[Coral-List] How can we "bleach proof" transplanted coral nurseries?

[Scott Wooldridge]

Dear fellow coral researchers,

I have received numerous emails asking to suggest possible ways that we may
be able to enhance the thermal bleaching resistance of transplanted corals
- if as i suggest, intracellular CO2-limitation of the endosymbiont
phyotosynthetic machinery is the underpinning mechanism.

Just drawing quickly attention back to the following manuscripts:

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

https://www.researchgate.net/publication/317100418_Instability_and_breakdown_of_the_coral-algae_symbiosis_upon_exceedence_of_the_interglacial_pCO2_threshold_260_ppmv_the_%27%27missing%27%27_Earth-System_feedback_mechanism

It is clear, that the biological challenge is to keep endosymbiont
(symbiodinium) levels at optimal levels (~1.5 x10^6 cells.cm2 host tissue
in branching corals). Two factors in combination promote 'excess' densities
- elevated pCO2 and dissolved inorganic nutrients (principally nitrogen)
concentrations.

Thus, the bioengineering challenge is also clear. We need to lower pCO2 and
DIN in the immediate vicinity of transplanted corals (or coral reefs in
general).

In my opinion (and being a pessimist/realist? in my belief that
governmental institutions have the political will to reduce pCO2 or DIN
runoff to the levels required for a stable coral symbiosis) the only hope
we have is to investigate the beneficial role that co-transplanted seaweeds
/ macro-algae / crustose coraline algae can play.

Active seaweed growth has the potential to draw down both DIN and pCO2 in
the seawater in their near vicinity. Note: seaweed precipitate no (or
little) CaCo3 (= source of CO2 to seawater)

The use of "algal scrubbers" to reduce DIN levels in aquarium systems is
common practice. Can we do something similar within our transplanted coral
nurseries and co-locate seaweeds?

Obviously, field research is needed here. But a couple of interesting
results are noteworthy, and suggest the successful outcomes may be possible.

Firstly, in the lab, Yuen et al. (2009) were able to demonstrate that algae
(in this case crustose coralline algae) when co-located with Acorpora
digitifera in small tanks were able to quickly (days) draw down DIN  (and
possibly pCO2?) levels. This resulted in the exact benefits expected if
intracellular CO2-limitation is a controlling feature of endosymbiont
behaviour. It increased photophysiology efficiency (Fv/Fm, ETR), and
reduced the level of bleaching and mortality compared to non treatment.

https://www.researchgate.net/publication/319255076_Published_manuscript_Yuen_et_al_2009_Effects_of_live_rock_on_the_reef-building_coral_Acropora_digitifera_cultured_with_high_levels_of_nitrogenous_compounds_Aquacultural_Engineering_4135-43

This is a promising result in the lab. But what of any field evidence?

There is an interesting dataset by Jompa and McCook (1998) who (quite by
accident) recorded much lower levels of bleaching and mortality in corals
that were surrounded by seaweed (particurlarly Sargassum spp) during the
1998 mass bleaching event on the inshore Great Barrier Reef. The authors
were at the time undertaking another unrelated experiment that involved
manually removing seaweed from some reef sites. They observed a dramatic
(2-3 fold) increase in the level of coral bleaching at sites from which
seaweeds had been removed.

https://www.researchgate.net/publication/319255226_The_beneficial_role_of_seaweeds_in_reducing_coral_bleaching_on_the_inshore_reefs_of_the_Great_Barrier_Reef_1998

The authors speculated that the seaweed may have been providing a shading
benefit (and this may be true). However, could it also have been the case
that the seaweed had reduced seawater DIN and pCO2 in the vicinity of the
corals? I think we need (with haste) to test this possibility.

One issue may be that we need the seaweed to be in an active growing state
to be of real benefit. For example, in artificial wetlands used to treat
nutrient effluent it is well know that nutrient uptake rates into plant
biomass are high initially and then tail off. This necessitates (for this
system) that the plants be harvested and removed at the end of each growing
season. Luckily for us, seaweeds tend to die back in the winter months
naturally and we may not need to worry about this? Anyway, now i am just
guessing.

I hope this may be of some benefit in sparking the challenge to that
eco-engineer/entrepreneur.

scott

https://www.researchgate.net/profile/Scott_Wooldridge

Cited Literature

Jompa and McCook (1998) Seeweeds save the reef. Unpublished report.

Yuen et al. (2009) Effects of live rock on the reef-building coral Acropora
digitifera cultured with high levels of nitrogenous compounds. Aquacultural
Engineering 41:35-43