Abstract
Temperature and nutrients are key factors affecting the growth, cell size, and physiology of marine phytoplankton. In the ocean, temperature and nutrient availability often co-vary because temperature drives vertical stratification, which further controls nutrient upwelling. This makes it difficult to disentangle the effects of temperature and nutrients on phytoplankton purely from observational studies. In this study, we carried out a factorial experiment crossing two temperatures (13°and 19°C) with two growth regimes (P-limited, semi-continuous batch cultures [“−P”] and nutrient replete batch cultures in turbidostat mode [“+P”]) for three species of common marine haptophytes (Emiliania huxleyi, Chrysochromulina rotalis and Prymnesium polylepis) to address the effects of temperature and nutrient limitation on elemental content and stoichiometry (C:N:P), total RNA, cell size, and growth rate. We found that the main gradient in elemental content and RNA largely was related to nutrient regime and the resulting differences in growth rate and degree of P-limitation, and observed reduced cell volume-specific content of P and RNA (but also N and C in most cases) and higher N:P and C:P in the slow growing −P cultures compared to the fast growing +P cultures. P-limited cells also tended to be larger than nutrient replete cells. Contrary to other recent studies, we found lower N:P and C:P ratios at high temperature. Overall, elemental content and RNA increased with temperature, especially in the nutrient replete cultures. Notably, however, temperature had a weaker–and in some cases a negative–effect on elemental content and RNA under P-limitation. This interaction indicates that the effect of temperature on cellular composition may differ between nutrient replete and nutrient limited conditions, where cellular uptake and storage of excess nutrients may overshadow changes in resource allocation among the non-storage fractions of biomass (e.g. P-rich ribosomes and N-rich proteins). Cell size decreased at high temperature, which is in accordance with general observations.
Highlights
Marine phytoplankton constitute nearly 50% of the global primary productivity (Field et al, 1998) and are an essential component in the biogeochemical cycling of carbon (C), nitrogen (N), and phosphorus (P) (Arrigo, 2005)
alkaline phosphatase activity (APA) was strongly elevated in the −P cultures for all species and temperatures, except for C. rotalis at 13 ◦C, where APA was similar for both P-regimes (Fig. S1 and Table S1; 58% of the variation in APA was explained by P-regime alone)
Temperature had no significant additive effect on APA (p > 0.05), but there was a significant interaction between temperature and species (p = 0.035) manifested by elevated APA at 19 ◦C in P-limited E. huxleyi
Summary
Marine phytoplankton constitute nearly 50% of the global primary productivity (Field et al, 1998) and are an essential component in the biogeochemical cycling of carbon (C), nitrogen (N), and phosphorus (P) (Arrigo, 2005). The latter will likely limit nutrient supply and reduce productivity in stratified tropical waters, but may increase productivity at higher latitudes, where phytoplankton growth is constrained by light and deep mixing (Behrenfeld et al, 2006; Boyce, Lewis & Worm, 2010). Knowledge of how phytoplankton growth, stoichiometry, and size structure respond to combined changes in these factors is important to predict how productivity, food web dynamics, and the biogeochemical cycling of C, N and P will be affected in a warmer and more stratified ocean (Arrigo et al, 1999; Arrigo, 2005; Litchman et al, 2015; Sommer et al, 2017)
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