• assessing
• biota
• coastal
• engineering
• environment
• geomorphology
• material
• MIT
• modeling
• rock
• science

Erosion and weathering occur as a result of the substantial non-equilibrium state in which rocky landforms are interfaced to a constantly shifting water and subaerial environments. The equation describing rocky coastal reshaping dynamics has many factors playing different roles, starting from the rock structure and type, the environment in which the landform is located and how much time the erosive agents have to remove the weathered materials and expose the fresh rock underneath.
The weathering and erosion occurring in any given location is, usually, the result of the simultaneous action of many different agents that are responsible for dismantling rock outer layers. The weathering pattern starts penetrating down in the rock mass following faults, discontinuities and joints attacking the exposed surfaces and later proceeding to the inner cores of the single blocks. This process produces weaker and porous rocks or disintegrate/transform the rock into finer and less dense materials (such as gravel, sand, silt and clay) that result to be more stable in these particular conditions.
As said, the time required by the erosive agents to remove a certain amount of altered material (known as erosion rate value) is a key information to understand how the whole process evolves. Usually this is driven mostly by the nature of the agent itself: those agents may be broadly summarized in two categories:
• Abiotic – described by the mechanical action of wind, water, waves, etc..
• Biotic - described by the action of biota living on the altered substrate and performing a direct or indirect action on it.
According to this, usually the cumulative erosion rate of a geologically uniform landmass may be evaluated quantitatively with specific tests (e.g. downwearing rates measured by micro erosion meter). The problem occurs when trying to disentangle the effects of one agent from the others; this is particularly difficult since different processes may act on very long time spans with different dynamics but with similar results. This study is focused on the biotic agents indirect role, exposing an innovative approach to the study of bio-remodeling, with the final intent to estimate the indirect mechanical interaction that may occur between biota and rocky substrates. Cyanobacteria is a widespread phylum that well represent some of the most common photosynthetic life forms in coastal regions. These microorganisms are usually found worldwide, ranging from any kind of climate and environment, and in some cases they may form biofilm covers on rock surfaces, beneath and above the sea level. In this work, that part of the rocky shore has been examined where the wetting and drying cycles occur between intertidal and lower supra-littoral bands, in which it is possible to identify a prevalent presence of biofilm covers (resembled by darker colors of rock).
The key hypothesis, which the study is based on, is suggested by the peculiar material properties of the biofilm itself: mainly composed by heavy polymeric biological compounds, it has been found to have interesting gelling properties (retaining up to 97% of water in the matrix) and stability at wide ranges of temperature, pH and salinity. These characteristics may influence the ability of the biofilm to expand and shrink, according to water content, inducing a cyclic stress on the rock surface, weakening it and improving faults generation and fragmentation of the outer layers. The base hypothesis, on which this work is focused, is that the wetting-drying cycles occurring in the rock-biofilm system may promote/attenuate the deterioration of the coastal rock surfaces enhancing/limiting the rock internal stress. Due to the extreme variability and difficulty of direct detection of such weak forces, occurring with expanding and shrinking coupled materials, a multiscale approach has been proposed. Merging computational molecular models and experimental measurements, able of describing the mechanical properties of the single molecule, it is possible to evaluate the mechanics of molecule ensembles and scale up to model bulk materials at macroscale. The computational models are rarely used in this kind of assessments, especially in bio-erosion studies where no similar procedures have been implemented yet. The models have been implemented using molecular dynamics (MD) softwares such as CHARMM and LAMMPS using high performance computational machines using message passing interface (MPI) parallel technology to speed up the computational process.
Starting from basic information about the chemical composition and structure of the biofilm and environmental parameters two simplified polymer models were built with two different chain lengths. Later more variants were implemented including random residue and anionic charges distribution along the side-chains in order to verify fundamental structure characteristics compared to natural ones. The molecules were solvated individually in seawater-like solutions and exposed to a complete drying cycle to measure shrinking mechanical properties.
Mapping dynamic atomic coordinates helps to further simplify the system into a coarse-grained (CG) model, widening the possibilities to implement more molecules into the system and understand how they interact with each other and with the ions present in the solution. Furthermore, the same dynamics information may be used to define a bulk material model of the biofilm. The bulk material model is particularly useful for a direct match with experimental data; in fact, it is possible to build a bulk material model of the same shape and dimensions of real samples and measure the same variables.
Validation is always an indispensable practice while working with models, if experimental and models data diverge too much or they exhibit an incompatible behavior it is probable that something in the model is not working properly or that an unknown variable has a not negligible role in the dynamics and has to be identified and implemented in the updated model.
In a fast growing interference between human usage of territory and the environment, understanding the dynamics of coastal environments is fundamental to perform accurate planning of human activities on the coastline, avoid hazards, preserve ecosystems and in general gain knowledge about the different interactions occurring between the abiotic and biotic spheres.
Moreover, many MD data may be really useful as starting point to design better performing construction materials, bioinspired materials with different applications or possible alternative and sustainable usages of biofilms in industry and wastewater treatment plants.