Phytoplankton sterol contents vary with temperature, phosphorus and silicate supply: a study on three freshwater species

Abstract

The understanding of environmentally induced changes in the biochemical composition of phytoplankton species is of great importance in both physiological studies and ecological food web research. In extensive laboratory experiments we tested the influence of two different temperatures (10 C and 25 C) and a phosphorus supply gradient on the sterol concentrations of the three freshwater phytoplankton species Scenedesmus quadricauda, Cryptomonas ovata and Cyclotella meneghiniana. The diatom C. meneghiniana was additionally exposed to a silicate gradient. In two separate experiments we analysed (1) possible interactive effects of temperature and phosphorus supply and (2) the effect of four phosphorus levels and three silicate levels on algal sterol concentrations. We observed that sterol concentrations were higher at 25 C than at 10 C in S. quadricauda and C. meneghiniana, but were not affected by temperature in C. ovata. Interactive effects of temperature and phosphorus supply on sterol concentrations were found in C. meneghiniana. This presumably was due to the bioconversion of one sterol (24-methylenecholesterol) into another (22-dihydrobrassicasterol). Increasing phosphorus supply resulted in species-specific effects on sterol concentrations, viz. an optimum curve response in S. quadricauda, a saturation curve response in C. meneghiniana and no change in sterol concentration in C. ovata. Effects of silicate supply on the sterols of C. meneghiniana equalled the effects of phosphorus supply. Albeit we did not observe a general trend in the three phytoplankton species tested, we conclude that sterol concentrations of phytoplankton are strongly affected by temperature and nutrient supply. Interactive effects point out the importance of taking into account more than just one environmental factor when assessing the effects of environmentally induced changes on phytoplankton sterol concentrations.

Daphnia's dilemma of adjusting carbon budgets when facing limitations by food quantity and the essential organic compound cholesterol

“…However, herbivore sterol limitation might not be restricted to cyanobacteria blooms as eukaryotic algae can also be poor in sterols. In particular, high light intensity and low nutrient availability (for instance in summer) can reduce sterol concentrations in algae below critical levels for herbivorous zooplankton such as Daphnia (Piepho et al, 2010;Piepho et al, 2012). Furthermore, not all phytosterols of eukaryotic algae are suitable precursors for cholesterol, and they vary in their conversion efficiency to cholesterol (Martin-Creuzburg and Von Elert, 2004).…”

Induction of carotenoid and phytosterol accumulation in microalgae

“…Statistical analysis confirmed that significant differences were observed between the sampling days but not for the different salinities (Table 5.2). As this effect was independent of changes in salinity, it was likely that the increase in total phytosterol was supported by the dilution of cells on day 11, leading to a lower cell density and a higher light exposure per cell as light was known to play a role in sterol biosynthesis by microalgae (Piepho et al 2012). Notably, the results show that microalgal phytosterol contents higher than 5% DW are achievable in P. lutheri.…”

“…Previous studies have shown that availability of nutrients and salility of the cultivation medium may influence phytosterol biosynthesis in microalgae (Francavilla et al, 2010;Piepho et al 2012;Peeler et al, 1989;Zelazny et al, 1995;). When P. lutheri was grown under high nitrate or phosphate concentrations, the sterol contents varied from 5.0 ±0.8 mg/g DW (6P on day 2) to 7.6 ± 0.6 mg/g DW (control on day 10) during the 10 days of cultivation ( Fig.…”

“…However, it has been reported that changes in growth conditions such as the renewal rate of semicontinuous cultures (Fabregas et al, 1997), salt concentration in the growth medium (Peeler et al, 1989;Zelazny et al, 1995;Francavilla et al, 2010), type of photobioreactor (Ponis et al, 2006), light and phosphorus content (Piepho et al, 2010), temperature and silicate content (Piepho et al, 2012) and the stage of growth (Xu et al, 2008) can affect the production of sterols in the different microalgae strains studied so far.…”

“…Through such existence, the sterols play significant roles in the maintenance of cellular structural stability, and acclimation to membrane temperature (Piepho et al, 2012). Sterols help maintain fluidity and permeability by controlling movement of fatty acid chains within the membrane (Dufourc, 2008).…”