3.4. Biochemical composition of the carcasses: Spectral
characterization.
There are two main hypotheses that consider the biochemical components
of the soft body as the key factor of preservation. The first one is
focused on the presence of recalcitrant tissues such as chitin (Butler
et al., 2015), while the second one emphasizes the stability of proteins
tanned by aluminium ions (Wilson, Butterfield, 2014; Naimark et al.,
2016a). To evaluate the importance of the two mechanisms, we assessed
the sustainability of chitin and proteins in the carcasses with
contrasting degrees of preservation (Figure 3 A, B) using FT-IR
spectroscopy (see Materials and Methods) (Figure 3 C-H).
In all carcasses (except for the sediment-free control), the FT-IR
profiles consisted of two parts: the bands corresponding to chemical
bonds in organic molecules, and the bands that correspond to the mineral
(inorganic) components of the sediment. We first considered the organic
part (Figure 3 C-E).
The spectra of all the studied A. salina samples from the
different sediments were similar. The spectral pictures of the
excellently preserved montmorillonite-hosted specimens and poorly
preserved clinochlore-hosted samples showed little to no difference
(Figure 3 D, E). In the carcasses from different clays with different
degree of preservation, the same peaks were present, and the differences
affected only the intensities of the peaks. Moreover, the best and worst
preserved specimens belonging to groups 1 and 4-5 from the kaolinite
also had very similar FT-IR spectra (Figure 4). The spectra also did not
differ much from the two controls: live nauplii and the decayed
sediment-free control (Figure 3 C). The protein characteristic bands
(known as Amid I and II between 1600 and 1700 cm-1 and
1510 and 1580 cm-1 respectively; Figure 3E) varied
little across the carcasses from different sediments. The same is true
for the chitin characteristic bands (between 3400 and 3500
cm-1 and 2950-2800 cm-1 (Negrea et
al., 2015)) (Figure 3D).
The second, inorganic part of the spectra occupies intervals at
wavelength <1300 cm-1. In all exhumed
carcasses, these profiles repeated the spectra for the corresponding
sediments (Figure 3F, G). In the SEM images, we could see small (less
than 1 µm) sediment particles that stayed attached to the organic
surface (Figure 2 G) even after multiple intensive rinsings. So, can the
similarity of the spectra of carcasses and sediments be explained by
mere adherence of inorganic particles to the carcasses? Probably not,
because the 1-day control specimens, which had been kept in the
kaolinite sediment for one day after death and then rinsed according to
the common protocol, showed a much less pronounced signature of
kaolinite (Figure 3 H). This means that mineral particles did not adhere
to a nauplial carcass immediately after deposition, but it took some
time for the minerals to form strong mineral-organic bonds or to become
incorporated into organic tissues.
Overall, the results presented in this section do not demonstrate any
simple relationship between the rate of decomposition of chitin and/or
proteins and the degree of preservation of the carcasses.