Environmental Engineering Reference
In-Depth Information
In contrast to the present situation, the entire Proterozoic ocean may have
consisted of sulfidic deep water covered by a possibly 100 m-thick oxygenated
surface layer. This has been inferred from the S isotopic composition of sed-
imentary sulfides, the fraction of reactive iron and the degree of pyritization,
the evidence for decreased organic carbon burial, and a limited abundance and
diversity of protists [1]. The sulfidic pelagial of the Proterozoic ocean may have
persisted over 1000 million years. Extended water column anoxia may have
occurred also during the Phanerozoic. Derivatives of bacteriochlorophylls and
carotenoids of the obligately anoxygenic phototrophic green sulfur bacteria
have been detected in sediments deposited throughout this period [40], starting
with late Ordovician marine black shales, and including Upper Devonian [24],
Permian and Mid-Triassic shales [21] and Messinian Marl [37] sediments. From
the presence and the distinct carbon isotopic composition of these bacterial bio-
markers, it has been deduced that the anoxic layers in paleoceans frequently
extended into the photic zone. These findings have sparked a considerable
interest in the structure and function of such large oceanic oxic/anoxic envi-
ronments. The Black Sea may represent the largest and closest contemporary
analogue to past sulfidic oceans [15], and hence constitutes a valuable model
system for the study of the biogeochemical cycles and the (micro)organisms
which become relevant in these environments.
Anoxygenic phototrophic bacteria typically occur where light reaches sul-
fidic water layers. Among the anoxygenic phototrophic bacteria, green sulfur
bacteria are especially well adapted to low-light habitats due to their large pho-
tosynthetic antennae, their lower maintenance energy requirements and higher
sulfide tolerance [56]. Green sulfur bacteria are significant for the understand-
ing of oxic/anoxic oceans in two respects. Firstly, like other phototrophic sulfur
bacteria, they may substantially alter the carbon and sulfur cycles by reoxidizing
a major portion of the biogenic sulfide, thereby efficiently recycling electrons
and feeding additional organic carbon into the pelagic carbon cycle [51]. In
order to be able to assess the role of green sulfur bacteria in anoxic oceans,
however, the regulation of photosynthetic activity by environmental factors like
light, sulfide and temperature first needs to be quantified for those types which
are typical for euxinic water bodies. Secondly, green sulfur bacteria have been
used as indicator organisms for past photic zone anoxia when reconstructing
oceanic paleoenvironments [40] (Coolen, this volume). Green sulfur bacteria
are especially well suited for this purpose since their photosynthetic pigments
are well preserved in anoxic sediments and since the origin even of pigment
derivatives can be verified by their distinct carbon isotopic signatures [37]. Still,
a correct interpretation of fossil green sulfur bacterial biomarkers and a detailed
reconstruction of environmental conditions in ancient oceans require a better
knowledge of the physiology of green sulfur bacteria typical for the marine
oxic/anoxic pelagial. From a more general perspective, green sulfur bacteria
