Species‐specific photosynthetic responses of symbiotic zoanthids to thermal stress and ocean acidification

Abstract

Increasing sea‐surface temperatures and ocean acidification (OA) are impacting physiologic processes in a variety of marine organisms. Many sea anemones, corals and jellies in the phylum Cnidaria form endosymbiotic relationships with Symbiodinium spp. (phylum Dinoflagellata) supply the hosts with fixed carbon from photosynthesis. Much work has focused on the generally negative effects of rising temperature and OA on calcification in Symbiodinium‐coral symbioses, but has not directly measured symbiont photosynthesis in hospite or fixed carbon translocation from symbiont to host. Symbiodinium species or types vary in their environmental tolerance and photosynthetic capacity; therefore, primary production in symbiotic associations can vary with symbiont type. However, symbiont type has not been identified in a large portion of Symbiodinium−cnidarian studies. Future climate conditions and OA may favor non‐calcifying, soft‐bodied cnidarians, including zoanthids. Here we show that two zoanthid species, Palythoa sp. and Zoanthus sp., harboring different symbiont types (C1 and A4), had very different responses to increased temperature and increased partial pressure of CO2 (pCO2), or dissolved CO2, and low pH. Thermal stress did not affect carbon fixation or fixed carbon translocation in the Zoanthus sp./A4 association, and high pCO2/low pH increased carbon fixation. In contrast, both thermal stress and high pCO2/low pH greatly inhibited carbon fixation in the Palythoa sp./C1 association. However, the combined treatment of high temperature and high pCO2 increased carbon fixation relative to the treatment of high temperature alone. Our observations support the growing body of evidence that demonstrates that the response of symbiotic cnidarians to thermal stress and OA must be considered on a host‐specific and symbiont‐specific basis. In addition, we show that the effects of increased temperature and pCO2 on photosynthesis may change when these two stressors are combined. Understanding how carbon fixation and translocation varies among different host−symbiont combinations is critical to predicting which Symbiodinium associations may persist in warm, acidified oceans.

Carbonic anhydrase activity changes in response to increased temperature and pCO2 in Symbiodinium–zoanthid associations

“…However, the effects of temperature and OA on both host and symbiont CA activity in cnidarian-Symbiodinium associations have not been examined. Species-specific changes in carbon fixation in zoanthid-Symbiodinium associations with increased temperature and OA were shown in a recent study, suggesting that differences in carbon fixation could be related to changes in CA activity (Table 1; Graham and Sanders, 2015). In the present study, the hypothesis that under normal environmental conditions, total CA activity differs between the zoanthids Palythoa sp.…”

“…in hospite that rely on their hosts for DIC acquisition and transport. Research has shown that DIC enrichment enhances primary production in several phytoplankton species (reviewed in Riebesell, 2004 andRost et al, 2008), however the effects of increased CO 2 on photosynthesis in symbiotic cnidarians have been mixed (Graham and Sanders, 2015;Anthony et al, 2008;Herfort et al, 2008;Langdon and Atkinson, 2005;Reynaud et al, 2003;Rodolfo-Metalpa et al, 2010;Schneider and Erez, 2006). One reason for this may be that the benefit of increased DIC associated with ocean acidification may be outweighed by the concomitant decrease in pH, which affects most physiological processes.…”

“…The effects of increased temperature and low pH on carbon fixation and percent fixed carbon translocated in Palythoa/C1 and Zoanthus/A4 associations, as reported in Graham and Sanders, 2015 Three target pH values were selected for pCO 2 experiments: 8.1 (control), 7.85, and 7.65. Temperature in all tanks was 27°C.…”

“…For details on extraction, primers and PCR conditions, see Graham and Sanders (2015). The Symbiodinium clade/s in each zoanthid species was determined by amplifying the internal transcribed spacer 2 region (ITS 2) and using denaturing gradient gel electrophoresis (DGGE).…”

Effects of seawater pCO2 and temperature on calcification and productivity in the coral genus Porites spp.: an exploration of potential interaction mechanisms

“…Symbiont photosynthesis may be limited by light or nutrients (rather than carbon) in the culture seawater (Chauvin et al 2011) or may be affected by symbiont type. The growth and photosynthetic response of Symbiodinium to elevated pCO 2 is type specific in culture (Brading et al 2011) and in hospite (Graham and Sanders 2016).…”