Rhodoliths (from 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, [1] 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. [2] [3] While corals are animals that are both autotrophic (photosynthesize via their symbionts) 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. [4]
Rhodoliths (including maërl) have been defined as calcareous nodules composed of more than 50% of coralline red algal material and consisting of one to several coralline species growing together. [5] [6]
Rhodolith beds have been found throughout the world's oceans, including in the Arctic near Greenland, in waters off British Columbia, Canada, the Gulf of California, Mexico, [7] the Mediterranean [8] as off New Zealand [9] and eastern Australia. [10] Globally, rhodoliths fill an important niche in the marine ecosystem, serving as a transition habitat between rocky areas and barren, sandy areas. Rhodoliths provide a stable and three-dimensional habitat onto and into which a wide variety of species can attach, including other algae, commercial species such as clams and scallops, and true corals. [4] Rhodoliths are resilient to a variety of environmental disturbances, but can be severely impacted by harvesting of commercial species. For these reasons, rhodolith beds deserve specific actions for monitoring and conservation. [11] [12] [13] [14] Rhodoliths come in many shapes, including laminar, branching and columnar growth forms. [15] In shallow water and high-energy environments, rhodoliths are typically mounded, thick or unbranched; branching is also rarer in deeper water, and most profuse in tropical, mid-depth waters. [1]
Rhodoliths are a common feature of modern and ancient carbonate shelves worldwide. [16] Rhodolith communities contribute significantly to the global calcium carbonate budget, and fossil rhodoliths are commonly used to obtain paleoecologic and paleoclimatic information. [17] [18] [19] Under the right circumstances, rhodoliths can be the main carbonate sediment producers, [20] [21] often forming rudstone or floatstone beds consisting of rhodoliths and their fragments in grainy matrix.
Rhodoliths are significant photosynthesizers, calcifiers, and ecosystem engineers, which raises an issue about how they might respond to ocean acidification. [23]
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 pH levels and a lower carbonate saturation in the seawater. These affect the calcification process. [24] Organisms like rhodoliths accrete carbonate as part of their physical structure, since precipitating CaCO3 would be less efficient. [25] [26] Ocean acidification presents a threat by potentially affecting their growth and reproduction. [27] [28] Coralline algae are particularly sensitive to ocean acidification because they precipitate high magnesium-calcite carbonate skeletons, the most soluble form of CaCO3. [29] [30] [23]
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. [31] Elevated CO2 levels might impair biomineralization due to decreased seawater carbonate (CO2−
3) availability as pH falls, but photosynthesis could be promoted as the availability of bicarbonate (HCO−
3) increases. [32] 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. [33] [23]
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. [34] [23]
In 2018 the first metagenomic analysis 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. [23]
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. [23]
Coccolithophores, or coccolithophorids, are single-celled organisms which are part of the phytoplankton, the autotrophic (self-feeding) component of the plankton community. They form a group of about 200 species, and belong either to the kingdom Protista, according to Robert Whittaker's five-kingdom system, or clade Hacrobia, according to a newer biological classification system. Within the Hacrobia, the coccolithophores are in the phylum or division Haptophyta, class Prymnesiophyceae. Coccolithophores are almost exclusively marine, are photosynthetic, and exist in large numbers throughout the sunlight zone of the ocean.
Coralline algae are red algae in the order Corallinales. They are characterized by a thallus that is hard because of calcareous deposits contained within the cell walls. The colors of these algae are most typically pink, or some other shade of red, but some species can be purple, yellow, blue, white, or gray-green. Coralline algae play an important role in the ecology of coral reefs. Sea urchins, parrot fish, and limpets and chitons feed on coralline algae. In the temperate Mediterranean Sea, coralline algae are the main builders of a typical algal reef, the Coralligène ("coralligenous"). Many are typically encrusting and rock-like, found in marine waters all over the world. Only one species lives in freshwater. Unattached specimens may form relatively smooth compact balls to warty or fruticose thalli.
Ocean acidification is the ongoing decrease in the pH of the Earth's ocean. Between 1950 and 2020, the average pH of the ocean surface fell from approximately 8.15 to 8.05. Carbon dioxide emissions from human activities are the primary cause of ocean acidification, with atmospheric carbon dioxide levels exceeding 410 ppm. CO2 from the atmosphere is absorbed by the oceans. This produces carbonic acid which dissociates into a bicarbonate ion and a hydrogen ion. The presence of free hydrogen ions lowers the pH of the ocean, increasing acidity. Marine calcifying organisms, such as mollusks and corals, are especially vulnerable because they rely on calcium carbonate to build shells and skeletons.
Maerl is a collective name for non-geniculate coralline red algae with a certain growth habit. Maerl grows at a rate of c. 1 mm per year. It accumulates as unattached particles and forms extensive beds in suitable sublittoral sites. The term maerl originally refers to the branched growth form of Lemoine (1910) and rhodolith is a sedimentological or genetic term for both the nodular and branched growth forms.
The spotted sea hare is a species of sea slug in the family Aplysiidae, the sea hares. It reaches a length of up to 20 cm (7.9 in) and is found in the northeast Atlantic, ranging from Greenland and Norway to the Mediterranean Sea.
Marine chemistry, also known as ocean chemistry or chemical oceanography, is influenced by plate tectonics and seafloor spreading, turbidity currents, sediments, pH levels, atmospheric constituents, metamorphic activity, and ecology. The field of chemical oceanography studies the chemistry of marine environments including the influences of different variables. Marine life has adapted to the chemistries unique to Earth's oceans, and marine ecosystems are sensitive to changes in ocean chemistry.
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The oceanic carbon cycle is composed of processes that exchange carbon between various pools within the ocean as well as between the atmosphere, Earth interior, and the seafloor. The carbon cycle is a result of many interacting forces across multiple time and space scales that circulates carbon around the planet, ensuring that carbon is available globally. The Oceanic carbon cycle is a central process to the global carbon cycle and contains both inorganic carbon and organic carbon. Part of the marine carbon cycle transforms carbon between non-living and living matter.
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Estuarine acidification happens when the pH balance of water in coastal marine ecosystems, specifically those of estuaries, decreases. Water, generally considered neutral on the pH scale, normally perfectly balanced between alkalinity and acidity. While ocean acidification occurs due to the ongoing decrease in the pH of the Earth's oceans, caused by the absorption of carbon dioxide (CO2) from the atmosphere, pH change in estuaries is more complicated than in the open ocean due to direct impacts from land run-off, human impact, and coastal current dynamics. In the ocean, wave and wind movement allows carbon dioxide (CO2) to mixes with water (H2O) forming carbonic acid (H2CO3). Through wave motion this chemical bond is mixed up, allowing for the further break of the bond, eventually becoming carbonate (CO3) which is basic and helps form shells for ocean creatures, and two hydron molecules. This creates the potential for acidic threat since hydron ions readily bond with any Lewis Structure to form an acidic bond. This is referred to as an oxidation-reduction reaction.
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