Influences of Infaunal Burrows on the Community Structure and Activity of Ammonia-Oxidizing Bacteria in Intertidal Sediments

ABSTRACT Influences of infaunal burrows constructed by the polychaete (Tylorrhynchus heterochaetus) on O2 concentrations and community structures and abundances of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) in intertidal sediments were analyzed by the combined use of a 16S rRNA gene-based molecular approach and microelectrodes. The microelectrode measurements performed in an experimental system developed in an aquarium showed direct evidence of O2 transport down to a depth of 350 mm of the sediment through a burrow. The 16S rRNA gene-cloning analysis revealed that the betaproteobacterial AOB communities in the sediment surface and the burrow walls were dominated by Nitrosomonas sp. strain Nm143-like sequences, and most of the clones in Nitrospira-like NOB clone libraries of the sediment surface and the burrow walls were related to the Nitrospira marina lineage. Furthermore, we investigated vertical distributions of AOB and NOB in the infaunal burrow walls and the bulk sediments by real-time quantitative PCR (Q-PCR) assay. The AOB and Nitrospira-like NOB-specific 16S rRNA gene copy numbers in the burrow walls were comparable with those in the sediment surfaces. These numbers in the burrow wall at a depth of 50 to 55 mm from the surface were, however, higher than those in the bulk sediment at the same depth. The microelectrode measurements showed higher NH4 + consumption activity at the burrow wall than those at the surrounding sediment. This result was consistent with the results of microcosm experiments showing that the consumption rates of NH4 + and total inorganic nitrogen increased with increasing infaunal density in the sediment. These results clearly demonstrated that the infaunal burrows stimulated O2 transport into the sediment in which otherwise reducing conditions prevailed, resulting in development of high NH4 + consumption capacity. Consequently, the infaunal burrow became an important site for NH4 + consumption in the intertidal sediment.

A new Psilonichnus ichnospecies attributed to mud-shrimp Upogebia in estuarine settings

Psilonichnus lutimuratus n. ichnosp. is described from a Pliocene estuarine-mouth depositional environment (Skolithos ichnofacies) of the Olympic Peninsula, Washington, U.S.A. These simple Y-, I-, and J-shaped, mud-lined burrows occur in situ as dense patches within alternating, wavy-bedded sandstone and mudstone in a storm and flood influenced coastal sequence from an active tectonic margin. The I- and J-shaped traces represent erosional modification of burrow tops during storm-flood events. The new ichnospecies differs from the two other Psilonichnus ichnospecies by the distinct mud-lining of the burrow wall. Comparison with living thalassinoidean shrimp burrows and shrimp ecology allow this new ichnospecies to be attributed to the extant mud shrimp Upogebia. Biological and behavioral characteristics of this living shrimp restrict it to the mouth of the open estuary, and these parameters can be used to narrowly define a shoreline environment in the stratigraphic record.

Denitrification activity in sediment surrounding polychaete (Ceratonereis aequisetis) burrows

The effect of burrow-dwelling fauna on sediment denitrification within the Swan River Estuary, Western Australia, was assessed by determining the spatial profile of potential denitrification activity surrounding individual burrows of a polychaete. This activity was described for Ceratonereis aequisetis and compared with uninhabited sediment. Potential porewater denitrification activity was measured as N’2O production in the presence of acetylene (which blocks N2O reduction and NH4+ oxidation) and supplementary NO3-(provided as a substrate for denitrification). Snap-freezing of sediment cores in liquid nitrogen allowed easy sectioning in both the vertical (perpendicular depth from surface sediment) and radial (depth from burrow wall) planes. Overall, potential denitrification activity was significantly greater in inhabited sediment than in uninhabited sediment, although uninhabited sediment had higher surficial (0–10 mm) potential denitrification activity. Potential denitrification activity was also greater closer to the burrow wall (0–9 mm) rather than further into the sediment (9–13 mm). Greater sampling resolution would be required to determine whether a thin oxygenated surface layer (of either the vertical or radial plane) exists in which denitrification is inhibited. Although this study accurately demonstrates the spatial effect of C. aequisetis on sediment potential denitrification, the reported denitrification intensity may not reflect the rate in situ.

Direct Peristaltic Progression and the Functional Significance of the Dermal Connective Tissues During Burrowing in the Polychaete Polyphysia Crassa (Oersted)

1. Polyphysia excavates its burrow in soft, sublittoral mud by sinusoidal waves of the septate anterior region of the body and the lateral scraping action of the prostomial horns. 2. Associated with discrete, direct peristaltic constrictions, in which the longitudinal muscles shorten to 40%, and the circulars to 80%, of their distended lengths, hydraulic deployment of coelomic fluid converts the periodic advance of the trunk segments into continuous head progression. 3. Direct peristaltic progression advances the body by one-fifth or less of a wavelength per cycle, which is comparable to the figure for earthworm locomotion. 4. A three-dimensional dermal collagen fibre lattice accommodates extensive folding of the cuticle and epidermis while permitting a three- to fourfold increase in the radial dimension of the body wall during peristaltic constriction. Elastic fibre columns oppose the radial distension and control the cuticular folding. 5. These features are seen as adaptations to burrowing in the soft mud habitat. The high degree of body-wall flexibility permits the transition from contracted to distended configuration within the length of one segment. Some three-quarters of the body surface may be in contact with the burrow wall at any one time. Unlike other soft-bodied burrowing animals the force exerted on the burrow wall is unidirectional and the applied pressure is probably small, relatively constant and spread over a wide area.