New branched Porolithon species (Corallinales,Rhodophyta) from the Great Barrier Reef,Coral Sea,and Lord Howe Island

Porolithon is one of the most ecologically important genera of tropical and subtropical crustose (non-geniculate) coralline algae growing abundantly along the shallow margins of coral reefs and functioning to cement reef frameworks. Thalli of branched, fruticose Porolithon specimens from the Indo-Pacific Ocean traditionally have been called P. gardineri, while massive, columnar forms have been called P. craspedium. Sequence comparisons of the rbcL gene both from type specimens of P. gardineri and P. craspedium and from field-collected specimens demonstrate that neither species is present in east Australia and instead resolve into four unique genetic lineages. Porolithon howensis sp. nov. forms columnar protuberances and loosely attached margins and occurs predominantly at Lord Howe Island; P. lobulatum sp. nov. has fruticose to clavate forms and free margins that are lobed and occurs in the Coral Sea and on the Great Barrier Reef (GBR); P. parvulum sp. nov. has short (<2 cm), unbranched protuberances and attached margins and is restricted to the central and southern GBR; and P. pinnaculum sp. nov. has a mountain-like, columnar morphology and occurs on oceanic Coral Sea reefs. A rbcL gene sequence of the isotype of P. castellum demonstrates it is a different species from other columnar species. In addition to the diagnostic rbcL and psbA marker sequences, the four new species may be distinguished by a combination of features including thallus growth form, margin shape (attached or unattached), and medullary system (coaxial or plumose). Porolithon species, because of their ecological importance and sensitivity to ocean acidification, need urgent documentation of their taxonomic diversity.     

Effects of elevated pCO2 on the metabolism of a temperate rhodolith Lithothamnion corallioides grown under different temperatures

Coralline algae are considered among the most sensitive species to near future ocean acidification. We tested the effects of elevated pCO₂ on the metabolism of the free‐living coralline alga Lithothamnion corallioides (“maerl”) and the interactions with changes in temperature. Specimens were collected in North Brittany (France) and grown for 3 months at pCO₂ of 380 (ambient pCO₂), 550, 750, and 1000 μatm (elevated pCO₂) and at successive temperatures of 10°C (ambient temperature in winter), 16°C (ambient temperature in summer), and 19°C (ambient temperature in summer +3°C). At each temperature, gross primary production, respiration (oxygen flux), and calcification (alkalinity flux) rates were assessed in the light and dark. Pigments were determined by HPLC. Chl a, carotene, and zeaxanthin were the three major pigments found in L. corallioides thalli. Elevated pCO₂ did not affect pigment content while temperature slightly decreased zeaxanthin and carotene content at 10°C. Gross production was not affected by temperature but was significantly affected by pCO₂ with an increase between 380 and 550 μatm. Light, dark, and diel (24 h) calcification rates strongly decreased with increasing pCO₂ regardless of the temperature. Although elevated pCO₂ only slightly affected gross production in L. corallioides, diel net calcification was reduced by up to 80% under the 1,000 μatm treatment. Our findings suggested that near future levels of CO₂ will have profound consequences for carbon and carbonate budgets in rhodolith beds and for the sustainability of these habitats.     

Diurnal fluctuations in seawater pH influence the response of a calcifying macroalga to ocean acidification

Coastal ecosystems that are characterized by kelp forests encounter daily pH fluctuations, driven by photosynthesis and respiration, which are larger than pH changes owing to ocean acidification (OA) projected for surface ocean waters by 2100. We investigated whether mimicry of biologically mediated diurnal shifts in pH—based for the first time on pH time-series measurements within a kelp forest—would offset or amplify the negative effects of OA on calcifiers. In a 40-day laboratory experiment, the calcifying coralline macroalga, Arthrocardia corymbosa, was exposed to two mean pH treatments (8.05 or 7.65). For each mean, two experimental pH manipulations were applied. In one treatment, pH was held constant. In the second treatment, pH was manipulated around the mean (as a step-function), 0.4 pH units higher during daylight and 0.4 units lower during darkness to approximate diurnal fluctuations in a kelp forest. In all cases, growth rates were lower at a reduced mean pH, and fluctuations in pH acted additively to further reduce growth. Photosynthesis, recruitment and elemental composition did not change with pH, but δ¹³C increased at lower mean pH. Including environmental heterogeneity in experimental design will assist with a more accurate assessment of the responses of calcifiers to OA.     

One‐year experiment on the physiological response of the Mediterranean crustose coralline alga,Lithophyllum cabiochae,to elevated pCO2 and temperature

The response of respiration, photosynthesis, and calcification to elevated pCO₂ and temperature was investigated in isolation and in combination in the Mediterranean crustose coralline alga Lithophyllum cabiochae. Algae were maintained in aquaria during 1 year at near‐ambient conditions of irradiance, at ambient or elevated temperature (+3°C), and at ambient (ca. 400 μatm) or elevated pCO₂ (ca. 700 μatm). Respiration, photosynthesis, and net calcification showed a strong seasonal pattern following the seasonal variations of temperature and irradiance, with higher rates in summer than in winter. Respiration was unaffected by pCO₂ but showed a general trend of increase at elevated temperature at all seasons, except in summer under elevated pCO₂. Conversely, photosynthesis was strongly affected by pCO₂ with a decline under elevated pCO₂ in summer, autumn, and winter. In particular, photosynthetic efficiency was reduced under elevated pCO₂. Net calcification showed different responses depending on the season. In summer, net calcification increased with rising temperature under ambient pCO₂ but decreased with rising temperature under elevated pCO₂. Surprisingly, the highest rates in summer were found under elevated pCO₂ and ambient temperature. In autumn, winter, and spring, net calcification exhibited a positive or no response at elevated temperature but was unaffected by pCO₂. The rate of calcification of L. cabiochae was thus maintained or even enhanced under increased pCO₂. However, there is likely a trade‐off with other physiological processes. For example, photosynthesis declines in response to increased pCO₂ under ambient irradiance. The present study reports only on the physiological response of healthy specimens to ocean warming and acidification, however, these environmental changes may affect the vulnerability of coralline algae to other stresses such as pathogens and necroses that can cause major dissolution, which would have critical consequence for the sustainability of coralligenous habitats and the budgets of carbon and calcium carbonate in coastal Mediterranean ecosystems.     

Characterization of a natural inducer of coral larval metamorphosis

External chemical signals used by scleractinian corals to recognize suitable substrata for larval settlement and metamorphosis were identified from crustose coralline red algae (CCA). A fragment of coral rubble with CCA induced larval metamorphosis of the scleractinian coral Pseudosiderastrea tayamai. A natural inducer and compounds that enhanced its effect in larval metamorphosis were isolated from the methanol extracts of coral rubble with CCA. A bromotyrosine derivative, 11-deoxyfistularin-3 (10^− 7 M) isolated from the CCA, induced the metamorphosis of P. tayamai larvae (27.5 ± 24.0%). In the presence of fucoxanthinol (10^− 9 M) and fucoxanthin (10^− 9 M), the percentage of metamorphosis induced by the bromotyrosine derivative was further enhanced to 87.8 ± 13.0 and 88.4 ± 17.8%, respectively. Both carotenoids are also found in the coral rubble with CCA. These results suggest that bromotyrosine derivative and carotenoids have a synergistic effect in the metamorphosis of P. tayamai larvae. The synergistic effect provides a higher selectivity for recruitment than a single-component natural inducer for recognizing suitable substrata for larval metamorphosis. Thus, the effect might offer a survival advantage for benthic marine invertebrates.     

Thanetian–Ilerdian coralline algae and benthic foraminifera from northeast India: microfacies analysis and new insights into the Tethyan perspective

Thanetian–Ilerdian carbonate deposits from the Lakadong Limestone in Assam Shelf, Meghalaya, northeast India, are studied with respect to microfacies distributions and controlling ecological factors on dominant biogenic components. Palaeoenvironmental implications are inferred following the detailed analysis of microfacies characterized by rich assemblages of coralline red algae and benthic foraminifera. The carbonate sediments have been interpreted as lagoonal to outer shelf facies. It is envisaged that the analysed benthic communities thrived in a meso‐oligotrophic regime above the fair‐weather wave base. The Lakadong Limestone constitutes a well‐preserved record of Late Palaeocene–earliest Eocene shallow marine carbonate ecosystem and has high potential to decipher its response to an interval of distinct changes in climate and tectonic settings. The abundance of oligotrophic larger benthic foraminifera in the Lakadong Limestone is comparable to the foraminiferal assemblages of west Tethys. The phylogenetic changes (‘Larger Foraminiferal Turnover’, LFT) and subsequent rapid radiation of typical Eocene larger benthic foraminifera (Alveolina, Nummulites) usually observed in the west Tethys have also been observed in the upper part of the Lakadong Limestone. The eastward migration of Eocene foraminifera from the west coincided with the India‐Asia collision and global warming events at the Palaeocene–Eocene boundary that may have produced a wide array of modifications in biogeography, seasonal run‐offs and ocean circulation pathways. The data indicate that rapid rate of migration from west before the onset of geographic barriers and/or timely restoration of pan‐Tethyan environmental conditions ensured the incidence of these forms in the earliest Eocene sediments.     

Diversity of carbon use strategies in a kelp forest community: implications for a high CO2 ocean

Mechanisms for inorganic carbon acquisition in macroalgal assemblages today could indicate how coastal ecosystems will respond to predicted changes in ocean chemistry due to elevated carbon dioxide (CO₂). We identified the proportion of noncalcifying macroalgae with particular carbon use strategies using the natural abundance of carbon isotopes and pH drift experiments in a kelp forest. We also identified all calcifying macroalgae in this system; these were the dominant component of the benthos (by % cover) at all depths and seasons while cover of noncalcareous macroalgae increased at shallower depths and during summer. All large canopy‐forming macroalgae had attributes suggestive of active uptake of inorganic carbon and the presence of a CO₂ concentration mechanism (CCM). CCM species covered, on average, 15–45% of the benthos and were most common at shallow depths and during summer. There was a high level of variability in carbon isotope discrimination within CCM species, probably a result of energetic constraints on active carbon uptake in a low light environment. Over 50% of red noncalcifying species exhibited values below ?30‰ suggesting a reliance on diffusive CO₂ uptake and no functional CCM. Non‐CCM macroalgae covered on average 0–8.9% of rock surfaces and were most common in deep, low light habitats. Elevated CO₂ has the potential to influence competition between dominant coralline species (that will be negatively affected by increased CO₂) and noncalcareous CCM macroalgae (neutral or positive effects) and relatively rare (on a % cover basis) non‐CCM species (positive effects). Responses of macroalgae to elevated CO₂ will be strongly modified by light and any responses are likely to be different at times or locations where energy constrains photosynthesis. Increased growth and competitive ability of noncalcareous macroalgae alongside negative impacts of acidification on calcifying species could have major implications for the functioning of coastal reef systems at elevated CO₂ concentrations.     

The Role of Encrusting Coralline Algae in the Diets of Selected Intertidal Herbivores

Kalk Bay, South Africa, has a typical south coast zonation pattern with a band of seaweed dominating the mid-eulittoral and between two molluscan-herbivore dominated upper and lower eulittoral zones. Encrusting coralline algae were very obvious features of these zones. The most abundant herbivores in the upper eulittoral were the limpet, Cymbula oculus (10.4 ± 1.6 individuals m⁻²; 201.65 ± 32.68 g.m⁻²) and the false limpet, Siphonaria capensis (97.07± 19.92 individuals m⁻²; 77.93 16.02 g.m⁻²). The territorial gardening limpet, Scutellastra cochlear, dominated the lower eulittoral zone, achieving very high densities (545.27 ± 84.35 m⁻²) and biomass (4630.17 ± 556.13 g.m⁻²), and excluded all other herbivores and most seaweeds, except for its garden alga and the encrusting coralline alga, Spongites yendoi (35.93 ± 2.26% cover). In the upper eulittoral zone, encrusting coralline algae were only present in the guts of the chiton Acanthochiton garnoti (30.5 ± 1.33%) and the limpet C. oculus (2.9 ± 0.34%). The lower eulittoral zone limpet, Scutellastra cochlear also had a large percentage of encrusting coralline algae in its gut with limpets lacking gardens having higher (45.1 ± 1.68%) proportions of coralline algae in their guts than those with gardens (25.6 ± 0.8%). Encrusting coralline algae had high organic contents, similar to those of other encrusting and turf-forming algae, but higher organic contents than foliose algae. Radula structure, grazing frequencies as a percentage of the area grazed (upper eulittoral 73.25 ± 3.60% d⁻¹; lower eulittoral 46.0 ± 3.29% d⁻¹), and algal organic content provided evidence to support the dietary habits of the above herbivores. The data show that many intertidal molluscs are actively consuming encrusting coralline algae and that these seaweeds should be seen as an important food source.     

Microbial to reef scale interactions between the reef-building coral Montastraea annularis and benthic algae

Competition between reef-building corals and benthic algae is of key importance for reef dynamics. These interactions occur on many spatial scales, ranging from chemical to regional. Using microprobes, 16S rDNA pyrosequencing and underwater surveys, we examined the interactions between the reef-building coral Montastraea annularis and four types of benthic algae. The macroalgae Dictyota bartayresiana and Halimeda opuntia, as well as a mixed consortium of turf algae, caused hypoxia on the adjacent coral tissue. Turf algae were also associated with major shifts in the bacterial communities at the interaction zones, including more pathogens and virulence genes. In contrast to turf algae, interactions with crustose coralline algae (CCA) and M. annularis did not appear to be antagonistic at any scale. These zones were not hypoxic, the microbes were not pathogen-like and the abundance of coral-CCA interactions was positively correlated with per cent coral cover. We propose a model in which fleshy algae (i.e. some species of turf and fleshy macroalgae) alter benthic competition dynamics by stimulating bacterial respiration and promoting invasion of virulent bacteria on corals. This gives fleshy algae a competitive advantage over corals when human activities, such as overfishing and eutrophication, remove controls on algal abundance. Together, these results demonstrate the intricate connections and mechanisms that structure coral reefs.