Metabolic Reliance on Photosynthesis Depends on Both Irradiance and Prey Availability in the Mixotrophic Ciliate, Strombidium cf. basimorphum

Abstract

Many species of the ciliate genus Strombidium can acquire functional chloroplasts from a wide range of algal prey and are thus classified as generalist non-constitutive mixotrophs. Little, however, is known about the influence of irradiance and prey availability on their ability to exploit the photosynthetic potential of the chloroplasts, and how this may explain their spatial and temporal distribution in nature. In this study, inorganic carbon uptake, growth, and ingestion rates were measured for S. cf. basimorphum under three different irradiances (10, 40, and 120 μmol photons m–2 s–1) when acclimated to three different prey densities (5 × 103, 1 × 104, and 4 × 104 cells mL–1), as well as when allowed to deplete the prey. After prey depletion, cultures survived without prey longest (∼6 days) at the medium irradiance treatment (40 μmol photons m–2 s–1), while ciliate density, inorganic carbon uptake rates, and cellular chl-a content declined fastest at the highest irradiance treatment. This indicates that the ciliates may be unable to maintain the chloroplasts functionally without replacement at high irradiances. Ingestion rates were not shown to be significantly influenced by irradiance. The maximum gross growth efficiency (GGE) in this study (1.1) was measured in cultures exposed to the medium test irradiance and lowest prey density treatment (5 × 103 cells mL–1). The relative contribution of inorganic carbon uptake to the ciliate carbon budget was also highest in this treatment (42%). A secondary GGE peak (0.99) occurred when cultures were exposed to the highest test irradiance and the medium prey density. These and other results suggest that S. cf. basimorphum, and other generalist non-constitutive mixotrophs, can flexibly exploit many different environmental conditions across the globe.

Competition between mixo‐ and heterotrophic ciliates under dynamic resource supply

“…GNCMs were shown to be more resistant under food limitation than their heterotrophic counterparts. In contrast, GNCMs are generally competitively inferior under food‐replenished conditions, also supporting the assumption of an energetic trade‐off (Hughes et al, 2021; Maselli et al, 2020; McManus et al, 2018; Schoener & McManus, 2017).…”

Effects of finfish farms on pelagic protist communities in a semi-closed stratified embayment

“…These groups are typically abundant in the surface of estuarine and freshwater habitats (Hadas & Berman 1998; Jiang et al 2021; Muylaert et al 2000; Urrutxurtu, Orive & de la Sota 2003), and in this study the ciliates Choreotrichia and Oligotrichia ( Strombidium ) were characteristic of the plankton community found in the surface waters. Their predominance in the surface waters of Macquarie Harbour may be due to a range of factors, including differences in salinity and temperature, availability of humic matter, or prey availability (e.g., cyanobacteria, small dinoflagellates and flagellates) (Carlsson & Granéli 1993; Hughes et al 2021; Jiang et al 2021; Yang et al 2020). Ciliates inhabiting the water column of the Harbour are likely to make a crucial contribution to nutrient cycling and plankton dynamics.…”

Fending for oneself or partnering up: Competition between mixo- and heterotrophic ciliates under dynamic resource supply

“…They postulated that the advantage of chloroplast retention is only high when food concentrations are low, but diminishes when food concentrations are high. Under food limitation, GNCMs were shown to be more resistant than their heterotrophic counterparts (McManus et al 2018, Maselli et al 2020, Hughes et al 2021). In contrast, GNCMs are generally competitively inferior under food replenished conditions, supporting the assumption of an energetic trade-off.…”