You’ll need the following items to complete this lab session. (The standard kit for this book, available from www.thehomescientist.com, includes the items listed in the first group.)

Microcosms are simplified artificial ecosystems that are used to simulate and observe the behavior of natural ecosystems under controlled conditions. Open or closed microcosms provide an experimental area for ecologists to study natural ecological processes. Microcosm studies can be very useful to study the effects of disturbance or to determine the ecological role of key species. A Winogradsky column is an example of a microbial microcosm.

Most of the other lab sessions in this book are organized into related groups, may be performed in any order you wish, and many (not all) can be completed over the course of one lab period to at most several lab periods over a week or two. Unfortunately—unlike chemistry, physics, and most other sciences—biology labs can’t all be broken down into self-contained single lab sessions. Living things run on their own schedules. Life cycles are what they are, and we’re often powerless to speed them up or slow them down.

Building and observing microcosms is an excellent example of one of these on-its-own-schedule activities. There is much to be learned by doing this activity, but pursuing it properly requires frequent and detailed observations over a period of weeks to months. Accordingly, rather than attempt to distribute these activities out over many lab session groups, we decided to consolidate them in this group of lab sessions, which we’ll begin at the start of the semester and continue throughout the semester.

Some of the individual sessions in this group will be repeated periodically throughout the semester, beginning early and continuing. For example, we’ll observe microscopically the populations of our pond water microcosms throughout the semester, noting changes that occur in the types and mix of different organisms as the microcosm life cycle progresses from juvenile to mature to senescent. Our microcosms will also serve as resources for some later topics. For example, we’ll observe succession in the microcosms, which properly belongs in ecology, but requires extensive and repeated observations over the course of many weeks. Similarly, we can use our microcosms as a source of live protists when we do the lab session on protists.

In this lab session we’ll build two types of microcosms based on water, sediment, and plant life gathered from a pond or stream. We’ll then observe these microcosms periodically over the course of several weeks, noting changes that occur through the life cycles of the various organisms present.

The first type of microcosm resembles a standard aquarium. We’ll use wide-mouth jars filled with a relatively thin layer of sediment, with pond water and aquatic plants above it.

These aquaria will provide the opportunity to observe a wide diversity of microlife, especially protists. We’ll build several of these microcosms initially, some of which we’ll modify later to observe the effects of environment changes on the populations present.

The second type of microcosm, called a Winogradsky column, is a tall column that contains a thick layer of pond sediment and pond water. We’ll build several Winogradsky columns using 500 mL or 1 L soft drink bottles, with different columns containing different nutrients, and observe the changes in these columns over the course of several months. Although many types of lifeforms will be present initially, the real purpose of the Winogradsky column is to produce a mixed bacterial culture and observe the different types of bacteria that thrive in different microenvironments within the columns.

The Winogradsky column has been used for decades in biology classes to demonstrate the metabolic diversity of prokaryotes (bacteria and archaea). All organisms require a carbon source and an energy source to live and grow. Some organisms, called autotrophs, obtain carbon from atmospheric carbon dioxide, while others, called heterotrophs, obtain carbon from the organic compounds such as carbohydrates that are produced by autotrophs. Similarly, some organisms, called phototrophs, obtain energy directly from sunlight via photosynthesis, while others, called chemotrophs, obtain energy from breaking down chemical compounds.

Different organisms use all four possible combinations of these methods to obtain carbon and energy:

All four of these strategies are represented in a typical Winogradsky column, which also provides a visible example of how different organisms occupy different ecological niches, according to how they obtain carbon and energy.

A Winogradsky column, from top to bottom, may include the following ecological micro-niches:

Each Winogradsky column is unique, even if it started with the same combination of water, sediment, and nutrients, so we’ll make several Winogradsky columns and observe and compare them as they mature.

Insofar as is possible, our goal is to make each of our aquarium microcosms identical, using the same amount of sediment, the same amount of water, and the same amount and mix of aquatic plant life.

Just as with the aquarium microcosms, our goal is to make each of our Winogradsky column microcosms as similar as possible, other than differences in added nutrients. With the exception of one control Winogradsky column, which contains only unenriched sediment, each of the other Winogradsky columns begins with sufficient enriched sediment to fill the column roughly half full. To that, we’ll add add about half as much unenriched sediment to fill the column to a total of about three quarters. Finally, we’ll fill the column with pond or stream water, leaving a small air gap.

Observing a Winogradsky column microcosm is a long-term project. Some visible changes occur soon after you build the microcosm, but the major changes take place gradually over weeks and months.

Q1: We emphasize safety in collecting and handling pond or stream specimens because of the possible presence of pathogens. Using the Internet or other resources, determine two common pathogens in each of the three classes: viruses, bacteria, and protozoa.

Q2: We built Winogradsky columns with sediment enriched by carbon, carbon+sulfur, and carbon+sulfur+calcium. Using the Internet or other resources, suggest several other elements we might have used to enrich the sediment.