[Coral-List] So you think you understand coral bleaching?

[Thomas Krueger]

Hi everyone,

in light of the resurging discussions on symbiont parasitism and carbon 
metabolism in corals, I felt like contributing three observations to 
this subject.

One, Baker et al. 2018 (https://doi.org/10.1038/s41396-018-0046-8) 
actually demonstrated that the host assimilation of symbiont-derived 
carbon and nitrogen under ambient and elevated temperature in shallow 
and deep corals is not significantly different, no matter what the 
symbiont population is doing in terms of their assimilation (Fig 1B, D). 
Rädecker et al 2018 (https://doi.org/10.3389/fphys.2018.00214) shows the 
same thing for the anemone host ExAiptasia CC7, harboring either 
symbiont ITS2 type A4 or B1. Host assimilation of C and N is not 
significantly different between harboring A4 or B1, despite the two 
symbiont types assimilating different amounts of carbon or nitrogen 
(Fig. 3 F, G, N, O) (albeit N=1 for each combination). My point here is 
to consider ALL of the raised data in a discussion about symbionts 
actively parasitizing their coral host. Labelling relationships as 
'mutualistic' or 'parasitic' is always scale-dependent and might vary 
for different aspects of the symbiosis.

Second, regarding the prior methodological limitations, I would like to 
highlight that it is possible to actually measure individual nutrient 
assimilation and cell metabolism (e.g. nicely done by Rädecker et al 
2018 for the anemone system using NanoSIMS) and finally fill conceptual 
paradigms with better, hard data. I definitely recommend the recent 
review by Decelle and co-authors 2020 as an overview on the 
methodological possibilities 
(https://doi.org/10.1016/j.tcb.2019.12.007). Admittedly, NanoSIMS data 
does not provide information on the actual amount of initially fixed 
material, but at least it quantifies remaining assimilated C and N in 
individuals cells of each partner. These methodologies have already been 
used for testing some of the pressing questions raised by Scott 
Woolridge and I love the discussion papers from him, because they 
provide actual testable hypotheses for the experimental folks. How much 
carbon does recycling of host respiratory CO2 contribute to symbiont C 
assimilation and where does it come from (i.e. host glycolysis vs. TCA 
cycle)? Using 13C labelled prey and pyruvate see Krueger et al. 2018 
https://doi.org/10.1038/s41598-018-31094-1 or Gibbin et al. 2020 
https://doi.org/10.1038/s42003-019-0742-6. Is there a limitation of 
seawater or respiratory CO2 for symbiont assimilation, depending on the 
overall density of symbiont cells in the host tissue? See Krueger et al. 
2020 under _ambient_ temperature conditions: 
https://doi.org/10.1098/rspb.2020.0049.

My third remark relates to the hypothesis of a "metabolic optimum" (i.e. 
the existence of an optimal density for coral "productivity"; sensu 
Woolridge 2017 https://doi.org/10.1007/s00338-017-1594-5). The core of 
this idea is really about the relationship between symbiont density and 
photosynthetic carbon assimilation. My problem with it (apart from the 
fundamental lack of a sufficiently large data set) was always that the 
whole literature and review discussions were almost exclusively based on 
oxygen data and P:R ratios; not on actual carbon fixation or 
assimilation measurements. Considering that some studies (Burris JE. 
1977 https://doi.org/10.1007/BF00391940, Crawley et al. 2010 
https://doi.org/10.1111/j.1365-2486.2009.01943.x, Schrameyer et al 2014 
https://doi.org/10.1371/journal.pone.0110814) have highlighted the 
potential importance of photorespiration in Symbiodiniaceae (not 
surprising in a type II RubisCO system with a carbon concentrating 
mechanisms), it is in my opinion extremely important to properly 
distinguish between photosynthetic measurements of oxygen evolution in 
photosystem II and actual carbon assimilation in the Calvin cycle when 
talking about coral productivity or density-related effects. Oxygen is 
not the primary currency in this nutritional symbiosis. When we talk 
about host sustenance from symbiont-derived photosynthates we mean 
primarily carbon.

All the best,

Thomas

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*Thomas Krueger */Research Associate/

Waller Lab | Department of Biochemistry | University of Cambridge

*P: *+44 (0)1223 768942