Algae & Microbial Components:
No short paragraph can describe the complex petrography of mineral deposits generated by algae, bacteria, archea, fungi, etc. These different groups are lumped here together because in nature they may occur together in complex consortia (mats and oncolites, for example) and it can be rather difficult to discriminate the particular contributions of each. Some algae (especially the red, "coralline" algae, certain green algae, and the minute planktonic forms known as coccolithophorids) secrete discrete skeletal structures that break down to yield silt-or sand-size detritus with distinct petrographic character. Other algae, and certain microbes as well, induce mineral precipitation (generally microcrystalline) in response to their metabolic processes that has little discernible morphological organization or simply makes passive coatings on cells and filaments. Sticky secretions (biofilms) created by algae and microbes also play a role in the trapping of mineral grains, adding a detrital component to mineral deposits created by these lifeforms.