Control Seep Site H. opuntia
0 200 400 600 800 1000
Control Seep Site
Net photosynthesis [µg O2 L-1 h-1 gFW-1] H. digitata
Figure 3.4: In-situ rates of net photosynthesis ofH. digitataandH. opuntiagrown at control and CO2 seep site
H. opuntia
*
Control Seep Site 14 **
18 22 26
H. digitata
**
0 5 10 15 20
**
4 6 8 10 12
Cinorg [%]
**
0 1 2 3 4
Control Seep Site Corg : CinorgCorg [%]Ctot [%]
Figure 3.5: Total-, organic- and inorganic carbon content and Corg: Cinorg ratio ofH. digitataandH.
opuntiagrown at control and CO2seep site. Brackets indicate significant differences in ANOVAs, with significance levels∗p<0.05,∗∗p<0.001,∗ ∗ ∗p<0.0001
the impacts of OA on Halimedaspp. also arrived at varying conclusions, with some suggesting that growth and calcification of severalHalimedaspp. may be impacted under future CO2conditions (Price et al. 2011; Ries et al. 2009; Sinutok et al. 2011), while others suggest that several others are unlikely to be impacted by OA alone (Comeau et al. 2013; Hofmann et al. 2014; Vogel et al. 2015). Morphological distinctions, such as surface area to volume ratio of phylloids may contribute to different responses of different Halimeda species to OA where thicker phylloids may reduce OA impacts. In addition, different morphologies affect diffusion of inorganic carbon to sites of calcification and photosynthesis.
***
Control Seep In−situ H. opuntia
*
CC CI
Transplant
**
−28
−26
−24
−22
−20
−18
Control Seep In−situ δ13C
H. digitata
**
−28
−26
−24
−22
−20
−18
CC CI
Transplant δ13C
Figure 3.6:δ13C andδ15N signatures ofH. digitataandH. opuntiagrown at control and CO2seep site and transplanted from control to control and control to seep site. Brackets indicate significant differences in ANOVAs, with significance levels∗ p<0.05,∗∗ p<0.001,∗ ∗ ∗p<0.0001
Moreover, different organisms possess different mechanisms of calcification. While aragonite deposition inHalimedatakes place in the interutricular spaces (Borowitzka 1989),Padinacalcification is initiated intracellular (Okazaki et al. 1986) and corals deposit CaCO3at their calicoblastic epithelium (Allemand et al. 2004). However, in this studyHalimedagrowing at the seep sites did not show any pattern related to their morphology, and included lightly and heavily calcifying species, as well as rock-anchoring and sand-dwelling species. Our measured seawater carbonate system parameters provide evidence for the existence of Halimedain high CO2 environments, suggesting several tropicalHalimedaspp. can acclimatize to future OA conditions. This observation is in agreement with a previous study on the slightly calcareous brown algae Padinasp., which occurs at volcanic CO2 seep sites in PNG and the at the Mediterranean (Johnson and Carpenter 2012). However, seep sites investigate the effects of OA in isolation and it is possible that other co-occurring factors predicted for the future (e.g. warming or increase of terrestrial runoff) may interact to affectHalimedaspp.
We investigated H. digitata and H. opuntia physiology in detail since they were most abundant
at both control and seep site. By selecting the most abundant species the potential of a bias towards more resilient species cannot be excluded. Nevertheless, occurrence ofH. cylindracea,H. hederacea, H. macroloba and H. undulata at the seep sites suggests that several other species can tolerate this particular environment. Net- and gross photosynthesis of both, H. digitata and H. opuntia, did not differ between control and seep site. Increased dissolved inorganic carbon (DIC) availability did not positively affect the photosynthesis ofHalimedaspp. grown at volcanic seep sites incubated in otherwise present environmental conditions (i.e. present light conditions). In contrast, previous studies observed increased productivity of benthic foraminifera at the Upa-Upasina seep site, suggesting endosymbiotic algae hosted by foraminifera may be carbon limited and thus benefit from increased DIC availability (Uthicke and Fabricius 2012). Similar results were observed in an experiment with coral Acropora eurystoma, which showed increased photosynthesis in elevated DIC concentrations, presuming carbon limitation of zooxanthellae in ambient water conditions (Chauvin et al. 2011). As shown by Borowitzka and Larkum (1976b)Halimeda tunaphotosynthesis saturates at DIC≤3 mmol L−1(DIC∼1900 µmol kgSW−1in present study, kgSW−1=per kg seawater).Halimedaphotosynthesis utilizes dissolved CO2 as the primary carbon source however HCO3– can also be used, but at a reduced rate (Borowitzka and Larkum 1976b). Moreover, in experimentsH. tunaphotosynthesis saturated at 27 µmol L−1 CO2and 2274 µmol L−1HCO3–(Borowitzka and Larkum 1976b), both indicating photosynthesis should be DIC limited under present environmental conditions at control sites (CO2=7.78 µmol kgSW−1, Table 3.1).
Potentially, ambient PAR level of experimental incubations forH. digitataandH. opuntia(39 and 281 µmol photons m−2s−1, respectively) were below light saturation and organisms were subjected to light limitation before DIC limitation could be observed.
In-situ calcification rates showed that both H. digitata andH. opuntiahad increased calcification rates in the light at the seep compared to the control site. This is an indication that calcification of some Halimedaspp. may benefit from increased DIC availability. Increased bicarbonate concentrations at the seep site may thus have relieved the organisms of limiting conditions for calcification. Borowitzka and Larkum (1976b) showed thatH. tuna calcification is saturated at about 5 mmol L−1 ∑CO2, indi-cating carbon limitation at control conditions of the present study (DIC = 1.892 mmol kgSW−1, Table 3.1). Calcification inHalimedaspp. is dependent on diffusion of CO32–and Ca2+into the intercellular space, suggesting the supply of DIC can become limiting (Borowitzka and Larkum 1977; Borowitzka and Larkum 1976a; Borowitzka and Larkum 1976b). Thus, elevated DIC concentrations at seep sites (DIC=2163 µmol kgSW−1, Table 3.1) may explain increased calcification rates ofH. digitataandH.
opuntia, compared to control sites. However, low water motion in chambers may also have increased the thickness of boundary layers on the organisms’ surface and thus exacerbated the positive effect of elevated DIC on calcification as discussed by Langdon and Atkinson (2005) and seen for coral
photosyn-thesis (Chauvin et al. 2011). Therefore, potentially a combination of DIC under-saturation at ambient seawater conditions (1.892 mmol kgSW−1) and increased boundary layers in incubation chambers may have resulted in increased calcification rates at the seep site, as presumed by Chauvin et al. (2011).
In contrast incubations in darkness showed calcification rates ofH. opuntiawere strongly and nega-tively impacted by decreased pH leading to decreased calcification and dissolution at the seep compared to the control site. Positive calcification rates were still observed at the control site in darkness, despite respiratory CO2 release. While Borowitzka (1986) showed some decalcification in ambient seawater conditions due to respiratory CO2during the night, he also showed much of the DIC, which is released into the intracellular space, can be refixed in the morning. A potential reason whyH. opuntiashowed significant impacts of elevated pCO2during darkness, butH. digitatadid not may emerge from the dif-ferent morphology of both species. H. opuntiaphylloids have a larger surface area to volume ratio and thus calcified areas are more exposed to their physical environment. This may have an advantage dur-ing the day, when a proportionally larger surface area facilitates diffusion processes and thus increases productivity and calcification. However, at night this property may be a disadvantage, where a higher exposure to elevated CO2conditions, may increase negative impacts, as seen in the present study. The observed CaCO3 dissolution of H. opuntia in the present study is in agreement with a laboratory ex-periment, which showed no negative effect of OA on two photosynthesizing and calcifying organisms (Acropora millepora andH. opuntia) in the light, but during the dark (Vogel et al. 2015). The latter study also observed this phenomenon in incubation conditions with water movement, suggesting low water motion did not exacerbate dissolution in darkness in the present study. Moreover, this observation agrees with results from Borowitzka and Larkum (1976b), which showed that respiration can inhibit calcification ofHalimedaby decreasing pH and [CO32–] and presumed that respiratory CO2production could lead to CaCO3dissolution. In contrast, during light no negative impacts of OA on calcification could be observed. Photosynthesis may thus offset impacts of OA by buffering pH during light, increase Ωarand therefore facilitate deposition of CaCO3(Al-Horani et al. 2003; Borowitzka and Larkum 1976b;
Goreau 1959; Vogel et al. 2015).
Calculated net calcification rates did not differ between control and seep site for neither H. digi-tata, norH. opuntia. Increased light calcification and decreased dark calcification rates at the seep site cancelled out each other to no difference of net calcification rates between sites. This observation is in agreement with results derived from laboratory experiments onH. opuntia(Vogel et al. 2015), and re-emphasizes our in-situ observations that showHalimedaspp. are capable to grow and calcify at high CO2.
Elevated CO2showed opposite effects on inorganic carbon content of the two species with increased CinorginH. digitata, but decreased values inH. opuntiaat the seep site, compared to controls. CaCO3
dissolution during the dark may lead to a marginally lowered Cinorg content ofH. opuntia. Similarly, increased Cinorgcontent ofH. digitataat the seep site may be explained by elevated calcification rates during the light at the seep site. Decreased Cinorgcontent (despite unaffected net calcification rates) of H. opuntiawas previously observed by Hofmann et al. (2014). Moreover, a previous study onPadina showed lower CaCO3 content at PNG seep sites compared to controls (Johnson and Carpenter 2012).
Increased Cinorgcontent ofH. digitatais in contrast to previously discussed observations, but is in agree-ment with the calcification rates measured, showing a trend (non-significant) towards slightly increased net calcification rates at the seep compared to control site. Decreased Corgand increased CinorgofH.
digitataalso reflected in a decreased Corg: Cinorgratio at the seep site compared to controls. Notably, despite changes in CinorgofH. digitataandH. opuntia, both were still capable to grow and to deposit CaCO3even in conditions temporary corrosive to aragonite (Ωarunder saturation).
BothH. digitataandH. opuntiatissues showed increased negativeδ13C signatures (i.e. increased fractionation of carbon isotopes) at the seep compared to the control site, indicating either13C depletion or proportionally higher12C in tissues. In addition, tissues of both species showed depletion in13C after 14 day transplantation to the seep site, while thalli that remained at the control site showed the same isotopic signatures as originally determined. Thus, we showed that the environment at the seep site led to a depletion of13C and an increased fractionation of carbon isotopes inHalimedaspp. tissue compared to controls and that these changes are detectable after as little as 14 days. This was most likely due to increased CO2availability at the seep site. Since CO2is isotopically light compared to HCO3–(∼10h) (Laws et al. 2002) an increased fractionation of carbon isotopes indicates an increased utilization of CO2 over HCO3– at the seep site. This observation is an indication that Halimedaspp. may benefit from increased CO2availability at the seep site for photosynthetic carbon acquisition and organic carbon assimilation in their tissue. InHalimedaspp. photosynthesis utilizes CO2as primary source of inorganic carbon. Therefore, elevated CO2availability at the seep sites may facilitate the diffusion process and thus the uptake of CO2compared to the control sites. This observation has also been demonstrated for non-calcifying algae (Carvalho et al. 2010). Theoretically, calcification may alter fractionation ofδin organic tissue due to supply of CO2for photosynthesis derived from heavier HCO3–during calcification (Ca2++ 2 HCO3– −−→CaCO3+CO2+H2O) (Laws et al. 2002). However, Laws et al. (2002) also provide evidence that calcification does not supply heavier CO2from calcification for photosynthesis (Buitenhuis et al. 1999; Riebesell and Wolf-Gladrow 1995). Unaltered rates of net photosynthesis suggested that both species did not benefit from elevated CO2at the seep site and thus were not DIC (i.e. CO2) limited under the experimental conditions. However, as discussed above, it is possible that the light conditions during incubations were below saturation explaining why DIC limitation in net photosynthesis was not detected. Nonetheless, carbon isotope signatures from transplants indicateHalimedaspp. may benefit
from increased CO2at the seep site, when integrated over several days.
With this study we provide evidence that several Halimedaspp. are tolerant of increasing pCO2. Some species (e.g.H. opuntia) that are found at the seep site are reported to be sensitive to OA. However, this conclusion is derived from laboratory experiments in artificial conditions, while the results from the present study are based on long-term exposure in a natural environment with natural light, nutrient and flow regimes. Therefore, we suggest re-evaluating the impact of OA as single stressor onHalimedaspp.
However, in future environmental conditions, organisms will not only have to deal with OA, but also with other environmental stressors, such as ocean warming and land runoff, which may have additive or synergistic effects. Additional investigations are necessary to evaluate impacts of several stressors combined.
Acknowledgments
We thank the crew of the M.V. Chertan for their sincere hospitality and their professional help in con-ducting this study. We are grateful to the local families at Dobu Island and Upa-Upasina for approving our work in their neighborhood. Many thanks to Craig Humphrey for his support during the field work.
We thank Peter Davern, Mick Donaldson and Peter Coumbis for their help concerning the shipment of our experimental equipment and legal advice. We thank the Leibniz Center for Tropical Marine Ecol-ogy, Dorothee Dasbach and Friedrich Meyer for helping with elemental and stable isotope analyzes.
The study was funded by the Australian Institute of Marine Science and conducted with the support of funding from the Australian Government’s National Environmental Research Program.
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Decreased light availability can amplify negative impacts of ocean acidification on calcifying coral reef organisms
Nikolas Vogel1,2,3, Friedrich Wilhelm Meyer2,3Christian Wild2,3and Sven Uthicke1
(1)Australian Institute of Marine Science, PMB 3, Townsville MC, Queensland 4810, Australia
(2)Leibniz Center for Tropical Marine Ecology, Fahrenheitstraße 6, 28359 Bremen, Germany
(3)Faculty of Biology and Chemistry (FB 2), University of Bremen, 28359 Bremen, Germany
Keywords: pH, turbidity, calcification, dissolution, photosynthesis, corals, algae, Acropora millepora, Halimeda opuntia
This chapter is accepted for publication in Marine Ecology Progress Series (2015) doi: 10.3354/meps11088
Abstract
Coral reef organisms are increasingly and simultaneously affected by global and local stressors, such as ocean acidification (OA) and reduced light availability. However, knowledge of the interplay between OA and light availability is scarce. We exposed two calcifying coral reef species (the scleractinian coral Acropora milleporaand the green algaHalimeda opuntia) to combinations of ambient and increased pCO2(427 and 1073 µatm, respectively), and two light intensities (35 and 150 µmol photons m−2s−1) for 16 days. We evaluated the individual and combined effects of these two stressors on weight increase, calcification rates, O2fluxes and chlorophyll content for the species investigated. Weight increase ofA.
milleporawas significantly reduced by OA (48%) and low light intensity (96%) compared to controls.
While OA did not affect coral calcification in the light, it decreased calcification in the dark by 155%, leading to dissolution of the skeleton.H. opuntiaweight increase was not affected by OA, but decreased (40%) at low light. OA did not affect algae calcification in the light, but decreased calcification in the dark by 164%, leading to dissolution. Low light significantly reduced gross photosynthesis (56 and 57%), net photosynthesis (62 and 60%) and respiration (43 and 48%) ofA. milleporaandH. opuntia, respectively. In contrast toA. millepora,H. opuntiasignificantly increased chlorophyll content by 15%
over the course of the experiment. No interactive effects of OA and low light intensity were found on any response variable for either organism. However,A. milleporaexhibited additive effects of OA and low light, whileH. opuntiawas only affected by low light. Thus, this study suggests that negative effects of low light and OA are additive on corals, which may have implications for management of river discharge into coastal coral reefs.