Effects of CO2 on growth rate, C:N:P, and fatty acid composition of seven marine phytoplankton species

Abstract

Carbon dioxide (CO 2 ) is the primary substrate for photosynthesis by the phytoplankton that form the base of the marine food web and mediate biogeochemical cycling of C and nutrient elements. Specific growth rate and elemental composition (C:N:P) were characterized for 7 cosmopolitan coastal and oceanic phytoplankton species (5 diatoms and 2 chlorophytes) using low density, nutrient-replete, semi-continuous culture experiments in which CO 2 was manipulated to 4 levels ranging from post-bloom/glacial maxima (< 290 ppm) to geological maxima levels (> 2900 ppm). Specific growth rates at high CO 2 were from 19 to 60% higher than in low CO 2 treatments in 4 species and 44% lower in 1 species; there was no significant change in 2 species. Higher CO 2 availability also resulted in elevated C:P and N:P molar ratios in Thalassiosira pseudonana (~60 to 90% higher), lower C:P and N:P molar ratios in 3 species (~20 to 50% lower), and no change in 3 species. Carbonate system-driven changes in growth rate did not necessarily result in changes in elemental composition, or vice versa. In a subset of 4 species for which fatty acid composition was examined, elevated CO 2 did not affect the contribution of polyunsaturated fatty acids to total fatty acids significantly. These species show relatively little sensitivity between present day CO 2 and predicted ocean acidification scenarios (year 2100). The results, however, demonstrate that CO 2 availability at environmentally and geologically relevant scales can result in large changes in phytoplankton physiology, with potentially large feedbacks to ocean biogeochemical cycles and ecosystem structure.

Phytoplankton as Key Mediators of the Biological Carbon Pump: Their Responses to a Changing Climate

“…If CO 2 emissions keep rising at current rates, it is predicted that the seawater pCO 2 would increase from the present value of~400 to 800-1000 ppm by the year 2100 [43]. CO 2 addition to seawater thus increases HCO 3 − , which brings about a decline in the ocean pH by increasing H + concentration [44].…”

“…CO 2 addition to seawater thus increases HCO 3 − , which brings about a decline in the ocean pH by increasing H + concentration [44]. Such changes in carbonate chemistry are referred to as ocean acidification, which has already led to a nearly 30% increase in seawater H + [45] and is expected to result in a mean pH drop from 8.2 to 7.8 by the year 2100 [43].…”

“…If CO2 emissions keep rising at current rates, it is predicted that the seawater pCO2 would increase from the present value of ~400 to 800-1000 ppm by the year 2100 [43].…”

“…Also, different species of phytoplankton have different requirements for inorganic carbon due to variation in CO 2 concentrating mechanisms (CCMs) in their cells [47]. CCMs increase the concentration of CO 2 , surrounding the carboxylating enzyme Rubisco [43]. In the presence of a CCM, the cell may remain carbon saturated, even when ambient CO 2 levels are low.…”

“…Such species are far more likely to exhibit stimulation of growth and photosynthesis at increased oceanic pCO 2 , as compared to species using HCO 3 − , or taking up CO 2 actively via a CCM [47]. Increased CO 2 availability could potentially alter phytoplankton community composition by favoring taxa having less efficient CCMs [43]. Ambient CO 2 levels can be quite variable in marine ecosystems even in the short-to mid-term [48].…”