Moxi Z Applied to Assessing Algal Growth
Measurement of algal growth parameters such as cell size, cell number, and growth rate is important for confirming optimal growth conditions and culture health. One of the best ways to simultaneously assess algal culture health is to take a series of cell size and number measurements during their growth cycle. The simplest method to do this is to use the Coulter Principle.
Other methods such as combining spectrophotometry (light scattering or absorbance assays) with manual hemocytometer counts are slow, cumbersome, and difficult to perform quickly, especially in the field. Until recently, simultaneously measuring cell size and number with the Coulter Principle required expensive stationary equipment. These systems were inaccessible for many laboratories and not amenable for field measurements.
Many algal species do not undergo binary fission, especially the model algal Chlamydomonas reinhardtii, which instead undergoes multiple fission, where cell growth and division are uncoupled. Moreover, being photosynthetic, many algal species, including Chlamydomonas, synchronize their cell cycles to the availability of light and divide in the dark. Consequently, when growing, Chlamydomonas cell numbers remains unchanged while cell size increases as much as 10-fold in a single day (Figure 3A).
Measurement of the growth of Chlamydomonas during light cycles is best performed with the Coulter method. The Moxi Z is being applied as an inexpensive portable instrument for measuring cell number and sizes in the laboratory at Kansas State University.
For example, in Figure 3A, a synchronous culture of Chlamydomonas was sampled just as growth had initiated and then six hours later of a total, 14 hour, growth cycle. While the number of cells in the synchronous sample has not changed (determined by the Moxi Z as 3.51e5 cells/mL), the volume of the cells has changed from ~50 µm3 to ~320 µm3, a fivefold increase in cell size in six hours.
In addition to being a valuable tool for measuring Chlamydomonas growth rates, the Moxi Z can also be used for determining physiological differences between species. For example the author’s lab also uses Chlamydomonas and a closely related colonial multicellular alga, Gonium pectorale, as a model system for multicellular evolution.
Chlamydomonas is a unicellular strain, while Gonium species are colonies of Chlamydomonas-like cells that remain attached through cytoplasmic bridges (see micrograph insets). The lab uses a variety of Gonium strains in the course of research. Using the Moxi Z, it is straightforward to determine if a particular culture is in fact Gonium.
The Moxi Z is also ideal for characterizing Gonium colony size and cells per colony. For example, Figure 3B shows the corresponding size profiles on the Moxi Z (Figure 3B blue versus green). From these data, the Gonium strain is ~850 µm3, suggesting it is composed of 8 cells of ~110 µm3, a fact confirmed by microscopy.