Experimental taphonomy of marine algae and cyanobacteria reveals the decoupling of morphological and chemical decay patterns

Algae are a major constituent of modern and ancient ecosystems. Investigating their evolutionary history relies on understanding their past morphologies as preserved in the fossil record. This is challenged by postmortem decay of algal cells before stabilizing as fossils. Yet, no work has documented the decay of algae in sediments under controlled experimental conditions and applied this information to interpretations of the fossil record. Here, 120 green algal samples belonging to three different morphogroups, and 40 red cyanobacteria samples were left to decay while buried in kaolinite for nine weeks. Multispectral macroimaging revealed that external morphologies were preserved long after death. Nevertheless, chemical compounds decayed over time, and pigments, such as phycoerythrin and chlorophyll became almost undetectable. Some algal morphogroups showed cell compressions, or taphonomic artifacts, which could resemble structures previously described as organelles in fossils, potentially challenging some interpretations in early eukaryotes.