Climate change and the aquatic continuum: A cyanobacterial comeback story

Billions of years ago, the Earth's waters were dominated by cyanobacteria. These microbes amassed to such formidable numbers, they ushered in a new era-starting with the Great Oxidation Event-fuelled by oxygenic photosynthesis. Throughout the following eon, cyanobacteria ceded portions of their global aerobic power to new photoautotrophs with the rise of eukaryotes (i.e. algae and higher plants), which co-existed with cyanobacteria in aquatic ecosystems. Yet while cyanobacteria's ecological success story is one of the most notorious within our planet's biogeochemical history, scientists to this day still seek to unlock the secrets of their triumph. Now, the Anthropocene has ushered in a new era fuelled by excessive nutrient inputs and greenhouse gas emissions, which are again reshaping the Earth's biomes. In response, we are experiencing an increase in global cyanobacterial bloom distribution, duration, and frequency, leading to unbalanced, and in many instances degraded, ecosystems. A critical component of the cyanobacterial resurgence is the freshwater-marine continuum: which serves to transport blooms, and the toxins they produce, on the premise that "water flows downhill". Here, we identify drivers contributing to the cyanobacterial comeback and discuss future implications in the context of environmental and human health along the aquatic continuum. This Minireview addresses the overlooked problem of the freshwater to marine continuum and the effects of nutrients and toxic cyanobacterial blooms moving along these waters. Marine and freshwater research have historically been conducted in isolation and independently of one another. Yet, this approach fails to account for the interchangeable transit of nutrients and biology through and between these freshwater and marine systems, a phenomenon that is becoming a major problem around the globe. This Minireview highlights what we know and the challenges that lie ahead.

Acquisition and use of bicarbonate by Emiliania huxleyi

•   Bicarbonate acquisition mechanisms and the kinetics of dissolved inorganic carbon (DIC) use in photosynthesis and calcification were investigated in Emiliania huxleyi. •   Photosynthesis was measured using O₂ evolution and ¹⁴ C incorporation and calcification was measured with ¹⁴ C. Noncalcifying (coccolith-free) cells were produced from calcifying (coccolith-bearing) cells of the same strain of E. huxleyi, so that photosynthesis could be monitored independently from calcification. •   Neither photosynthesis nor calcification was saturated at the ambient DIC concentration of seawater. In coccolith-bearing cells, both processes showed biphasic kinetics with DIC concentration, with a hiatus located at 1 mM. The same biphasic pattern and similar rates of photosynthesis were found in the coccolith-free cells. Inhibitor experiments showed that E. huxleyi acquires bicarbonate mainly by an anion exchange protein, but external carbonic anhydrase can be activated at low concentrations of DIC. •   We conclude that the biphasic kinetics of photosynthesis and calcification are caused by the presence of two bicarbonate acquisition mechanisms and also, since calcification does not enhance photosynthesis in this coccolithophore, we question the current view that the two processes are tightly coupled.