The Importance of Ecological Accommodation Space and Sediment Supply for Cold-Water Coral Mound Formation, a Case Study From the Western Mediterranean Sea

The formation of cold-water coral (CWC) mounds is commonly seen as being the result of the sustained growth of framework-forming CWCs and the concurrent supply and deposition of terrigenous sediments under energetic hydrodynamic conditions. Yet only a limited number of studies investigated the complex interplay of the various hydrodynamic, sedimentological and biological processes involved in mound formation, which, however, focused on the environmental conditions promoting coral growth. Therefore, we are still lacking an in-depth understanding of the processes allowing the on-mound deposition of hemipelagic sediments, which contribute to two thirds of coral mound deposits. To investigate these processes over geological time and to evaluate their contribution to coral mound formation, we reconstructed changes in sediment transport and deposition by comparing sedimentological parameters (grain-size distribution, sediment composition, accumulation rates) of two sediment cores collected from a Mediterranean coral mound and the adjacent seafloor (off-mound). Our results showed that under a turbulent hydrodynamic regime promoting coral growth during the Early Holocene, the deposition of fine siliciclastic sediments shifted from the open seafloor to the coral mounds. This led to a high average mound aggradation rate of >130 cm kyr–1, while sedimentation rates in the adjacent off-mound area at the same time did not exceed 10 cm kyr–1. Thereby, the baffling of suspended sediments by the coral framework and their deposition within the ecological accommodation space provided by the corals seem to be key processes for mound formation. Although, it is commonly accepted that these processes play important roles in various sedimentary environments, our study provided for the first time, core-based empirical data proving the efficiency of these processes in coral mound environment. In addition, our approach to compare the grain-size distribution of the siliciclastic sediments deposited concurrently on a coral mound and on the adjacent seafloor allowed us to investigate the integrated influence of coral mound morphology and coral framework on the mound formation process. Based on these results, this study provides the first conceptual model for coral mound formation by applying sequence stratigraphic concepts, which highlights the interplay of the coral-framework baffling capacity, coral-derived ecological accommodation space and sediment supply.

The influence of Atlantic climate variability on the long-term development of Mediterranean cold-water coral mounds (Alboran Sea, Melilla Mound Field)

This study provides a detailed reconstruction of climatic events affecting a cold-water coral mound located within the East Melilla Coral Province (Southeast Alboran Sea) over the last 300 ky. Based on benthic foraminiferal assemblages, macrofaunal quantification, grain size analysis, sediment geochemistry, and foraminiferal stable isotope compositions, a reconstruction of environmental conditions prevailing in the region is proposed. The variations in planktonic and benthic δ18O values indicate that cold-water coral mound formation follows global climatic variability. Cold-water corals develop during both interglacial and glacial periods, although interglacial conditions would have allowed better proliferation. Environmental conditions during glacial periods, particularly during the Last Glacial Maximum, appear to better suit the ecological requirements of the erect cheilostome bryozoan Buskea dichotoma. Benthic foraminiferal assemblages suggest that high organic carbon flux characterized interglacial periods. Results from this study imply that increased influence of warm and moist Atlantic air masses during interglacial periods led to increased fluvial discharge, providing nutrients for cold-water corals. Important interglacial Atlantic Water mass inflow further promoted strong Alboran Gyres, and thus mixing between surface and intermediate water masses. Increased turbulence and nutrient supply would have hence provided suitable conditions for coral development. In contrast, benthic foraminiferal assemblages and grain size distributions suggest that the benthic environment received less organic matter during glacial periods, whilst bottom flow velocity was reduced in comparison to interglacial periods. During glacial periods, arid continental conditions combined to more stratified water masses caused a dwindling of coral communities in the southeastern Alboran Sea, although aeolian dust input may have allowed these to survive. In contrast to Northeast Atlantic counterparts, coral mound build-up in the southeastern Alboran Sea occurs during glacial as well as during interglacial periods and at very low aggradation rates (between 1 and 9 cm ky−1). We propose that Buskea dichotoma plays an important role in long-term mound formation at the East Melilla Coral Province, noticeably during glacial periods.

The dualism between adatom- and vacancy-based single crystal growth models

In homoepitaxial crystal growth, four basic growth morphologies (idealized growth modes) have been established that describe the deposition of atoms on single crystal surfaces: step-flow, layer-by-layer, mound formation, and random/self-affine growth. Mound formation leads to nano-scale surface patterning. However, the formation of (nano)-islands, patterns, and roughness occurs also during ion bombardment, electrochemical etching and oxidation/reduction cycling. Here we show, in analogy to many particle/anti-particle formalisms in physics, the existence of the dualism between individual adatom and single vacancy growth modes. We predict that all standard adatom growth modes do exist also in their counter, vacancy version. For the particular case of mound formation, we derive the theoretical equations and show the inverse similarity of the solution. We furthermore treat simultaneous growth by adatoms and vacancies, and derive the analytical solution of the growth shape evolution of the mounds. Finally, we present an experimental verification, in which both adatom and vacancy mound formation are active. The theoretically predicted mound shape nicely fits the experimental observation.

Ultrasensitive Response of DevelopingMyxococcus xanthusto the Addition of Nutrient Medium Correlates with the Level of MrpC

ABSTRACTUpon depletion of nutrients,Myxococcus xanthusforms mounds on a solid surface. The differentiation of rod-shaped cells into stress-resistant spores within mounds creates mature fruiting bodies. The developmental process can be perturbed by the addition of nutrient medium before the critical period of commitment to spore formation. The response was investigated by adding a 2-fold dilution series of nutrient medium to starving cells. An ultrasensitive response was observed, as indicated by a steep increase in the spore number after the addition of 12.5% versus 25% nutrient medium. The level of MrpC, which is a key transcription factor in the gene regulatory network, correlated with the spore number after nutrient medium addition. The MrpC level decreased markedly by 3 h after adding nutrient medium but recovered more after the addition of 12.5% than after 25% nutrient medium addition. The difference in MrpC levels was greatest midway during the period of commitment to sporulation, and mound formation was restored after 12.5% nutrient medium addition but not after adding 25% nutrient medium. Although the number of spores formed after 12.5% nutrient medium addition was almost normal, the transcript levels of “late” genes in the regulatory network failed to rise normally during the commitment period. However, at later times, expression from a reporter gene fused to a late promoter was higher after adding 12.5% than after adding 25% nutrient medium, consistent with the spore numbers. The results suggest that a threshold level of MrpC must be achieved in order for mounds to persist and for cells within to differentiate into spores.IMPORTANCEMany signaling and gene regulatory networks convert graded stimuli into all-or-none switch-like responses. Such ultrasensitivity can produce bistability in cell populations, leading to different cell fates and enhancing survival. We discovered an ultrasensitive response ofM. xanthusto nutrient medium addition during development. A small change in nutrient medium concentration caused a profound change in the developmental process. The level of the transcription factor MrpC correlated with multicellular mound formation and differentiation into spores. A threshold level of MrpC is proposed to be necessary to initiate mound formation and create a positive feedback loop that may explain the ultrasensitive response. Understanding how this biological switch operates will provide a paradigm for the broadly important topic of cellular behavior in microbial communities.

Report on excavation of a shell mound site at Mandjungaar, western Cape York Peninsula

This short report presents results of excavation and analysis of a shell mound deposit at Mandjungaar, near Weipa, Cape York Peninsula. This study was initiated as a cultural heritage management project focused on a shell mound site damaged by unauthorised clearing of access tracks. This study included a small research component to establish a baseline understanding of longer-term use history of the Mandjungaar area at the request of Ndrua’angayth custodians. This included excavation and analysis of a test pit at the site. Results of the study are presented and contextualised in relation to previous research on the Weipa Peninsula in order to expand our understanding of the wider cultural history of the southern Weipa Peninsula. These results provide further support for the assertion that shell mound formation in the Albatross Bay region involved food production activities that were strategically focused on estuarine mud and sandflat ecosystems. In doing so, this dataset provides additional support for the previously proposed niche production model of shell mound formation.