Reproduction of Mediterranean zooxanthellate and azooxanthellate scleractinian corals along environmental gradients

Despite extensive studies focus mainly on sexual reproductive characteristics in tropical scleractinian species, there is limited knowledge on temperate regions. The Mediterranean is a biodiversity hotspot under intense pressure from anthropogenic impacts. Climatic models further predict that the Mediterranean basin will be one of the most impacted regions by the ongoing warming trend. This makes it a potential model of more global patterns to occur in the world’s marine biota, and a natural focus of interest for research on climate. The present research contributed to increase data on reproductive modes and sexuality of temperate scleractinian corals, highlighting their developmental plasticity, showing different forms of propagation and different responses to environmental change. For the first time, sexuality and reproductive mode in Caryophyllia inornata were determined. An unusual embryogenesis without a clear seasonal pattern was observed, suggesting the possibility of an asexual origin. Sexual reproduction of Astroides calycularis was governed by annual changes in seawater temperature, as observed for other Mediterranean dendrophylliids. Defining the reproductive biology of these species is the starting point for studying their potential response to variations of environmental parameters, on a global climate change context. The results on the influence of temperature on reproductive output of the zooxanthellate (symbiosis with unicellular algae) Balanophyllia europaea and the non-zooxanthellate Leptopsammia pruvoti suggest that the latter may be quite tolerant to temperature increase, since the zooxanthellate species resulted less efficient at warm temperatures. A possible explanation could be related to their different trophic system. In B. europaea thermal tolerance is primarily governed by the symbiotic algae, making it more sensitive to temperature changes. On the contrary, the absence of symbionts in L. pruvoti might make it more resistant to temperature. In a progressively warming Mediterranean, the efficiency on scleractinian reproduction could be influenced in different ways, reflecting their extraordinary adaptability.

Parameter estimation in a growth model for a biological population

The motivating problem concerns the estimation of the growth curve of solitary corals that follow the nonlinear Von Bertalanffy Growth Function (VBGF). The most common parameterization of the VBGF for corals is based on two parameters: the ultimate length L∞ and the growth rate k. One aim was to find a more reliable method for estimating these parameters, which can capture the influence of environmental covariates. The main issue with current methods is that they force the linearization of VBGF and neglect intra-individual variability. The idea was to use the hierarchical nonlinear model which has the appealing features of taking into account the influence of collection sites, possible intra-site measurement correlation and variance heterogeneity, and that can handle the influence of environmental factors and all the reliable information that might influence coral growth. This method was used on two databases of different solitary corals i.e. Balanophyllia europaea and Leptopsammia pruvoti, collected in six different sites in different environmental conditions, which introduced a decisive improvement in the results. Nevertheless, the theory of the energy balance in growth ascertains the linear correlation of the two parameters and the independence of the ultimate length L∞ from the influence of environmental covariates, so a further aim of the thesis was to propose a new parameterization based on the ultimate length and parameter c which explicitly describes the part of growth ascribable to site-specific conditions such as environmental factors. We explored the possibility of estimating these parameters characterizing the VBGF new parameterization via the nonlinear hierarchical model. Again there was a general improvement with respect to traditional methods. The results of the two parameterizations were similar, although a very slight improvement was observed in the new one. This is, nevertheless, more suitable from a theoretical point of view when considering environmental covariates.

Effects of temperature and acidity on Mediterranean benthic calcifiers, in the face of globale climate change

The Mediterranean Sea is expected to react faster to global change compared to the ocean and is already showing more pronounced warming and acidification rates. A study performed along the Italian western coast showed that porosity of the skeleton increases with temperature in the zooxanthellate (i.e. symbiotic with unicellular algae named zooxanthellae) solitary scleractinian Balanophyllia europaea while it does not vary with temperature in the solitary non-zooxanthellate Leptopsammia pruvoti. These results were confirmed by another study that indicated that the increase in porosity was accompanied by an increase of the fraction of the largest pores in the pore-space, perhaps due to an inhibition of the photosynthetic process at elevated temperatures, causing an attenuation of calcification. B. europaea, L. pruvoti and the colonial non-zooxanthellate Astroides calycularis, transplanted along a natural pH gradient, showed that high temperature exacerbated the negative effect of lowered pH on their mortality rates. The growth of the zooxanthellate species did not react to reduced pH, while the growth of the two non-zooxanthellate species was negatively affected. Reduced abundance of naturally occurring B. europaea, a mollusk, a calcifying and a non-calcifying macroalgae were observed along the gradient while no variation was seen in the abundance of a calcifying green alga. With decreasing pH, the mineralogy of the coral and mollusk did not change, while the two calcifying algae decreased the content of aragonite in favor of the less soluble calcium sulphates and whewellite (calcium oxalate), possibly as a mechanism of phenotypic plasticity. Increased values of porosity and macroporosity with CO2 were observed in B. europaea specimens, indicating reduces the resistance of its skeletons to mechanical stresses with increasing acidity. These findings, added to the negative effect of temperature on various biological parameters, generate concern on the sensitivity of this zooxanthellate species to the envisaged global climate change scenarios.

Biomineralization in calcifying marine organisms

The objective of this theses is to contribute to the wide discussion about the biological control level on the biomineralization operated by calcifying organisms. In particular the intra-crystalline organic matrix associated with different coral species was studied and its role in the process was investigated. The main goals obtained from the research on corals included: (i) the discovery of the species specific role of the intra-crystalline organic matrix molecules in the precipitation of calcium carbonate; (ii) the definition of the role of magnesium ions in the control of the macromolecules assembly/aggregation and in the consequent calcium carbonate polymorphic selectivity; (iii) the discovery that in corals the biomineralization process is not affected by the sea water acidity, as consequence corals are able to construct their skeletons independently from the environmental conditions as far they survive. At the same time, investigations on different kind of vaterite, biogenic and synthetic, were also carried out and confirm the importance of the organism control on the biomineralization process and in particular on the co-existence of different crystalline structures of vaterite for enabling optimization of specific functions, through the employment of OM and acidic macromolecules.

Relationship between phenotype and environment in marine calcifying organisms: exploring growth and shell properties of different mollusks species in past and modern scenario

Coastal ecosystems represent an inestimable source of biodiversity, being among the most productive areas on the planet. Despite the great ecological and economic value of those environments, many threats endanger the species living in this ecosystem, like the rapid warming and the sea acidification, among many other. Benthic calcifying organisms (e.g. mollusks, corals and echinoderms) in particular, are among the most exposed to those hazards. These organisms use calcium carbonate as a structural and protective material through the biomineralization process, biologically controlled by the organism, but nevertheless, strongly influenced by the environmental surroundings. Evaluating how a changing environment can influence the process of biomineralization is critical to understand how those species of great ecological and economic importance will face the ongoing climate change. This thesis investigates the mechanism of biomineralization in different mollusks’ species of the Adriatic Sea, providing detailed descriptions of shells skeletal, biometric and growth parameters. Applying a multidisciplinary and multi-scale research approach, the influence of external environmental factors on the process of shell formation has been investigated. To achieve this purpose analysis were conducted both on current populations and on fossil remain, which allows to investigate ecological responses to past climate transitions. Mollusks’ shells in fact are one of the best tools to understand climate change in the past, present and future, since they record the environmental conditions prevailed during their life, reflected on the geochemical properties, microstructure and growth of the shell. This approach allowed to overcome the time scale limit imposed by field and laboratory survey, and better understand species long term adaptive response to changing environment, a crucial issue to define proper conservation and management strategies. Furthermore, the investigation of fossil record of mollusks assemblages offered the opportunity to evaluate the long-term biotic response to anthropogenic stressors in the north Adriatic Sea.