High temperature decreases the PIC / POC ratio and increases phosphorus requirements in <i>Coccolithus pelagicus</i> (Haptophyta)

Abstract

Abstract. Rising ocean temperatures will likely increase stratification of the water column and reduce nutrient input into the photic zone. This will increase the likelihood of nutrient limitation in marine microalgae, leading to changes in the abundance and composition of phytoplankton communities, which in turn will affect global biogeochemical cycles. Calcifying algae, such as coccolithophores, influence the carbon cycle by fixing CO 2 into particulate organic carbon through photosynthesis (POC production) and into particulate inorganic carbon through calcification (PIC production). As calcification produces a net release of CO 2 , the ratio of PIC to POC production determines whether coccolithophores act as a source (high PIC / POC) or a sink (low PIC / POC) of atmospheric CO 2 . We studied the effect of phosphorus (P-) limitation and high temperature on the physiology and the PIC / POC ratio of two subspecies of Coccolithus pelagicus. This large and heavily calcified species is a major contributor to calcite export from the photic zone into deep-sea reservoirs. Phosphorus limitation did not influence exponential growth rates in either subspecies, but P-limited cells had significantly lower cellular P-content. One of the subspecies was subjected to a 5 • C temperature increase from 10 • C to 15 • C, which did not affect exponential growth rates either, but nearly doubled cellular P-content under both high and low phosphate availability. This temperature increase reduced the PIC / POC ratio by 40-60 %, whereas the PIC / POC ratio did not differ between P-limited and nutrientreplete cultures when the subspecies were grown near their respective isolation temperature. Both P-limitation and elevated temperature significantly increased coccolith malformations. Our results suggest that a temperature increase may intensify P-limitation due to a higher P-requirement to maintain growth and POC production rates, possibly reducing abundances in a warmer ocean. Under such a scenario C. pelagicus may decrease its calcification rate relative to photosynthesis, thus favouring CO 2 sequestration over release. It seems unlikely that P-limitation by itself causes changes in the PIC / POC ratio in this species.

Physiology regulates the relationship between coccosphere geometry and growth phase in coccolithophores

“…This represents a significant increase of 4 to 7 % on exponential-phase mean ∅ and an increase of 10 to 27 % on exponential-phase mean C N (t test, p<0.0001). C N is not a frequently recorded variable, but ∅ and C N in both nutrient-replete and nutrient-deplete cultures can sometimes be inferred from supplementary information (Balch et al, 1993;Paasche, 1998;Gerecht et al, 2014Gerecht et al, , 2015. These are consistent with the extensive observations from our experiments for Calcidiscus and H. carteri and those of Gibbs et al (2013) for C. braarudii.…”

“…Given the tendency of coccolithophores to show strong species-and strain-specific responses to external factors, extending this application to other fossil species might be seen as highly speculative based on data from only two modern species. The new experimental data presented here for Calcidiscus and Helicosphaera, in combination with previous results for Coccolithus and Emiliania (Balch et al, 1993;Paasche, 1998;Gibbs et al, 2013;Gerecht et al, 2014Gerecht et al, , 2015, provide validity that coccosphere geometry persistently responds to growth phase in a common manner, regardless of species, and notably that mean population C N increases under slowed growth in Calcidiscus, Helicosphaera, Coccolithus and Emiliania.…”

“…An increase in cell size has also previously been observed in response to nutrient limitation in Coccolithus and Helicosphaera (Gerecht et al, 2014.…”

Phosphorus limitation and heat stress decrease calcification in &lt;i&gt;Emiliania huxleyi&lt;/i&gt;

“…In a (semi-)continuous culturing setup, growth rate is constant over the course of the experiment and production rates can be calculated (Paasche and Brubak, 1994;Paasche, 1998;Riegman et al, 2000;Borchard et al, 2011). Ratio data such as coccolith morphology, on the other hand, should be comparable between batch and (semi-)continuous culture experiments (Langer et al, 2013b), as has been shown for C. pelagicus (Gerecht et al, 2014. However, the only strain of E. huxleyi (B92/11) that was tested in both batch and continuous culture was not analysed for coccolith morphology and the PIC / POC ratio showed a markedly different response to P limitation in batch and in continuous culture (Borchard et al, 2011;Langer et al, 2013b).…”

“…non-constant growth rates), nutrientlimited production cannot be determined in the batch approach (e.g. Müller et al, 2008;Langer et al, 2012Langer et al, , 2013bGerecht et al, 2014;Oviedo et al, 2014;Perrin et al, 2016). In a (semi-)continuous culturing setup, growth rate is constant over the course of the experiment and production rates can be calculated (Paasche and Brubak, 1994;Paasche, 1998;Riegman et al, 2000;Borchard et al, 2011).…”