imprint:
[Erscheinungsort nicht ermittelbar]: The University of Hong Kong (Pokfulam, Hong Kong), 2016
Language:
English
Identifier:
Origination:
University thesis:
Dissertation, The University of Hong Kong (Pokfulam, Hong Kong), 2016
Footnote:
Description:
Nearly a third of all anthropogenic 〖CO〗_2 emissions from human activities in the last hundred years has been absorbed by the world's oceans, the greatest natural carbon sink. This has led to a reduction in the carbonate ion concentration in coastal seawaters, which is rapidly reducing the global ocean surface pH, imposing various harmful effects on the marine environment and animals. This global phenomenon is generally referred to as ocean acidification" (OA). OA causes marine calcifiers to produce shells with altered chemical composition, impaired ultrastructure and reduced protection against predators. Generally, common marine calcifiers undergo a process of "larval metamorphosis" starting out as swimming larvae that attach onto benthic substrates, followed by the formation of protective calcareous shells through biomineralization. The protective shell is generally considered to be more complex in composition and structurally more sophisticated compared to artificially constructed or geological materials. However, biomineralization can be a highly energy-consuming process, during which more than 70% of the larvae fail to survive. Presently, an even more worrisome circumstance is that these marine calcifying organisms have to face challenges brought about by OA. Extensive studies have been carried out on how OA can affect the physiological pattern and performance (e.g., larval growth rate, settlement rate, survivalship) of marine organisms including oysters, mussels, sea urchins. However, only a small portion of these studies have investigated the effect of OA at the biomineral level, including ultrastructure, mineral composition, mineral packing orientation and density, and the resistance of the biomineral products against predatory attack. These biomineral aspects can be highly relevant to the animal's competitiveness and evolutionary pathway, which needs to be carefully examined and justified. This thesis explored the impact of OA on the biomineralization of serpulid tubeworm, Hydroides elegans, using a multi-disciplinary approach involving larval biology, mechanical engineering and materials science. There were four major experiments in the study: (1) Projection of OA's impact (pH 8.1 and 7.8) on biomineral structure formation and their properties including tube size and volume, ultrastructure and mineral density, spatial distribution of mechanical properties and simulation of the in-field performance against predatory attack. (2) Assessment of tube recovery ability and physiological resilience at the biomineral level by returning the worms reared in decreased pH (pH 7.8) conditions to ambient conditions (pH 8.1). (3) Investigation of the synergistic effects of decreased pH (pH 8.1 and 7.8), reduced salinity (33‰ and 27‰) and elevated temperature (25°C and 29°C) on biomineral formation, structural design and product performance. (4) Inspection of the response of the calcification site facing near-future ocean acidity (pH 8.1 and 7.8) using microscopy. The key findings of each experiment were: (1) Under near-future OA conditions, the tube size and volume, ultrastructure, mineral density and mineral material hardness were detrimentally affected by the decreased ocean pH level. The produced tubes had lower resistance to counteract simulated predatory attack. (2) The serpulid tubeworm exhibited remarkable capacity in recovering tube chemical composition, tube mineral density, tube material hardness and tube material elasticity when a suitable pH window (~8.1 in this study) was reopened for calcification. (3) Warming rescued the biomineral products from adverse effects brought about by OA. Warming had the least effect at the early juvenile stage when OA harms the biomineral products the most. (4) Serpulid tubeworms failed to maintain a higher pH level at the calcification site when subjected to near-future decreased pH, potentially resulting in mechanically weaker tubes found in previous studies mentioned in the thesis. Throughout the four studies, I developed a systematic and generalizable methodology to analyze the biomineral-associated materials from a nano-structural and nano-mechanical perspective. The results offer new insights and provide new directions for future studies on OA's impact on marine organisms at the biomineral level.