Contribution of Soil Ciliates to Biomass and Secondary Production in a Subtropical Forest of Puerto Rico

Protists, ciliates in particular, play a role in ecosystem functioning by recycling microbial biomass, and thus, nutrients and energy. Studies on their secondary production in aquatic ecosystems suggest that their overall contribution to annual secondary production is relatively minor, only becoming significant during bloom episodes. It remains unclear if the ecological role of soil ciliates, as estimated by their secondary production, is similarly minor. Temporal data on ciliate abundance and biovolume among the rhizosphere of three flowering plant species were used to estimate biomass and potential secondary production of soil ciliates in a subtropical forest of Puerto Rico. Ciliate abundance, biomass, and secondary production were estimated in the top 5 cm of soil during both the dry and wet seasons. Mean ciliate abundances in these months were 5 ± 10.3 and 16 ± 16.1 (n = 93 soil samples) for nanociliates and microciliates, respectively. Mean ciliate biomass was 41,854 ± 37,055 pg C g-1 of dry weight soil. Mean nano- and microciliate biomass were 418 ± 704.4 pg C g-1 of dry weight soil and 45,315 ± 39,466.8 pg C g-1 of dry weight soil, respectively. The season in which the ciliates were collected significantly affected ciliate abundance and biomass, but this effect showed no predictable pattern, suggesting a lack of seasonality. Mean daily ciliate secondary production (the product of biomass and growth rate) was 1.74 × 104 ± 1.95 × 104 pg C g-1 of dry weigth soil d-1 (0.002 kJ m-2 d-1 or 0.047 kJ m-3 d-1), ranging from 0 to 107,804 pg C g-1 of dry weigth soil d-1. Based on monthly precipitation data, annual ciliate secondary production was estimated to be 40.0 × 105 pg C g-1 of dry weight soil yr-1, equivalent to 1.1 kJ m-2 yr-1 or 21.7 kJ m-3 yr-1. Reduced secondary production estimates, in addition to low mean consumption of microbial biomass (0.002%), indicates that ciliates have a relatively minor role in this subtropical soil ecosystem.