Shell composition and microstructure variation with pH in time and space

Lead Research Organisation: NERC British Antarctic Survey
Department Name: Science Programmes

Abstract

Over the last 200 years human activity has increased CO2 in the atmosphere by around 40%, roughly 25% of which has been absorbed by the oceans. This has increased oceanic acidity by around 30%. Many studies have shown negative effects of lowered pH on biological functions in a wide range of marine animals and algae. There is widespread concern from scientists, policymakers and conservationists over the effects this change is having, and will increasingly have, on marine life and on the stability of marine ecosystems. This is especially so for species with high requirements for CaCO3 to make skeletons (Royal Society 2005, IPCC 2007). There is thus a need to understand better how marine species can cope with lowered pH, how those currently living in environments of different pH are adapted to those conditions, and how these groups have coped with varying pH in the past both since industrialisation and in deeper geological time. The best way to address questions of this type is to study a marine group that is heavily calcified, has widespread distributions in sites of different pH and has a long and well represented fossil record. In this respect living articulated brachiopods are, if not the best candidate group, then certainly one of the best. They inhabit all of the world's oceans from the poles to the tropics, and from the deep sea to the intertidal. They are possibly the most calcium carbonate dependent on Earth. Over 90% of their dry mass (in some species over 97%) is accounted for by calcareous skeleton. They also have one of the best fossil records in terms of representation and abundance over long geological periods of any marine animal group. There are excellent museum collections for this group, including repeat samples of the same species over the last 150 years and extensive collections at the family level for several major geological periods from single sites. They are, therefore ideal for investigating questions associated with changing environmental pH. We will use up to date SEM and ion probe techniques to quantify articulated brachiopod skeletal characteristics (shell thickness, primary & secondary layer thickness, crystal morphology, major & minor elemental composition) to address questions in four main areas. Firstly we will investigate the effects of varying pH in current environments by sampling populations of key species living in sites of different pH. Terebratulina retusa is distributed from the Mediterranean to Svalbard, with populations living in sealochs and harbours where pH is lower than offshore. Calloria inconspicua inhabits a similar range of sites around New Zealand. We will sample populations living in different pH conditions and analyse their shells. We will also monitor pH in the areas sampled for at least a year. This will allow us to identify skeletal responses to being raised in reduced pH in the natural environment. Secondly we will quantify changes in skeletons that have occurred since the industrial revolution, when CO2 levels have been consistently rising. Both our key species have good museum collections from given localities covering the last 50 years, and T. retusa collections date back to 1870 in the BM Nat Hist. Collections of the Antarctic L. uva also date back to the 1960's. We plan to exploit these collections to identify skeletal changes over the recent past as oceanic CO2 has risen. Thirdly we will analyse shell characteristics in Articulated brachiopods from different geological periods when CO2 levels in the environment were markedly different from today. This will allow evolutionary scale responses to be addressed. Finally we will hold our key species in culture systems with altered pH conditions and assess changes in skeletal composition and structure. These approaches should provide a very good understanding of how marine species have and can respond to acidification over as wide a range of time and spatial scales as possible.

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