Chitin degradation by Synechococcus WH7803.

Chitin is an abundant, carbon-rich polymer in the marine environment. Chitinase activity has been detected in spent media of Synechococcus WH7803 cultures-yet it was unclear which specific enzymes were involved. Here we delivered a CRISPR tool into the cells via electroporation to generate loss-of-function mutants of putative candidates and identified ChiA as the enzyme required for the activity detected in the wild type.

Chitin utilization by marine picocyanobacteria and the evolution of a planktonic lifestyle.

Marine picocyanobacteria Prochlorococcus and Synechococcus, the most abundant photosynthetic cells in the oceans, are generally thought to have a primarily single-celled and free-living lifestyle. However, while studying the ability of picocyanobacteria to supplement photosynthetic carbon fixation with the use of exogenous organic carbon, we found the widespread occurrence of genes for breaking down chitin, an abundant source of organic carbon that exists primarily as particles. We show that cells that encode a chitin degradation pathway display chitin degradation activity, attach to chitin particles, and show enhanced growth under low light conditions when exposed to chitosan, a partially deacetylated soluble form of chitin. Marine chitin is largely derived from arthropods, which underwent major diversifications 520 to 535 Mya, close to when marine picocyanobacteria are inferred to have appeared in the ocean. Phylogenetic analyses confirm that the chitin utilization trait was acquired at the root of marine picocyanobacteria. Together this leads us to postulate that attachment to chitin particles allowed benthic cyanobacteria to emulate their mat-based lifestyle in the water column, initiating their expansion into the open ocean, seeding the rise of modern marine ecosystems. Subsequently, transitioning to a constitutive planktonic life without chitin associations led to cellular and genomic streamlining along a major early branch within Prochlorococcus. Our work highlights how the emergence of associations between organisms from different trophic levels, and their coevolution, creates opportunities for colonizing new environments. In this view, the rise of ecological complexity and the expansion of the biosphere are deeply intertwined processes.

Shifts in predator behaviour following climate induced disturbance on coral reefs.

Coral reefs are increasingly ecologically destabilized across the globe due to climate change. Behavioural plasticity in corallivore behaviour and short-term trophic ecology in response to bleaching events may influence the extent and severity of coral bleaching and subsequent recovery potential, yet our understanding of these interactions in situ remains unclear. Here, we investigated interactions between corallivory and coral bleaching during a severe high thermal event (10.3-degree heating weeks) in Belize. We found that parrotfish changed their grazing behaviour in response to bleaching by selectively avoiding bleached Orbicella spp. colonies regardless of bleaching severity or coral size. For bleached corals, we hypothesize that this short-term respite from corallivory may temporarily buffer coral energy budgets by not redirecting energetic resources to wound healing, and may therefore enable compensatory nutrient acquisition. However, colonies that had previously been heavily grazed were also more susceptible to bleaching, which is likely to increase mortality risk. Thus, short-term respite from corallivory during bleaching may not be sufficient to functionally rescue corals during prolonged bleaching. Such pairwise interactions and behavioural shifts in response to disturbance may appear small scale and short term, but have the potential to fundamentally alter ecological outcomes, especially in already-degraded ecosystems that are vulnerable and sensitive to change.

Filter Plating Method for Rendering Picocyanobacteria Cultures Free of Heterotrophic Bacterial Contaminants and Clonal.

Isolates of the marine picocyanobacteria, Prochlorococcus and Synechococcus, are often accompanied by diverse heterotrophic "contaminating" bacteria, which can act as confounding variables in otherwise controlled experiments. Traditional microbiological methods for eliminating contaminants, such as direct streak-plating, are often unsuccessful with this particular group of microorganisms. While they will grow in pour plates, colonies often remain contaminated with heterotrophic bacteria that can migrate through the soft agar. Additionally, axenic clones of picocyanobacteria can be recovered via dilution-to-extinction in liquid medium, but the efficiency of recovery is low, often requiring large numbers of 96-well plates. Here, we detail a simple and effective protocol for rendering cultures of Synechococcus and Prochlorococcus strains free of bacterial contaminants while at the same time yielding clonal isolates. We build on the fact that co-culture with specific heterotrophs-"helper heterotrophs"-is often necessary to grow colonies of picocyanobacteria from single cells in agar. Suspecting that direct physical contact between the helper and the picocyanobacterial cells was not necessary for the "helper effect," we developed a protocol in which the helper cells are embedded in soft agar pour plates, a filter overlaid on the surface, and a picocyanobacterial culture is diluted and then spotted on top of the filter. With this approach, motile contaminants cannot swim to the colonies, and it is possible to obtain the expected number of colonies from a given input (i.e., a Poisson distribution of colonies with an expected value equal to the input number of cells), thus ensuring clonal colonies. Using this protocol, we rendered three strains of Synechococcus, two strains of Prochlorococcus, and 19 new strains of Synechococcus from coastal seawater clonal and free of heterotrophic bacteria. The simplicity of this approach should expand the repertoire of axenic picocyanobacterial strains available for controlled physiological experiments. It will also enable the study of microdiversity in populations of picocyanobacteria by facilitating large-scale isolation of picocyanobacterial clones from a single source, including direct isolation from natural seawater.