Although calcareous algae are known both in present environments and fossil records, from shelf and upper slope settings, either in warm seas for green algae or at all latitudes for red algae, we still need models to quantify their abundance in space and time. Calcareous algae are an important component of biogenic carbonate production but they are very sensitive to marine acidification and rise in temperature, as illustrated by the effects of ongoing global change. Contributions herein were first presented at the 6^th Regional (European) Symposium of the International “Fossil Algae” Association: they include two reviews respectively devoted to the carbonate production of red and green algae, and a suite of investigations covering quantification, facies delineation, and controlling factors spanning the present-day Mediterranean Sea and Eastern Pacific going back to the Jurassic of Romania.

The most important groups of modern red calcareous algae are the Mg-calcite secreting Corallinales and Sporolithales, and the aragonitic Peyssonneliales and Nemaliales. They are common on the world's shelves and are vulnerable to the global warming and the lowering of pH of sea water, caused by the ongoing increase in anthropogenic CO₂. Among them, coralline algae are ecosystem engineers and major producers of carbonate sediment, of particular importance in temperate and cold seas. Corallines respond to marine acidification and rising temperature showing decreased net calcification, decreased growth and reproduction, as well as reduced abundance and diversity, leading to death and ecological shift to dominant non-calcifying algae. Despite their key ecological and sedimentological role, and because of their vulnerability to marine warming and acidification, our knowledge of the distribution of coralline-dominated habitats and the quantification of their carbonate production is not adequate to allow proper environmental management and confident modelling of a global carbon budget. Locating the algal carbonate factories around the world, then describing them, e.g., evaluating their extent and their production, are a priority for future research.

Calcareous green algae (CGA) are an artificially united but highly heterogeneous group of large unicellular benthic algae with one character in common: all have the capability of secreting a calcareous coating on the outer side of the cytoplasmic envelope. Today, they are a major contributor to carbonate sedimentation at all scales from clay-sized particles (aragonitic needles) to coarser grains (sand and gravel) and even to plurimetric sedimentary structures. There are fossil analogues to the features listed above. Phycologists know best Halimeda, Penicillus, Acetabularia and Cymopolia; micropaleontologists and carbonate sedimentologists are most knowledgeable about Acicularia, Clypeina, Neoteutloporella, Salpingoporella, Anthracoporella, Boueina, and Eugonophyllum. The CaCO₃ precipitated to form the coating is generally aragonite (the orthorhombic form) but there are short periods in the geologic record during which its calcite variant (the rhombohedric form) existed contemporaneously in discrete species. Recent studies on Halimeda have shown that some of the Bryopsidales have the capability to calcify strongly in the lower portion of the euphotic zone (where respiration becomes more important than photosynthesis in the process of mineralization) and to produce positive sedimentary reliefs (bioherms) in situ below the fair-weather wave base. Previous models of paleoenvironments considered the presence of Dasycladales or Bryopsidales to indicate shallow-water, that is the upper euphotic zone (from the sea surface down to -25 m), and predominantly low-energy, protected, lagoonal environments. When the algal remains were found in grain-supported facies, they were taken to have been subjected to dynamic transport and therefore indicative of high-energy environments of deposition. The new deeper-water finds have changed interpretations of the environments ascribed fossil algae. A current conception is that ancestral inarticulated Bryopsidales could have grown at depths as great as -120 m (near the base of the lower euphotic zone). This preliminary review concludes with suggestions about fields for continuing investigations.

This contribution discusses current carbonate deposition in the channel between the Ponza and Palmarola islands and along the western coast of Palmarola, focusing on the quantitative contribution of biogenic carbonate and in particular on the production and accumulation of carbonates by coralline algae. Seventeen out of 150 grab samples have been selected as representative of the main sedimentary features facies of the seafloor between 30 and 100 m of water depth (wd). They were analyzed using XRD and EDTA titration. Thin sections of the same samples were made to identify their biogenic components and to quantify the contribution of calcareous algae to the total sediment. Maximum carbonate production takes place between 40 and 70 m wd, with the percentage of carbonates ranging between 83.1 and 95.7%. of the total. Multidimensional statistical analyses found two different carbonate facies: the coralline algae facies (CA) occupies 38 km² between 30 and 70 m wd, and the carbonate matrix facies (CM) covers 24 km² between 70 and 100 m wd. An estimate of the current contribution of algal carbonates to seafloor sediments is about 80% in the CA facies and 15% in the CM facies. The accumulation of total carbonate sediments in the uppermost 2 cm interval of the CA facies is calculated to be 20 566 g m^-2 and the fraction of coralline carbonate in the superficial 2-cm layer of the CM facies is calculated as 16452 g m^-2. The production rate of the mean 7% live coralline covers ranges between 7.91 and 31.64 g m^-2 yr^-1.

On the continental shelf off the Cilento peninsula (eastern Tyrrhenian Sea) the occurrence of more than 13 km² of maerl beds was documented through acoustic surveys. Swath bathymetric data along with a dense grid of chirp-sonar profiles were acquired over more than 180 km². The maerl facies was characterized on the basis of the components analysis of 32 grab samples collected at selected sites. Mapped maerl-beds are predominant on submerged terraces located at variable water depth (wd) between 42 and 52 m. This preferred distribution on submerged terraces is probably associated with relatively vigorous bottom currents generated by local circulation that hinders the deposition of terrigenous sediments. Calcareous red algae result to be the most important producers of carbonates from 40 down to 60 m wd. We calculated the coralline carbonate accumulation from the percentage cover of coralline algae (thin section mapping) × 1 cm-thick layer of sediment × measured coralline density. The total coralline cover (living plus dead) in the Cilento area is 13.96 km², with a total 316800 tons of algal carbonate in the surface 1 cm layer, that correspond to 20430 g m^-2. Living maerl is recorded at a depth of 47 m, with a live coralline cover of about 40% over a minimum area of about 1.2 km². This live maerl has a thickness of about 1 cm and is composed mainly of unattached branches of Lithothamnion corallioides (P.L.Crouan & H.M.Crouan) P.L.Crouan & H.M.Crouan, 1867. The molluscan association of the maerl bed is dominated by characteristic species of the Coastal Detritic Biocoenosis. The production of carbonate by living coralline algae has been calculated as weight of live corallines in 1 cm-thick layer × 100 y^-1 × total area^-1 and corresponds to 90.8 g m^-2 y^-1.

Rhodolith-dominated carbonate environments, characterized by high abundances of free-living coralline algae, have been described globally from a wide range of Recent and fossil shallow marine settings. In the present-day warm-temperate Gulf of California, Mexico, rhodolith-dominated systems are important contributors to carbonate production. One of the most prolific rhodolith factories is located on the Punta Chivato shelf, in the central Gulf of California, where due to a lack of input of terrigenous material from the arid hinterland, carbonate content averages 79%. Punta Chivato rhodoliths thrive above the shallow euphotic zone under normal saline, warm-temperate and meso- to eutrophic conditions. A detailed sedimentologic study combined with acoustic seafloor mapping indicates the presence of extensive rhodolith-dominated facies at subtidal water depth covering an area of >17 km². Additional facies, surrounding the rhodolith-dominated facies include a fine-grained molluscan, a transitional bivalve-rhodolith and a bivalve facies. While the Punta Chivato shelf yields average abundances of 38% rhodolith-derived coralline algal components in the gravel-sized sediment fraction, the rhodolith facies itself is characterized by more than 60% coralline algal components. Other important carbonate producers at Punta Chivato include bivalves (35%), bryozoa (11%) and gastropods (8%). The present study shows that acoustic sediment mapping yields highly resolved continuous coverage of the seafloor and can distinguish modern rhodolith facies from surrounding sediment. This has important implications for quantifying rhodolith-dominated settings globally, as well as for ecological and conservation studies.

Thick rhodolith beds occur in the transgressive and highstand systems tracts of the early Pliocene sequence in the Carboneras Basin in SE Spain. Rhodolith beds accumulated in mid-to-outer ramp settings and in the leeside of a spit platform during the transgressive interval, whereas rhodolith concentrations in the highstand deposits only formed in the mid-to-outer ramp at the southern margin of the basin. The elevation of rhodolith beds compared with coeval shore deposits suggests that the beds developed at water depths of several tens of metres (probably less than 50m). This palaeodepth estimate is consistent with the composition of algal assemblages, which are dominated by melobesioids common in relatively deep platform environments in the modern Mediterranean Sea. Lithothamnion minervae Basso, 1995, L. philippii Foslie, 1909, and Mesophyllum alternans (Foslie) Mendoza & Cabioch, 1998 with subordinate Phymatolithon calcareum (Pallas) Adey & McKibbin, 1970 and M. macroblastum (Foslie) Adey, 1970 and lithophylloids of the Lithophyllum incrustans Philippi, 1837-L. racemus (Lamarck) Foslie, 1901 complex are the most common of a total of 21 recorded species. All the identified algal species are also living in the present-day Mediterranean except for the extinct Lithothamnion ramosissimum (Reuss) Piller, 1994 and for the recent Spongites decipiens (Foslie) Chamberlain, 1993, which has been reported in the Indo-Pacific and South Adantic, but not in the Mediterranean. Moderate energy and low sedimentation rates promoted development of rhodolith beds in the transgressive deposits, but they are not exclusive to the transgressive systems tract as they continued to accumulate at the southern margin during the highstand, beyond the influx of siliciclastics that reduced carbonate production in the rest of the basin. Isolated pillars (up to 90 cm high) or irregular patches (up to 2 m high and 7 m wide) of coralline algal-bryozoan-bivalve bioconstructions occur in outerramp fine-grained calcarenites. Despite their similarities to pillars and low relief buildups constructed by coralline algae (“coralligène de plateau”) in the present-day Mediterranean Sea, these bioconstructions are unique as bryozoans are the main builders and coralline algae (L. philippii and M. lichenoides) play only a secondary role. In contrast with modern Mediterranean coralline algal buildups, the lack of bioclastic debris derived from the build-ups in the surrounding fine-grained sediments and their general morphology suggest that the Pliocene bioconstructions in the Carboneras Basin did not create significant positive relief on the seafloor.

Digital photographs of the surface of Serravallian rhodolith-bearing strata from Stazzano (Tertiary Piedmont Basin) Italy have been elaborated by Image-J opensource software (U.S. National Institutes of Health — NIH), to obtain a map of the algal surface in each photograph. The 33 images in the 7 m-thick outcrop, covering a total of 11.53 m², were complemented by data on rhodolith shape, structure, composition and taphonomy. The Serravallian rhodolith body was produced in a long-lasting infralittoral sedimentary environment and then transported into deeper water by slumping. Although biological and geological definitions of rhodolith-dominated facies are inconsistent in some respects, the image analysis described here allows direct comparison of percentages of rhodolith dominance in discrete fossil rhodolith facies and their living counterparts. The procedures involved are both rapid and inexpensive, so the method appears very useful for carbonate quantification. The calculated algal cover, based only on rhodoliths > 2 cm, ranges from 11.9 to 59.7% (mean 27.3%). The carbonate production rate of the Serravallian rhodolith bed was probably in the range 55 to 136.3 g CaCO₃ m^-2yr^-1 as calculated for a Brazilian present-day analogue.

We investigated the distribution of the red algae assemblages along the depositional profile of the Attard carbonate ramp of Malta (Chattian). The Attard member is ascribed to the Lower Coralline Limestone Formation characterized by 4 members: Maghlaq, Attard, Xlendi and Il Mara. Corallinealgae are present in the inner and middle ramp environments of the Attard member. Sporolithaceans and melobesioids dominate the inner ramp, while mastophoroids and peyssonneliaceans are subordinate. In the middle ramp the association of red algae is characterized by an increase of sporolithacenas and a decrease of melobesioids, mastophoroids and peyssonneliaceans. These assemblages are related to the depth gradient existing from the inner relatively shallow to the progressively deeper middle ramp. However, transportation of red algae down slope may have had an effect on the red algae associations. The shape, morphology and structure of rhodoliths in the inner ramp environment are indicative of high-energy conditions. Nevertheless localized sectors of inner ramp are characterized by morphologies typical of low energy probably related to the presence of areas colonized by seagrass. Rhodoliths from the middle ramp have characteristics of moderately highenergy. The presence of Sporolithon Heydrich and Lithoporelk (Foslie) Foslie indicates that the production of carbonate took place under tropical conditions. We suggest that the Mg/Ca ratios may had a control on the flourishing of coralline algae in the Oligocene carbonate factories situated in oligo- to mesotrophic conditions, whereas during Early to Middle Miocene times the trophic conditions were one of the main controlling factor.

The presence of non-geniculate coralline red algae and bryozoans (rhodalgal lithofacies) in association with rudists has been mentioned only rarely in the literature. Nevertheless, because of the significance of rhodalgal facies in the characterization of shallow-water carbonate factories, a correct interpretation of the related ecological factors may improve the palaeo-environmental reconstruction of some rudist-bearing carbonate depositional systems. Uppermost Coniacian-Santonian rhodolith-rich rudist-bearing carbonate successions in the Nurra region (northwestern Sardinia, Italy) record several discrete depositional settings, from autochthonous shallow-water mobile skeletal deposits including coralline algal fragments and rhodoliths, to re-mobilized deposits rich in skeletal components with rhodoliths. The rudist-bearing rhodalgal limestone studied is part of an uppermost Turonian-Campanian transgressive sequence that covers a tectonically-modelled Lower Cretaceous substrate. The recovery of the Upper Cretaceous carbonate factories followed an interval of time during which the Lower Cretaceous carbonate systems, dominated by chlorozoan assemblages and non-skeletal grains, had experienced “Mid-Cretaceous” worldwide crises presumed to have been caused by global climatic/oceanographic perturbations. In particular, Early-Middle Turonian times, characterized by the hyper-greenhouse conditions then prevailing, witnessed a significant reduction or even complete demise of highly productive carbonate factories. In carbonate settings, biotic assemblages grew in mesotrophic/eutrophic environments. Cyanobacterial consortia, with variable contributions from rudists, largely prevailed in shallow-water domains. Thus far, the latest Turonian-Coniacian recovery of Sardinian carbonate factories with “impoverished chlorozoan assemblages” might be considered as an indication of ameliorated environmental conditions. However, ecological constraints did not allow the tropical “chlorozoan assemblages” to thrive in the Late Cretaceous low-latitudinal carbonate shelves of Sardinia. A Santonian shift toward foramol/rhodalgal depositional systems occurred with sciaphile- (shadow preferring), and mesotrophic-adapted (“rudist-bearing rhodalgal”) assemblages flourishing in the new shallow-water domains. In Santonian times relatively cool and mesotrophic to eutrophic conditions are presumed to have become dominant in the water mass impinging on the marginal sectors of the shelf or distal ramp of the Nurra carbonate system. Deterioration in the quality of water presumably caused the demise of large sectors of the Nurra carbonate factory, which underwent local drowning episodes controlled by tectonic activity.

The lithology of Cretaceous to Pleistocene shallow-water carbonates, which were collected from 29 sites on 24 submerged seamounts in the northwestern Pacific Ocean using the Deep-sea Boring Machine System, are described. The shallow-water carbonate deposits examined in the present study can be roughly divided into three types based on their composition: Cretaceous, Eocene (to lowest Oligocene?), and Oligocene to Pleistocene. The Cretaceous type is characterized by an abundance of molluscs (including rudists), smaller foraminifers, microencrusters, non-skeletal grains (e.g., peloids, cortoids, and intraclasts), and microbial sediments. Most components have been micritized and possess thick micrite envelopes. The Eocene type is characterized by the dominance of larger foraminifers, Halimeda spp., nongeniculate and geniculate coralline algae, bryozoans, and dasycladacean algae. Scleractinian corals are very minor components. The Oligocene to Pleistocene type is similar in composition to the Eocene type, but it differs from the latter by the abundant occurrence of scleractinian corals and nongeniculate coralline algae. Corals, nongeniculate coralline algae, and Halimeda spp., which precipitate carbonates within closed to semi-closed spaces in and around their bodies (intra-tissue), are major components of the Eocene and Oligocene to Pleistocene types. In contrast, the Cretaceous-type sediments contain relatively more carbonates of extra-tissue origin (i.e. carbonates deposited in relatively open spaces around the bodies of organisms, such as rudists, as well as microbialite and ooids) than the Eocene and Oligocene to Pleistocene types. The changes in the major constituents of the carbonate factory depend on local environments, such as nutrient availability, as well as a global factor: seawater chemistry in the surface waters. Temporal variations in the abundance of the shallow-water carbonates on the examined seamounts suggest that carbonate accumulation was not necessarily controlled by climatic conditions; instead, it was related to the volcanism and tectonics that served as the foundations for reef/carbonate-platform formation.

Three species of dasycladalean algae are described: Petrascula piai Bachmayer, 1944, P. bursiformis (Etallon, 1859) and Steinmanniporella kapelensis (Sokač & Nikler, 1973). Two of them, Petrascula piai and Steinmanniporella kapelensis, are relatively seldom reported and consequently poorly known algae. They were found at two locations in the Apuseni Mountains: Sănduleşti quarry (Trascău Mountains) and Şerbota Hill (near Aştileu, Pădurea Craiului Mountains). The two species of Petrascula were essentially identified from sections of the stalk. Their study provides, especially for P. piai, useful supplementary data related to the morphology of the laterals and the structure of the stalk. Regarding Steinmanniporella kapelensis, Sănduleşti is only the second locality (after the type locality) where this alga is known to occur as numerous well preserved specimens. Microfacies and sedimentological data suggest that these algae were restricted to micro-environments within the external part of the carbonate platforms.