[Coral-List] Can water quality improvements leading to zooxanthellae reductions in corals increase bleaching thresholds by 1-2 degree Celsius?

[Scott Wooldridge]

Dear CoralListers,

At the fear of becoming the resident Coral List Jester, i challenge you to
think on a this previous post suggestion.

In that, i suggested that my empirical analyses indicates that ~ 1-2
degrees celsius can be added to the upper thermal bleaching threshold of
coral reefs, if they are nutrient-limited (oligotrophic). More
specifically, if  zooxanthellae densities fall below key symbiotic
thresholds (~1.0-1.6x10^6 cells/cm2 of host tissue for thin-tissue
branching/plating corals). Or considered differently, roughly 1 zoox per
host coral gastrodermal cell.

https://www.researchgate.net/publication/308746785_Excess_seawater_nutrients_enlarged_algal_symbiont_densities_and_bleaching_sensitive_reef_locations_1_Identifying_thresholds_of_concern_for_the_Great_Barrier_Reef_Australia

One interesting place to consider this idea, is to look at seasonal
differences in bleaching thresholds. For the reefs nearby Townsville (Palm
Island group, GBR), it is well established that zooxanthellae densities
roughly double in size between summer and winter for branching corals like
P.damicornis and A.millepora. (Berkelmans and Willis 1999; Pillay et al.
2005). This can be seen graphically for the field data of Pillay et al.
(2005):

https://www.researchgate.net/publication/325032990_Figure_2_from_Pillay_et_al_2005_Trends_in_the_Density_of_Zooxanthellae_in_Acropora_millepora_Ehrenberg_1834_at_the_Palm_Island_Group_Great_Barrier_Reef_Australia_Symbiosis_38_209-226

Zoox densities ~2x10^6/cm2 host tissue (summer) versus ~4-5x10^6/cm2 host
tissue (winter).

INTERESTINGLY when Berkelmans and Willis (1999) thermally challenged these
same corals in summer vs winter, they found that the upper thermal
bleaching threshold was ~1 -2 deg lower in winter than summer. So in less
than 6 months, the upper thermal threshold drops by more than 1 degree.

Now critics will rightly say that other factors might contribute to this
response (e.g. MAAs, etc). But how intriguing.

And there are many other experiments that show similar (fast) responses
based on changes in zoox densities. I directly cut and paste the following
from the discussion in my 2010 published BioEssays manuscript.

'The capacity of the host to enhance thermal bleaching resistance by
constraining absolute densities of zooxanthellae (independent of the
zooxanthellae type) is demonstrated by the known acclimatisation response
of corals in which pre-exposure to environmental conditions that induce a
lowering of the endosymbiont population helps to reduce the future
susceptibility to bleaching. For example: (i) Coles and Jokiel [100]
highlight that pre-treatments which left Montipora verrucosa semi-bleached
assisted survival at higher temperatures; (ii) Brown et al. [101]
demonstrate that exposure to a high solar radiation event (that lowered
symbiont densities by 15–20%) prior to maximal seasonal sea water
temperatures helped to limit thermal bleaching in Goniastrea aspera; (iii)
Ulstrup et al. [102] demonstrate that regional-scale differences in
densities of zooxanthellae directly relate to the bleaching susceptibility
in Pocillopora damicornis, with the corals that hosted larger zooxanthellae
densities in the field showing the earliest and severest thermal stress in
the laboratory and (iv) Berkelmans and Willis [103] found that during
winter (when symbiont densities are typically higher by 20–50%) [104], the
bleaching threshold for P. damicornis was 1.8C lower than the summer
threshold. In each of these examples, the zooxanthellae type was presumed
to be uniform across treatments, highlighting that within reasonable
functional limits, lower densities of zooxanthellae may confer greater
bleaching resistance on many modern symbiotic corals. Moreover, it alludes
to the loss of optimal host control of endosymbiont densities given modern
environmental conditions. Indeed, the emerging envelope of environmental
conditions – characterised by elevated sea surface temperatures, rising
pCO2 and increased levels of inorganic nutrients – are clearly antagonistic
to the outlined host processes that contribute demographic control to the
endosymbiont population'.

for these references, see my reference list in

https://www.researchgate.net/publication/44644540_Is_the_coral-algae_symbiosis_really_'mutually_beneficial'_for_the_partners

If you are a young experimental scientist, these results should be like a
red rag to a bull. Its time for some good, targeted experiments in this
research space. In the field if possible, or at the very least with flow
through tank systems with natural light (>900 umol.m-2.s-1). that is,
typical summer-winter irradiance levels

https://www.researchgate.net/publication/319178185_Summer_and_winter_irradiance_levels_-_Dry_Tortugas


scott

cited Literature

Berkelmans and Willis (1999) Seasonal and local spatial patterns in the
upper thermal limits of corals on the inshore Central Great Barrier Reef.
Coral Reefs 18, 219-228.

Pillay et al. (2005) Trends in the Density of Zooxanthellae in Acropora
millepora (Ehrenberg, 1834) at the Palm Island Group, Great Barrier Reef,
Australia. Symbiosis 38, 209-226.