Marine coccolithophorid phytoplankton are major producers of biogenic calcite, playing a significant role in the global carbon cycle. Predicting the impacts of ocean acidification on coccolithophore calcification has received much recent attention and requires improved knowledge of cellular calcification mechanisms. Uniquely amongst calcifying organisms, coccolithophores produce calcified scales (coccoliths) in an intracellular compartment and secrete them to the cell surface, requiring large transcellular ionic fluxes to support calcification. In particular, intracellular calcite precipitation using HCO₃⁻ as the substrate generates equimolar quantities of H⁺ that must be rapidly removed to prevent cytoplasmic acidification. We have used electrophysiological approaches to identify a plasma membrane voltage-gated H⁺ conductance in Coccolithus pelagicus ssp braarudii with remarkably similar biophysical and functional properties to those found in metazoans. We show that both C. pelagicus and Emiliania huxleyi possess homologues of metazoan H_v1 H⁺ channels, which function as voltage-gated H⁺ channels when expressed in heterologous systems. Homologues of the coccolithophore H⁺ channels were also identified in a diversity of eukaryotes, suggesting a wide range of cellular roles for the H_v1 class of proteins. Using single cell imaging, we demonstrate that the coccolithophore H⁺ conductance mediates rapid H⁺ efflux and plays an important role in pH homeostasis in calcifying cells. The results demonstrate a novel cellular role for voltage gated H⁺ channels and provide mechanistic insight into biomineralisation by establishing a direct link between pH homeostasis and calcification. As the coccolithophore H⁺ conductance is dependent on the trans-membrane H⁺ electrochemical gradient, this mechanism will be directly impacted by, and may underlie adaptation to, ocean acidification. The presence of this H⁺ efflux pathway suggests that there is no obligate use of H⁺ derived from calcification for intracellular CO₂ generation. Furthermore, the presence of H_v1 class ion channels in a wide range of extant eukaryote groups indicates they evolved in an early common ancestor. The production of calcium carbonate structures by marine organisms has a major influence on the Earth's carbon cycle and is responsible for the eventual formation of sedimentary rocks such as chalk and limestone. The major contributors to marine calcification are the coccolithophores, a family of unicellular algae which surround themselves in calcified plates known as coccoliths. Unlike many other calcifying organisms, coccolithophores produce their calcified structures inside the cell, enabling precise control of this process. However, the other product resulting from the calcification reaction, H⁺, must be rapidly removed to maintain the pH inside the cell. In this study, we show that coccolithophores possess a voltage-gated H⁺ channel, which removes H⁺ rapidly from the cell during calcification and helps maintain a constant pH. We identify the gene encoding this H⁺ channel, HVCN1, and find that it is a distant relative of those recently identified in animal cells, suggesting that H⁺ channels may be present in many other types of eukaryote organism. As calcifying organisms may be affected by ocean acidification, the identification of an H⁺ channel in coccolithophores gives us an important mechanistic understanding of cellular pH regulation during the calcification process, and may give insight into the response of coccolithophores to future changes in ocean pH.

Coccolithophores occupy a special position within the marine phytoplankton because of their production of intricate calcite scales, or coccoliths. Coccolithophores are major contributors to global ocean calcification and long-term carbon fluxes. The intracellular production of coccoliths requires modifications to cellular ultrastructure and metabolism that are surveyed here. In addition to calcification, which appears to have evolved with a diverse range of functions, several other remarkable features that likely underpin the ecological and evolutionary success of coccolithophores have recently been uncovered. These include complex and varied life cycle strategies related to abiotic and biotic interactions as well as a range of novel metabolic pathways and nutritional strategies. Together with knowledge of coccolithophore genetic and physiological variability, these findings are beginning to shed new light on species diversity, distribution, and ecological adaptation. Further advances in genetics and functional characterization at the cellular level will likely to lead to a rapid increase in this understanding.

Both Ca²⁺ and reactive oxygen species (ROS) play critical signaling roles in plant responses to biotic and abiotic stress. However, the positioning of Ca²⁺ and ROS (in particular H₂O₂) after a stress stimulus and their subcellular interactions are poorly understood. Moreover, although information can be encoded in different patterns of cellular Ca²⁺ signals, little is known about the subcellular spatiotemporal patterns of ROS production or their significance for downstream responses. Here, we show that ROS production in response to hyperosmotic stress in embryonic cells of the alga Fucus serratus consists of two distinct components. The first ROS component coincides closely with the origin of a Ca²⁺ wave in the peripheral cytosol at the growing cell apex, has an extracellular origin, and is necessary for the Ca²⁺ wave. Patch-clamp experiments show that a nonselective cation channel is stimulated by H₂O₂ and may underlie the initial cytosolic Ca²⁺ increase. Thus, the spatiotemporal pattern of the Ca²⁺ wave is determined by peripheral ROS production. The second, later ROS component localizes to the mitochondria and is a direct consequence of the Ca²⁺ wave. The first component, but not the second, is required for short-term adaptation to hyperosmotic stress. Our results highlight the role of ROS in the patterning of a Ca²⁺ signal in addition to its function in regulating cell wall strength in the Fucus embryo.

Biomineralization by marine phytoplankton, such as the silicifying diatoms and calcifying coccolithophores, plays an important role in carbon and nutrient cycling in the oceans. Silicification and calcification are distinct cellular processes with no known common mechanisms. It is thought that coccolithophores are able to outcompete diatoms in Si-depleted waters, which can contribute to the formation of coccolithophore blooms. Here we show that an expanded family of diatom-like silicon transporters (SITs) are present in both silicifying and calcifying haptophyte phytoplankton, including some globally important coccolithophores. Si is required for calcification in these coccolithophores, indicating that Si uptake contributes to the very different forms of biomineralization in diatoms and coccolithophores. Significantly, SITs and the requirement for Si are absent from highly abundant bloom-forming coccolithophores, such asEmiliania huxleyi. These very different requirements for Si in coccolithophores are likely to have major influence on their competitive interactions with diatoms and other siliceous phytoplankton.