International Journal of Marine Science 2014, Vol.4, No.42, 1-11
8
these anoxic sediment samples suggest that some
benthic eukaryotes (protozoa and metazoa) in this area
can tolerate anoxic and sulfidic conditions. As well,
ciliates, gromiids and foraminiferans were found in
the upper 0-5 cm of the anoxic sediments. Altogether,
these results confirm our early conclusions that some
forms of benthos can possibly adapt to hypoxia/anoxia
and sulfide-rich environments (Sergeeva et al., 2013).
It is considered that the permanent reducing
conditions of anoxic sediments can preserve dead
organisms and their protein for a long time, so that
microscopic analyses of fixed bottom sediments do
not provide proof of the viability of fauna (Zaitsev et
al., 2007, 2008). To proof that benthos is alive,
viability must be tested, in particular the metabolic
activity (Danovaro et al., 2010) and the ability to
reproduce (Kolesnikova and Sergeeva, 2011). Our
observations of alive fauna proved the presence of
eukaryotic life in the anoxic and sulfidic sediments of
the Black Sea. It is known that meiobenthos can be
unevenly distributed in sediments (Sergeeva et al.
2012, 2013) and this can explain the different
distribution of fauna in cores sampled with a TV MUC
at similar depth (Figure.14). This uneven distribution
of meiobenthos visible is likely related to variable
environmental
conditions,
such
as
oxygen
concentration, presence of hydrogen sulfide in bottom
water and sediment, but also the heterogeneity of
habitats and possible differences in the degree of
bioturbation. One explanation might be the special
characteristic of the Bosporus outflow area, where
oxygenated Mediterranean water is injected into the
anoxic water of the Black Sea. As a results, the more
saline and heavier Mediterranean water sinks rapidly
and traces of oxygen can be found occasionally at
depths of up to 300 m (Özsoy et al., 2001), so more
than 100 m below the zone were oxygen usually is
depleted in the Black Sea. However, during our
sampling campaign in April 2010 no signs (neither in
the oxygen nor in the salinity signal) of an active
Bosporus inflow were recorded in the 59 CTD casts
that were performed within 7 days. Despite the regular
Mediterranean inflow carrying dissolved oxygen into
deeper waters and their penetration to considerable
depths of the Black Sea, in this study we show that
periodic anoxic conditions can prevail, leading to a
buildup of sulfide close to the sediment-water
interface at 200 m or even sulfide efflux at 250 m
(Figure 4).
Presence of some species of ciliates and
harpacticoides in sediment containing sulfide is
supported by the studies conducted in the last decades.
It is known from previous studies, that specific groups
of protozoa, fungi and lower metazoa can be found in
the sediments of coastal “sulfidic systems”.
Populations of the harpacticoides
Darcythompsonia
fairlensis
(T. Scott. 1899) were found in sulfidic
patches in different shallow areas of the Black Sea.
The population disappeared with sediment
oxygenation, which reflects adaptation to the sulfidic
environment.
D. fairlensis
was recognized for the first
time in this basin (Kolesnikova and Sergeeva, 2011)
and is one of the few species of the phylum
Harpacticoida, which has been found to inhabit
hypoxic or even anoxic sediments in other water
bodies (Kunz, 1961). The discovered harpacticoides
population included adult females, males, and
copepodites at different stages, ready to breed. These
might indicate that even the complete life cycle takes
place in these sediments, subject to anoxia and sulfidic
conditions.
7 Conclusion
Direct microscopy observations of alive and actively
moving ciliates and free-living nematodes provides
evidence that sediments underlying a sulfidic water
column (200~300 m water depth) can be a natural
habitat for eukaryotes (i.e. metazoan: Nematoda,
Polychaeta, Kinorhyncha and Harpacticoida and
protozoan: Gromiida, Foraminifera (soft-shelled),
Ciliophora). These eukaryotic organisms seem to be
indigenous habitants of the anoxic/sulfidic sediments.
However, the environmental factors and the specific
physiological and biochemical processes of the
benthic fauna to retain the metabolically activity and
facilitate survival are currently still unknown. As well
further studies have to be conducted on the
distribution of benthos as a whole and its individual
taxa in sediments underlying the sulfidic water
column in the Black Sea. The presented observations
of alive benthic organisms are opening new
perspectives for the study of metazoan and protozoan
life in Black Sea habitats depleted in oxygen and
enriched in sulfide.
