The Enduring Questions: What's for Dinner? Where's My Knife? …and Can I Use My Fingers?

Heterotrophic microbial communities play key roles in processing and remineralizing organic matter in marine sediments: they are the “final gatekeepers” that determine the types and quantity of organic matter that is ultimately buried in sediments—processes important to our understanding of past global environments, as well as to the production of petroleum products that still fuel much of modern society. These communities’ capabilities also help us understand the energetic and metabolic boundaries of life. Work over the past decades has revealed much information about sedimentary microbial communities: their overall composition (Bacteria and Archaea), the sequence of terminal respiration processes occurring with progressive burial depth in sediments, and the depth to which they can be detected; research on the members and metabolism in the “deep biosphere” has assumed a central position in organic geochemistry, environmental microbiology, and molecular ecology.

Despite this gain in knowledge, we are left with important questions about heterotrophic microbial communities in sediments, some of which can be summarized simply: What do they eat? How do they gain access to their substrates? How do they sense that specific substrates are out there? Our understanding is particularly incomplete with regard to the types of complex organic matter accessed by these communities. At a recent workshop¹, we discussed a number of key questions with respect to these issues, and here we focus on some of the questions raised in our discussion, in the hope that they will spur further thought and research.