3.1 Introduction
If you happen to be an outdoorsy type of person, chances are that you have stepped
into a river or lake and found the rocks of its bottom to be slippery. Or maybe visiting
a mudflat at low tide, you saw thousands of crabs and periwinkle snails coming out
to the mud surface and shorebirds probing the mud to pick up worms with their tubelike bills. Have you ever wondered what the crabs and worms feed upon? What fuels
these ecosystems? Let me break the news to you: it is myriads of microscopic life
forms that harvest the energy from the sun and convert it into different forms of
organic matter. Simply put, sediments are not inert, but rather thriving with millions
of microbes per cm
2 . Multiple and complex interactions between microbes and
sediments have taken place for millions of years, a relationship that probably is as
old as the existence of microbial life itself. Throughout this time, the rocks and
sediment particles that cover the surface of the Earth have provided a viable substrate
for these opportunistic critters, as is testified by a continuous rock record dating back
to 3500 million years. Yet, to many inattentive dwellers like you or me, these
vegetated sediments in well-illuminated shallow waters may have remained a “secret
garden” (MacIntyre et al. 1996).
This essay is an account of the intricate relationships that microbes have developed with sediments over millions of years of evolution on Earth. It starts by
providing a concise summary of the evolution of the early microbial lineages, the
key metabolic pathways they devised, and how the association with sedimentary
habitats has been a constant feature throughout microbial evolution. Microbial mats
from hypersaline environments and their associated microbially induced sedimentary structures (MISS) are treated to some further extent as being the archetypical
kind of biosedimentary ecosystems that have transcended since the Archaean eon
and as such deserve a special mention in a volume dedicated to microbes as the
foundation stone of the biosphere. Salient ecological features and the metabolism of
their integrating microbes are briefly discussed. Then the emergent properties of
microbial-sediment interactions are outlined and illustrated by present-day examples
that prove how these same processes have played a significant role in past times.
GeoBiology is the discipline that explores the interactions between the physical
Earth and the biosphere often with an interdisciplinary approach grounded in
modern-day processes (i.e., an actualistic approach). Hopefully this essay will
illustrate how seemingly disparate disciplines such as sedimentology,
paleoenvironmental reconstruction, microbial ecology, microbiology, and geochemistry can reconcile at disciplinary crossroads, when it comes to studying the lifelong
relationships of microbes and marine sediments. Let us now start by the very
beginning.
58
J. Pan
If you happen to be an outdoorsy type of person, chances are that you have stepped
into a river or lake and found the rocks of its bottom to be slippery. Or maybe visiting
a mudflat at low tide, you saw thousands of crabs and periwinkle snails coming out
to the mud surface and shorebirds probing the mud to pick up worms with their tubelike bills. Have you ever wondered what the crabs and worms feed upon? What fuels
these ecosystems? Let me break the news to you: it is myriads of microscopic life
forms that harvest the energy from the sun and convert it into different forms of
organic matter. Simply put, sediments are not inert, but rather thriving with millions
of microbes per cm
2 . Multiple and complex interactions between microbes and
sediments have taken place for millions of years, a relationship that probably is as
old as the existence of microbial life itself. Throughout this time, the rocks and
sediment particles that cover the surface of the Earth have provided a viable substrate
for these opportunistic critters, as is testified by a continuous rock record dating back
to 3500 million years. Yet, to many inattentive dwellers like you or me, these
vegetated sediments in well-illuminated shallow waters may have remained a “secret
garden” (MacIntyre et al. 1996).
This essay is an account of the intricate relationships that microbes have developed with sediments over millions of years of evolution on Earth. It starts by
providing a concise summary of the evolution of the early microbial lineages, the
key metabolic pathways they devised, and how the association with sedimentary
habitats has been a constant feature throughout microbial evolution. Microbial mats
from hypersaline environments and their associated microbially induced sedimentary structures (MISS) are treated to some further extent as being the archetypical
kind of biosedimentary ecosystems that have transcended since the Archaean eon
and as such deserve a special mention in a volume dedicated to microbes as the
foundation stone of the biosphere. Salient ecological features and the metabolism of
their integrating microbes are briefly discussed. Then the emergent properties of
microbial-sediment interactions are outlined and illustrated by present-day examples
that prove how these same processes have played a significant role in past times.
GeoBiology is the discipline that explores the interactions between the physical
Earth and the biosphere often with an interdisciplinary approach grounded in
modern-day processes (i.e., an actualistic approach). Hopefully this essay will
illustrate how seemingly disparate disciplines such as sedimentology,
paleoenvironmental reconstruction, microbial ecology, microbiology, and geochemistry can reconcile at disciplinary crossroads, when it comes to studying the lifelong
relationships of microbes and marine sediments. Let us now start by the very
beginning.
58
J. Pan