Abstract
Phytoplankton are exposed to different concentrations of nutrients in different waters along with changing light levels during diurnal and seasonal cycles. We grew the coccolithophorid Gephyrocapsa oceanica semi-continuously at different nitrate levels under indoor low and outdoor high light conditions, and found that reduced nitrate availability significantly increased its production of particulate inorganic carbon (PIC), with its growth being reduced. High light treatment suppressed the growth of nitrate-limited cells and their efficiency of N assimilation by up to 63% compared to low light treatment. The combination of high light and nitrate limitation increased contents of PIC per cell due to sustained photochemical energy transfer, resulting in faster sinking rates by up to 82% in comparison with nitrate-repleted cells. Additionally, the sinking rates were positively correlated with ratios of PIC to particulate organic carbon (POC). These results imply that coccolithophores distributed in oligotrophic waters could be more effective as the ballast in aggregates, facilitating particulate organic carbon flux to deeper waters.
Highlights
Particulate organic carbon (POC) produced by phytoplankton and their grazers is sunk out the euphotic layer with particles and partly sequestered in the sediments, efficiently facilitating the absorption of atmospheric CO2 by the oceans (Falkowski et al, 1998; Boyd and Trull, 2007)
Our results indicated that G. oceanica cells grown under N-limited conditions increased their particulate inorganic carbon (PIC) quotas with thicker coccoliths, resulting in faster sinking rates, which were positively correlated with PIC/POC ratios regardless of indoor constant light and outdoor fluctuating sunlight conditions
The microalgal cells sustained their photochemical performances with nonaffected light use efficiency for photosynthetic electron transport even under nitrate deficient conditions (Table 2), so that energy required for PIC production was sufficiently provided
Summary
Particulate organic carbon (POC) produced by phytoplankton and their grazers is sunk out the euphotic layer with particles and partly sequestered in the sediments, efficiently facilitating the absorption of atmospheric CO2 by the oceans (Falkowski et al, 1998; Boyd and Trull, 2007). Recent findings highlight the ballast effect in particles sinking, e.g., opal and CaCO3 produced by diatoms and coccolithophores can accelerate the sinking rate by increasing the specific gravity of particles during the sinking process (Armstrong et al, 2002; Klaas and Archer, 2002). It is known that the calcification of coccolithophores decreases under influence of ocean acidification, leading to a reduction of the CaCO3 precipitation (Riebesell et al, 2000; Raven and Crawfurd, 2012), which decreased the sinking rates of particles (Riebesell et al, 2016)
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