The symbiotic relationship between green algae and freshwater sponges: quantification of archaeocyte-associated algae
Freshwater sponges often contain symbiotic algae, genus Chlorella, within their cells, yet there is no quantitative data available on either the number of algae present in sponge host cells or the proportion of the host cell occupied by algae. The objectives of my study were (1) to determine the number of algae in archaeocytes, the sponge cell type containing most of the symbiotic algae, and the ratio of algal volume both to total host cell volume and to host cytoplasmic volume and (2) to determine the effect of light deprivation on the number of algae per host cell and the ratio of algal volume to host cell volume. Samples of the freshwater sponge, Ephydatia muelleri, were collected during the summer from Salmon Lake in Western Montana. The samples were dissociated and observed using brightfield and epifluorescence microscopy. There was an average of 20.5 + 16.5 algae per host cell, which occupy on average 3.8% + 6.5% of the host cell volume and 4.0% + 7.0% of the total cytoplasmic volume. The effect of light deprivation on the algae was a continuous decrease in algae numbers per host cell over time (numbers remain almost constant over time when sponges are kept in alternating light and dark). Unexpectedly, the ratio of algal volume to host cell volume in light deprived sponges remained almost constant over time. Further study is needed to reconcile the decrease in the number of algae with the constancy of the volume ratio.