Intra-crystalline amino acids in deep-sea corals: a first look at organic skeletal controls

Deep-sea corals are found in oceans around the world. However, there are
emerging environmental challenges, like ocean acidification and climate change,
that could threaten deep-sea corals. Previous research into deep-sea corals and
how they respond to environmental change has been focused on biogeographic
habitat mapping and paleoclimate work, exploring their environmental history
using their skeletal remains. Most of this research has focused on inorganic
geochemistry – including dating and trace metal and isotopic compositions.
Despite the potential importance of biomineralisation in calcifying corals, there is
no existing information on the amino acid composition of deep-sea corals.
This study is the first analysis of amino acids in deep-sea corals, helping to fill
this knowledge gap. Using ultra-high-performance liquid chromatography
(UHPLC), intra-crystalline amino acids were extracted from two groups of deep-
sea corals - stylasterids and Scleractinia. One hundred and thirty-six deep-sea
corals were analysed from eight locations. First, the analytical and preparative
reproducibility were assessed. Then the internal heterogeneity was established,
showing that some coral tips have lower total hydrolysable amino acid
concentration (THAA) than branch sections. Overall, high levels of acidic amino
acids (such as aspartic acid and glutamic acid) support previous findings of coral
acid-rich proteins found within shallow water corals. Amino acid differences
were observed between stylasterid and scleractinian corals, with further
differences between aragonitic and calcitic stylasterid corals.
The new amino acid data was also compared to co-located hydrographic data,
(aragonite saturation, pH, dissolved inorganic carbon, temperature and salinity).
This comparison indicated that coral amino acids may be influenced by some
environmental conditions. Two clear correlations were found: (i) THAA increased
with decreasing pH within the scleractinian coral (Caryophyllia), and (ii), the
percentage of aspartic acid was higher in stylasterid corals living in water that
was undersaturated in aragonite compared to saturated conditions.
Together, these are the first THAA data from deep-sea corals and show variations
of amino acids between genera, mineralogies and correlations with
environmental parameters.