Boring microorganisms and microborings in carbonate substrates

Boring or endolithic microorgani sms discussed herein are photosynthetic cyanophytes, eucary—otic green and red algae, and heterotrophic fungi that actively penetrate carbonate substrates. Although their existence has been known since the mid-nineteenth century, new techniques for preparation and study developed within the last decade have brought about significant progress in our understanding of them. Boring microorganisms have been studied in a variety of carbonate substrates, including the shells and skeletons of living organisms or their fragmented remains, and within coastal limestones. Endolithic microorganisms exhibit a wide but discontinuous, horizontal and vertical distribution, and accordingly indicate geographical, climatic, and ecological conditions. Their activity within carbonate substrates produces characteristic boring patterns, which permit taxonomic identification. Precision of this identification is restricted by the variability and convergent morphology of endolithic taxa. Bio-logically specific boring behavior is often modified by environmental factors, such as light intensity and water supply in the habitat; such modifications may be used as indicators of local ecological conditions. Orientation of the borings and their surface outlines are partly a function of biologically specific modes of penetration; they are also partly controlled by properties of the bored substrate. Microboring activity has a cumulative destructive effect on coastal lime-stones and on carbonate sediments. Post-mortem changes within endolithic microborings include (1) evacuation of the boring; (2) secondary carbonate precipitation or secondary leaching within the boring; (3) diagenetic alteration of host substrates, such as replacement, dolomitization, or silicification; and (4) infilling of microborings by phosphatized chalk or limonite to form natural casts. Study of recent boring microorganisms strongly suggests that microborings may be suitable as paleoecological indicators. Paleoecological application of endoliths must be tested initially within stratigraphically and paleoecologically well known ancient environments, proceeding from Quaternary or Tertiary deposits backward in geologic time.