Heterogeneity on a small scale

Marine Biology Research will from time to time publish articles that provide valuable reviews of recent developments in marine biology. In this issue Ronnie N. Glud presents a review of recent studies on the distribution and dynamics of dissolved oxygen in marine sediments (Glud 2008). Most people consider soft sediments to be rather homogeneous material notwithstanding the presence of burrowing invertebrates. Using a number of recently developed advanced techniques, it is now possible to map oxygen tension in situ at a spatial scale of a few microns and to follow changes in real time.

With these methodological advances it is shown that the upper several centimetres of sediments may present a mosaic of patches with the partial pressure of oxygen (pO₂) ranging from zero to several hundred per cent atmospheric super-saturation; these extremes may occur at a scale of a few millimetres connected by steep gradients of O₂. This heterogeneity is due to high reaction rates within patches of intense microbial photosynthesis or microbial O₂-consumption, and to transport limitation set by molecular diffusion. Other important factors that shape the O₂-landscape in marine sediments include: ventilated invertebrate burrows; motile behaviour of microorganisms; and advective transport within porous sediments caused by water currents above sediment surfaces.

Figure A.  In situ vertical distribution of pO₂ in a 4 cm-deep slice of shallow-water sediment in Øresund, Denmark, on a summer day. Micropatches of photosynthetic algae and cyanobacteria, and ventilation from worm burrows [Nereis diversicolor (Müller, 1776)] determine the distribution of dissolved O₂ in the sediment (Wenzhöfer & Glud 2004). For temporal variation within 24 h and the day-night pattern see the supplementary video clip available at www.informaworld.com/mpp/uploads/dissolved_oxygen_in_marine_sediments.avi (red means about 200% supersaturation, green means around atmospheric O₂-tension, purple about 20% saturation and black means anoxia).

Figure B. Patterns formed by cells of the colourless sulphur bacterium Thiovulum on the surface of sulphidic sediment in a glass dish with a diameter of 15 cm. The patterns develop because the cells draw aerobic water downwards and consume the O₂ to oxidise sulphide. The resulting anoxic water is, in turn, expelled upwards through voids in the bacterial film; cf. Figure 9 in Glud (2008). Photograph: T. Fenchel.

These findings indicate a spatially complex distribution of microorganisms of different functional types that have important implications for rates of mineralisation and other biogeochemical processes of marine sediments. On a sub-centimetre scale marine sediments appear as spatially complex and exciting as do coral reefs.