UNDERMAT MINING IN CYANOBACTERIAL MATS

Microbial mats and biofilms represent some of the oldest ecological communities on the Earth. They are also one of the most diverse and biogeochemically-coupled communities, consisting of a suite of prokaryotic and eukaryotic, photoautotrophic, chemoautotrophic, and heterotrophic bacteria and small meiofaunal and megafaunal organisms. The exact nature of the relationship between microbial mat bacterial communities and small metazoan organisms has garnered considerable interest as it has possible bearing on the decline of mat communities and biofilms and the rise of the early metazoans during the Precambrian.

This study represents a systematic investigation of the relationships among cyanobacterial mats and the meiofauna and small macrofaunal annelids that inhabit the mats. Specifically, this research focuses on identifying the preferred feeding location of the infauna – above, within, or underneath the microbial mat. It also identifies traces left by the organisms within the mats and the likelihood of preservation of these traces. Finally, the study compares these traces with those reported from Precambrian sediments.

Sediments and mats were collected as cores from Little Sippewissett salt marsh. Cores were photographed and x-rayed to document mat structures. Thin sections were prepared from several mats and examined under an electron microprobe for the presence of fecal pellets and burrows. Some cores were used to establish a laboratory mat population that was used in subsequent feeding experiments. In these, small plexiglas tm aquaria were layered with sand and cyanobacterial mats and covered with seawater. Twenty small worms were introduced into each chamber and allowed to acclimate for one week. Photographs and x-rays were taken of the chambers prior to the introduction of the worms, immediately after worm additions, and at various intervals over the course of four months.

Photographs and x-radiographs of the experimental chambers reveal the presence of extremely small burrows within the mats and abundant fecal pellets and potential feeding trails at the base of the mats. These traces are very similar to those identified in Precambrian sediments by various authors.

The results of this work lend support to the concept of undermat mining by triploblastic metazoans during the Precambrian.