Abstract
Coral reefs are endangered by constantly rising water temperature due to global warming. This triggers a breakdown of the nutritional symbiosis between cnidarian hosts and their Symbiodiniaceae symbionts, resulting in the loss of the algal partner. In the Symbiodiniaceae exists a high genetic diversity with broad physiological plasticity within and between species, resulting in large thermal tolerance. While these variations have been studied in individual taxa, comprehensive comparative experimental data on numerous species are still rare. In the present study, the photosynthetic performance and tolerance as function of light and temperature of nine Symbiodiniaceae genetic types of four different clades were determined. The data indicate significant differences in the response patterns. Almost all algal isolates exhibited low to moderate light requirements for photosynthesis without photoinhibition, and a photosynthetic efficiency between 20 and 80% in the temperature range 20–34°C, indicating a broad thermal tolerance to temperature fluctuations in tropical regions. The presented data clearly point to a broad photophysiological tolerance and thermal plasticity of genetically different Symbiodiniaceae, which contributes as an important finding to a better understanding of host-symbiont response to an increasing sea surface temperature.
Highlights
Coral reefs are one of the most important ecosystems in the marine environment, as they are highly diverse and productive ecosystems (Connell, 1978; Moberg and Folke, 1999)
The average respiration rate at 20◦C ranged from −38.33 μmol O2 mg−1 Chl a h−1 in strain SSB 01 to −127.40 μmol O2 mg−1 Chl a h−1 in strain SSA 01, while the maximum photosynthesis rate NNPmax in the light saturated range was between 47.21 μmol O2 mg−1 Chl a h−1 in strain SSA 01 and 233.45 μmol O2 mg−1 Chl a h−1 in strain 1,046 (Figure 1)
The results indicate that all investigated strains have a broad temperature tolerance for photosynthesis and respiration
Summary
Coral reefs are one of the most important ecosystems in the marine environment, as they are highly diverse and productive ecosystems (Connell, 1978; Moberg and Folke, 1999). The high productivity of coral reef systems is based on a mutualistic symbiosis between unicellular dinoflagellates (Symbiodiniaceae) and different invertebrate reef species, including corals, sea anemones and sponges among many others (Trench, 1993; Coffroth and Santos, 2005; Decelle et al, 2018). The success of this symbiosis depends on complementary intracellular solute exchange such as nutrients and carbon compounds. The flexibility of the coral host, in particular, to associate with different photosymbiont strains at the same time could be critical to cope with rapid environmental fluctuations (Baker, 2001; Berkelmans and Van Oppen, 2006; Abrego et al, 2008; Lewis et al, 2019; Qin et al, 2019)
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