Overcoming adversity through diversity: aquatic carbon concentrating mechanisms.

Carbon concentrating mechanism (CCM) systems, asso- ciated with evolutionarily diverse aquatic photosynthetic organisms, make a major contribution to global net primary productivity and marine carbon sequestration. Here, an overview of these global contributions is pre- sented from their evolutionary origins, including a pos- sible trigger for their diversi cation when the aqueous O2/CO2 ratio rose above parity, and a re-de nition of the paradox of phytoplankton. The reviews and research in the special issue also include molecular physiology and ecology of CCMs, through to future potential applications for sustaining carbon sequestration and supporting ter- restrial crop productivity

The influence of elevated SiO2 (aq) on intracellular silica uptake and microbial metabolism.

Microbes are known to accumulate intracellular SiO2 (aq) up to 100s of mmol/l from modern seawater (SiO2 (aq) <100 µmol/l), despite having no known nutrient requirement for Si. Before the evolution of siliceous skeletons, marine silica concentrations were likely an order of magnitude higher than the modern ocean, raising the possibility that intracellular SiO2 (aq) accumulation interfered with normal cellular function in non-silicifying algae. Yet, because few culturing studies have isolated the effects of SiO2 (aq) at high concentration, the potential impact of elevated marine silica on early microbial evolution is unknown. Here, we test the influence of elevated SiO2 (aq) on eukaryotic algae, as well as a prokaryote species. Our results demonstrate that under SiO2 (aq) concentrations relevant to ancient seawater, intracellular Si accumulates to concentrations comparable to those found in siliceous algae such as diatoms. In addition, all eukaryotic algae showed a statistically significant response to the high-Si treatment, including reduced average cell sizes and/or a reduction in the maximum growth rate. In contrast, there was no consistent response to the high-Si treatment by the prokaryote species. Our results highlight the possibility that elevated marine SiO2 (aq) may have been an environmental stressor during early eukaryotic evolution

Large variation in the Rubisco kinetics of diatoms reveals diversity among their carbon-concentrating mechanisms

While marine phytoplankton rival plants in their contribution to global primary productivity, our understanding of their photosynthesis remains rudimentary. In particular, the kinetic diversity of the CO2-fixing enzyme, Rubisco, in phytoplankton remains unknown. Here we quantify the maximum rates of carboxylation (kcatc), oxygenation (kcato), Michaelis constants (Km) for CO2 (KC) and O2 (KO), and specificity for CO2 over O2 (SC/O) for Form I Rubisco from 11 diatom species. Diatom Rubisco shows greater variation in KC (23–68 µM), SC/O (57–116mol mol−1), and KO (413–2032 µM) relative to plant and algal Rubisco. The broad range of KC values mostly exceed those of C4 plant Rubisco, suggesting that the strength of the carbon-concentrating mechanism (CCM) in diatoms is more diverse, and more effective than previously predicted. The measured kcatc for each diatom Rubisco showed less variation (2.1–3.7s−1), thus averting the canonical trade-off typically observed between KC and kcatc for plant Form I Rubisco. Uniquely, a negative relationship between KC and cellular Rubisco content was found, suggesting variation among diatom species in how they allocate their limited cellular resources between Rubisco synthesis and their CCM. The activation status of Rubisco in each diatom was low, indicating a requirement for Rubisco activase. This work highlights the need to better understand the correlative natural diversity between the Rubisco kinetics and CCM of diatoms and the underpinning mechanistic differences in catalytic chemistry among the Form I Rubisco superfamily

Does the life cycle stage matter for distinguishing phytoplankton via fluoro-electrochemical microscopy?

Phytoplankton have species-specific responses toward electrogenerated oxidants, allowing high-throughput species analysis. Herein, a fluoro-electrochemical method is used to expose single Chlamydomonas concordia vegetative cells at different points within their life cycle to electro-generated oxidants from seawater. The resulting decay in fluorescence from chlorophyll-a is measured as a function of time and drops to zero for phytoplankton adjacent to the electrode over a period of a few seconds. The chlorophyll-a transient timescale allows mother cells, which are distinctively larger and require a larger quantity of oxidants, to be distinguished from either zoospores or “growing” cells, while all the cells show the same intrinsic susceptibility modulated only by the size of the phytoplankton. These observations are essential for the future automated characterization of the speciation of phytoplankton populations as they show that there is no need to manually identify the life cycle stage

Why marine phytoplankton calcify.

Calcifying marine phytoplankton-coccolithophores- are some of the most successful yet enigmatic organisms in the ocean and are at risk from global change. To better understand how they will be affected, we need to know "why" coccolithophores calcify. We review coccolithophorid evolutionary history and cell biology as well as insights from recent experiments to provide a critical assessment of the costs and benefits of calcification. We conclude that calcification has high energy demands and that coccolithophores might have calcified initially to reduce grazing pressure but that additional benefits such as protection from photodamage and viral/bacterial attack further explain their high diversity and broad spectrum ecology. The cost-benefit aspect of these traits is illustrated by novel ecosystem modeling, although conclusive observations remain limited. In the future ocean, the trade-off between changing ecological and physiological costs of calcification and their benefits will ultimately decide how this important group is affected by ocean acidification and global warming