Paper Reviewed
Klein, S.G., Pitt, K.A., Nitschke, M.R., Goyen, S. Welsh, D.T., Suggett, D.J. and Carroll, A.R. 2017. Symbiodinium mitigate the combined effects of hypoxia and acidification on a noncalcifying cnidarian. Global Change Biology 23: 3690-3703.

The upside down jellyfish (Cassiopea sp.) tends to inhabit warm shallow coastal waters and lagoons, which waters typically exhibit large fluctuations in dissolved oxygen and pH in consequence of the relatively high input of nutrients from land/water runoff, which nutrients foster a disproportionate production of organic matter and microbial remineralization processes that deplete oxygen (hypoxia) from the water column and reduce seawater pH (acidification). However, not much is known regarding the biological responses of this and other organisms to the combined effects of hypoxia and ocean acidification, because most experiments to date, in the words of Klein et al. (2017), have "not accurately replicated the water chemistry associated with hypoxia [and ocean acidification]." Such failures include (1) reducing the dissolved oxygen (DO) content of the water but not the pH and/or (2) maintaining a constant DO and/or pH value instead of replicating the inherent variation in these parameters, which typically exhibit a widely fluctuating range on a daily basis.

In an attempt to overcome these experimental biases, Klein et al. set out to examine the impact of reduced DO and pH on Cassiopea sp. in a full-factorial design that mimicked the daily variability of pH and DO in current-day hypoxic and low-pH ecosystems. Their experiment was conducted over a period of 22 days under controlled laboratory conditions where the "absolute values and extent of diel variability for pH and DO of control treatments were replicated based on 24-hr field measurements taken in October 2014 in Moreton Bay, Australia." The daily mean values for pH and DO were 7.95 and 6.14 mg/L under control conditions and 7.63 and 2.09 mg/L under acidic and hypoxic conditions, respectively. The upside down jellyfish was chosen as the investigative marine species so that the authors could see if its host symbiotic dinoflagellates (e.g., Symbiodinium spp., photosynthetic marine phytoplankton) would respond positively to elevated pCO2 (reduced pH) and release higher amounts of DO that could potentially offset or mitigate the oxygen deficit that exists during hypoxic events. Specifically, the work involved examining the fitness responses of both symbiotic and aposymbiotic (without symbionts) polyps of Cassiopea sp. under the varying experimental conditions of DO and pH. And what did their study reveal?

According to Klein et al., host fitness response (asexual reproduction of Cassiopea sp. polyps) "was considerably higher when Symbiodinium [were] present, but the magnitude of the difference was greater in low pH conditions." Quantitatively, they report that "acidification alone resulted in 58% more symbiotic polyps than aposymbiotic polyps and enhanced numbers of Symbiodinium cells per polyp." They also found that hypoxia alone reduced host fitness regardless of acidification and symbiont status (with or without symbionts), suggesting that "Symbiodinium photosynthetic activity did not mitigate the negative effects of hypoxia." More importantly, however, they found that "hypoxia and acidification in combination produced as many symbiotic polyps as the aposymbiotic polyps kept under ambient conditions." Hence, they say that "by enhancing photosynthetic activity, exposure to elevated CO2 appears to [have] offset the negative effects of hypoxia in taxa that host Symbiodinium." And they thus conclude that their observations "suggest that Cassiopea sp., and perhaps other symbiotic noncalcifying cnidarians, may still thrive when hypoxia and acidification co-occur," adding that "noncalcifying cnidarians may be particularly advantaged in productive coastal waters that are subject to simultaneous hypoxia and acidification."