake mechanism for the remaining 17–44% of the NPS unaccounted. An in silico search for carbon-concentrating mechanism elements in expressed sequence tag libraries of Ulva found putative light-dependent transporters to which the remaining NPS can be attributed. The shift in δ13C signatures from –22‰ toward –10‰ under saturating light but not under elevated CO2(aq) suggest preference and substantial use to support photosynthesis and growth. U. rigida is Ci saturated, and growth was primarily controlled by light. Therefore, increased levels of CO2(aq) predicted for the future will not, in isolation, stimulate Ulva blooms.

Bicarbonate uptake via an anion exchange protein is the main mechanism of inorganic carbon acquisition by the giant kelp Macrocystis pyrifera (Laminariales, Phaeophyceae) under variable pH

Macrocystis pyrifera is a widely distributed, highly productive, seaweed. It is known to use bicarbonate (HCO3−) from seawater in photosynthesis and the main mechanism of utilization is attributed to the external catalyzed dehydration of HCO3− by the surface‐bound enzyme carbonic anhydrase (CAext). Here, we examined other putative HCO3− uptake mechanisms in M. pyrifera under pHT 9.00 (HCO3−: CO2 = 940:1) and pHT 7.65 (HCO3−: CO2 = 51:1). Rates of photosynthesis, and internal CA (CAint) and CAext activity were measured following the application of AZ which inhibits CAext, and DIDS which inhibits a different HCO3− uptake system, via an anion exchange (AE) protein. We found that the main mechanism of HCO3− uptake by M. pyrifera is via an AE protein, regardless of the HCO3−: CO2 ratio, with CAext making little contribution. Inhibiting the AE protein led to a 55%–65% decrease in photosynthetic rates. Inhibiting both the AE protein and CAext at pHT 9.00 led to 80%–100% inhibition of photosynthesis, whereas at pHT 7.65, passive CO2 diffusion supported 33% of photosynthesis. CAint was active at pHT 7.65 and 9.00, and activity was always higher than CAext, because of its role in dehydrating HCO3− to supply CO2 to RuBisCO. Interestingly, the main mechanism of HCO3− uptake in M. pyrifera was different than that in other Laminariales studied (CAext‐catalyzed reaction) and we suggest that species‐specific knowledge of carbon uptake mechanisms is required in order to elucidate how seaweeds might respond to future changes in HCO3−:CO2 due to ocean acidification.

Long-Term Conditioning to Elevated pCO2 and Warming Influences the Fatty and Amino Acid Composition of the Diatom Cylindrotheca fusiformis

The unabated rise in anthropogenic CO₂ emissions is predicted to strongly influence the ocean’s environment, increasing the mean sea-surface temperature by 4°C and causing a pH decline of 0.3 units by the year 2100. These changes are likely to affect the nutritional value of marine food sources since temperature and CO₂ can influence the fatty (FA) and amino acid (AA) composition of marine primary producers. Here, essential amino (EA) and polyunsaturated fatty (PUFA) acids are of particular importance due to their nutritional value to higher trophic levels. In order to determine the interactive effects of CO₂ and temperature on the nutritional quality of a primary producer, we analyzed the relative PUFA and EA composition of the diatom Cylindrotheca fusiformis cultured under a factorial matrix of 2 temperatures (14 and 19°C) and 3 partial pressures of CO₂ (180, 380, 750 μatm) for >250 generations. Our results show a decay of ~3% and ~6% in PUFA and EA content in algae kept at a pCO₂ of 750 μatm (high) compared to the 380 μatm (intermediate) CO₂ treatments at 14°C. Cultures kept at 19°C displayed a ~3% lower PUFA content under high compared to intermediate pCO₂, while EA did not show differences between treatments. Algae grown at a pCO₂ of 180 μatm (low) had a lower PUFA and AA content in relation to those at intermediate and high CO₂ levels at 14°C, but there were no differences in EA at 19°C for any CO₂ treatment. This study is the first to report adverse effects of warming and acidification on the EA of a primary producer, and corroborates previous observations of negative effects of these stressors on PUFA. Considering that only ~20% of essential biomolecules such as PUFA (and possibly EA) are incorporated into new biomass at the next trophic level, the potential impacts of adverse effects of ocean warming and acidification at the base of the food web may be amplified towards higher trophic levels, which rely on them as source of essential biomolecules.

Seaweed Responses to Ocean Acidification

Ocean acidification (OA) is the decline in seawater pH caused by the sustained absorption by the oceans of anthropogenically produced atmospheric CO2. The consequences of OA to seaweed-based coastal ecosystems range from organismal to community levels of biological organization. Organismal responses can be species specific, depending on their carbon physiology, mode of calcification, and morphology (functional form). At the community scale, changes in community structure and function can have severe consequences on trophic dynamics. Biologically driven fluctuations in seawater carbonate chemistry are observed from micro- (diffusion boundary layer, DBL) to mesoscales (e.g., within a kelp forest), and such fluctuations may be exacerbated by OA. The synergistic effects of elevated CO2 with other human-induced environmental stressors (e.g., warming, eutrophication, and UVR) could make the primary producers of coastal ecosystems vulnerable to global climate change; some species may perform better than others under “greenhouse” conditions, leading to community phase shifts.

Effect of Ocean Acidification and pH Fluctuations on the Growth and Development of Coralline Algal Recruits, and an Associated Benthic Algal Assemblage

Coralline algae are susceptible to the changes in the seawater carbonate system associated with ocean acidification (OA). However, the coastal environments in which corallines grow are subject to large daily pH fluctuations which may affect their responses to OA. Here, we followed the growth and development of the juvenile coralline alga Arthrocardia corymbosa, which had recruited into experimental conditions during a prior experiment, using a novel OA laboratory culture system to simulate the pH fluctuations observed within a kelp forest. Microscopic life history stages are considered more susceptible to environmental stress than adult stages; we compared the responses of newly recruited A. corymbosa to static and fluctuating seawater pH with those of their field-collected parents. Recruits were cultivated for 16 weeks under static pH 8.05 and 7.65, representing ambient and 4× preindustrial pCO2 concentrations, respectively, and two fluctuating pH treatments of daily x~ = 8.05 (daytime pH = 8.45, night-time pH = 7.65) and daily x~ = 7.65 (daytime pH = 8.05, night-time pH = 7.25). Positive growth rates of new recruits were recorded in all treatments, and were highest under static pH 8.05 and lowest under fluctuating pH 7.65. This pattern was similar to the adults’ response, except that adults had zero growth under fluctuating pH 7.65. The % dry weight of MgCO3 in calcite of the juveniles was reduced from 10% at pH 8.05 to 8% at pH 7.65, but there was no effect of pH fluctuation. A wide range of fleshy macroalgae and at least 6 species of benthic diatoms recruited across all experimental treatments, from cryptic spores associated with the adult A. corymbosa. There was no effect of experimental treatment on the growth of the benthic diatoms. On the community level, pH-sensitive species may survive lower pH in the presence of diatoms and fleshy macroalgae, whose high metabolic activity may raise the pH of the local microhabitat.

Interspecific Resource Competition—Combined Effects of Radiation and Nutrient Limitation on Two Diazotrophic Filamentous Cyanobacteria

The cyanobacterial blooms in the Baltic Sea are dominated by diazotrophic cyanobacteria, the potentially toxic species Aphanizomenon sp. and the toxic species Nodularia spumigena. The seasonal succession with peaks of Aphanizomenon sp., followed by peaks of N. spumigena, has been explained by the species-specific niches of the two species. In a three-factorial outdoor experiment, we tested if nutrient and radiation conditions may impact physiological and biochemical responses of N. spumigena and Aphanizomenon sp. in the presence or absence of the other species. The two nutrient treatments were f/2 medium without NO3− (−N) and f/2 medium without PO43− (−P), and the two ambient radiation treatments were photosynthetic active radiation >395xa0nm (PAR) and PAR + UV-A + UV-B >295xa0nm. The study showed that Aphanizomenon sp. was not negatively affected by the presence of N. spumigena and that N. spumigena was better adapted to both N and P limitation in interaction with ultraviolet radiation (UVR, 280–400xa0nm). In the Baltic Sea, these physical conditions are likely to prevail in the surface water during summer. Interestingly, the specific growth rate of N. spumigena was stimulated by the presence of Aphanizomenon sp. We suggest that the seasonal succession, with peaks of Aphanizomenon sp. followed by peaks of N. spumigena, is a result from species-specific preferences of environmental conditions and/or stimulation by Aphanizomenon sp. rather than an allelopathic effect of N. spumigena. The results from our study, together with a predicted stronger stratification due to effects of climate change in the Baltic Sea with increased temperature and increased precipitation and increased UV-B due to ozone losses, reflect a scenario with a continuing future dominance of the toxic N. spumigena.

Meiospores produced in sori of nonsporophyllous laminae of Macrocystis pyrifera (Laminariales, Phaeophyceae) may enhance reproductive output

Different lamina of Macrocystis pyrifera sporophytes (i.e., sporophylls, pneumatocyst‐bearing blades, and apical scimitars) in a wave‐sheltered site were found to be fertile. We quantified their sorus surface area, reproductive output (number of spores released) and the viability of released spores (germination rate). Sorus area was greatest on the sporophylls, with sporangia developing on >57% of the total area and smallest on the pneumatocyst‐bearing blades with 21% of the total area bearing sporangia. The apical scimitar released the greatest number of meiospores (cells · mL−1 · cm−2) and the sporophylls the least. Meiospores produced from all types of fertile laminae were equally viable. This reproductive plasticity may enhance reproductive output, and contribute to short and long‐distance spore dispersal and the cryptic gametophyte propagule bank for the next generation of sporophytes.