Abstract
Phytoplankton play a central role in the regulation of global carbon and nutrient cycles, forming the basis of the marine food webs. A group of biogeochemically important phytoplankton, the coccolithophores, produce calcium carbonate scales that have been hypothesized to deter or reduce grazing by microzooplankton. Here, a meta-analysis of mesocosm-based experiments demonstrates that calcification of the cosmopolitan coccolithophore, Emiliania huxleyi, fails to deter microzooplankton grazing. The median grazing to growth ratio for E. huxleyi (0.56 ± 0.40) was not significantly different among non-calcified nano- or picoeukaryotes (0.71 ± 0.31 and 0.55 ± 0.34, respectively). Additionally, the environmental concentration of E. huxleyi did not drive preferential grazing of non-calcified groups. These results strongly suggest that the possession of coccoliths does not provide E. huxleyi effective protection from microzooplankton grazing. Such indiscriminate consumption has implications for the dissolution and fate of CaCO3 in the ocean, and the evolution of coccoliths.
Highlights
Coccolithophores are small (2–20 μm), unicellular marine algae which form calcium carbonate (CaCO3) scales that adorn their cells, making them a key player in the global production of CaCO3 and its export from the upper ocean to deep-sea sediments (Berelson et al, 2007; Broecker and Clark, 2009; Balch, 2018)
Dilution experiments were conducted during mesocosm experiments over three years to determine phytoplanktongroup specific growth and microzooplankton grazing rates
The intensity of microzooplankton predation is based on a myriad of factors, including the ability to capture prey and prey palatability (Montagnes et al, 2008)
Summary
Coccolithophores are small (2–20 μm), unicellular marine algae which form calcium carbonate (CaCO3) scales (coccoliths) that adorn their cells, making them a key player in the global production of CaCO3 and its export from the upper ocean to deep-sea sediments (Berelson et al, 2007; Broecker and Clark, 2009; Balch, 2018). The coccolithophore Emiliania huxleyi (Lohmann) Hay and Mohler, forms large-scale blooms that may span over 250,000 km (Holligan et al, 1993) and is considered one of the most widely distributed and globally abundant algae in the contemporary ocean (Winter et al, 1994; Read et al, 2013). Coccolithophores such as E. huxleyi have key roles in several biogeochemical cycles, through the ballasting of organic matter fluxes to the deep-sea (Bach et al, 2016), influencing air-sea CO2 exchange (Shutler et al, 2013), and atmospheric sulfur production (Simó, 2001). These mechanisms include protection against calcium poisoning (Müller et al, 2015), to mitigate against stressful light and UV radiation (Xu et al, 2016), and to reduce host susceptibility to viral infection (Johns et al, 2019). Monteiro et al (2016) suggested that protection from zooplankton predation could be a significant driver of calcification but acknowledged that additional benefits may exist and that there is little or no direct field data to support these hypotheses
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