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Rhodoliths (from Ancient Greek for red rocks) are colorful, unattached calcareous nodules, composed of crustose, benthic marine red algae that resemble coral. Rhodolith beds create biogenic habitat for diverse benthic communities. The rhodolithic growth habit has been attained by a number of unrelated coralline red algae, organisms that deposit calcium carbonate within their cell walls to form hard structures or nodules that resemble beds of coral.
Rhodoliths do not attach themselves to the rocky seabed. Rather, they roll like tumbleweeds along the seafloor until they become too large in size to be mobilised by the prevailing wave and current regime. They may then become incorporated into a semi-continuous algal mat or form an algal build-up. While corals are animals that are both autotrophic (photosynthesize via their ) or heterotrophic (feeding on plankton), rhodoliths produce energy solely through photosynthesis (i.e. they can only grow and survive in the photic zone of the ocean).
Scientists believe rhodoliths have been present in the world's oceans since at least the Eocene epoch, some 55 million years ago. Science Daily, September 23, 2004
Changes in ocean carbonate chemistry driven by increasing anthropogenic carbon dioxide emissions promotes ocean acidification. Increasing the ocean carbon dioxide uptake results in increases in pCO2 (the partial pressure of carbon dioxide in the ocean) as well as lower and a lower carbonate saturation in the seawater. These affect the calcification process. Organisms like rhodoliths accrete carbonate as part of their physical structure, since precipitating CaCO3 would be less efficient. Ocean acidification presents a threat by potentially affecting their growth and reproduction. Coralline algae are particularly sensitive to ocean acidification because they precipitate high magnesium-calcite carbonate skeletons, the most soluble form of CaCO3.Bischoff, W.D., Bishop, F.C. and Mackenzie, F.T. (1983) "Biogenically produced magnesian calcite; inhomogeneities in chemical and physical properties; comparison with synthetic phases". American Mineralogist, 68(11–12): 1183–1188
Calcification rates in coralline algae are thought to be directly related to their photosynthetic rates, but it is not clear how a high-CO2 environment might affect rhodoliths. Elevated CO2 levels might impair biomineralization due to decreased seawater carbonate () availability as pH falls, but photosynthesis could be promoted as the availability of bicarbonate () increases. This would result in a parabolic relationship between declining pH and coralline algal fitness, which could explain why varied responses to declining pH and elevated pCO2 have been recorded to date.
The widespread distribution of rhodoliths hints at the resilience of this algal group, which have persisted as chief components of benthic marine communities through considerable environment changes over geologic times.
In 2018 the first metagenomic of live rhodoliths was published. Whole genome shotgun sequencing was performed on a variety of rhodolith bed constituents. This revealed a stable live rhodolith microbiome thriving under elevated pCO2 conditions, with positive physiological responses such as increased photosynthetic activity and no calcium carbonate biomass loss over time. However, the seawater column and coralline skeleton biofilms showed significant microbial shifts. These findings reinforce the existence of a close host-microbe functional entity, where the metabolic crosstalk within the rhodolith as a holobiont could be exerting reciprocal influence over the associated microbiome.
While the microbiome associated with live rhodoliths remained stable and resembled a healthy holobiont, the microbial community associated with the water column changed after exposure to elevated pCO2.
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