Efficient CO2 fixation by surface Prochlorococcus in the Atlantic Ocean

Nearly half of the Earth’s surface is covered by the ocean populated by the most abundant photosynthetic organisms on the planet—Prochlorococcus cyanobacteria. However, in the oligotrophic open ocean, the majority of their cells in the top half of the photic layer have levels of photosynthetic pigmentation barely detectable by flow cytometry, suggesting low efficiency of CO2 fixation compared with other phytoplankton living in the same waters. To test the latter assumption, CO2 fixation rates of flow cytometrically sorted 14C-labelled phytoplankton cells were directly compared in surface waters of the open Atlantic Ocean (30°S to 30°N). CO2 fixation rates of Prochlorococcus are at least 1.5–2.0 times higher than CO2 fixation rates of the smallest plastidic protists and Synechococcus cyanobacteria when normalised to photosynthetic pigmentation assessed using cellular red autofluorescence. Therefore, our data indicate that in oligotrophic oceanic surface waters, pigment minimisation allows Prochlorococcus cells to harvest plentiful sunlight more effectively than other phytoplankton

Adaptive divergence in pigment composition promotes phytoplankton biodiversity

The dazzling diversity of the phytoplankton has puzzled biologists for decades(1-5). The puzzle has been enlarged rather than solved by the progressive discovery of new phototrophic microorganisms in the oceans, including picocyanobacteria(6,7), pico-eukaryotes(8), and bacteriochlorophyll-based(9-11) and rhodopsin-based phototrophic bacteria(12,13). Physiological and genomic studies suggest that natural selection promotes niche differentiation among these phototrophic microorganisms, particularly with respect to their photosynthetic characteristics(14-16). We have analysed competition for light between two closely related picocyanobacteria of the Synechococcus group that we isolated from the Baltic Sea(17). One of these two has a red colour because it contains the pigment phycoerythrin, whereas the other is blue-green because it contains high contents of the pigment phycocyanin. Here we report theory and competition experiments that reveal stable coexistence of the two picocyanobacteria, owing to partitioning of the light spectrum. Further competition experiments with a third marine cyanobacterium, capable of adapting its pigment composition, show that this species persists by investing in the pigment that absorbs the colour not used by its competitors. These results demonstrate the adaptive significance of divergence in pigment composition of phototrophic microorganisms, which allows an efficient utilization of light energy and favours species coexistence

Modeling the Fitness Consequences of a Cyanophage-Encoded Photosynthesis Gene

Background: Phages infecting marine picocyanobacteria often carry a psbA gene, which encodes a homolog to the photosynthetic reaction center protein, D1. Host encoded D1 decays during phage infection in the light. Phage encoded D1 may help to maintain photosynthesis during the lytic cycle, which in turn could bolster the production of deoxynucleoside triphosphates (dNTPs) for phage genome replication. Methodology / Principal Findings: To explore the consequences to a phage of encoding and expressing psbA, we derive a simple model of infection for a cyanophage/host pair — cyanophage P-SSP7 and Prochlorococcus MED4— for which pertinent laboratory data are available. We first use the model to describe phage genome replication and the kinetics of psbA expression by host and phage. We then examine the contribution of phage psbA expression to phage genome replication under constant low irradiance (25 µE m[superscript −2] s[superscript −1]). We predict that while phage psbA expression could lead to an increase in the number of phage genomes produced during a lytic cycle of between 2.5 and 4.5% (depending on parameter values), this advantage can be nearly negated by the cost of psbA in elongating the phage genome. Under higher irradiance conditions that promote D1 degradation, however, phage psbA confers a greater advantage to phage genome replication. Conclusions / Significance: These analyses illustrate how psbA may benefit phage in the dynamic ocean surface mixed layer.United States. Dept. of Energy. Genomic Science ProgramGordon and Betty Moore Foundation Marine Microbiology InitiativeNational Science Foundatio

The EU Humanitarian Border and the Securitization of Human Rights:The “Rescue-through-Interdiction/Rescue-without-Protection” Paradigm

This article looks at securitization/humanitarianization dynamics in the EU external sea borders to track and critique the substantial transformation of the role played by human rights in the Mediterranean. Mapping the evolution of maritime engagement up to the ‘refugee crisis’, it is revealed how the invocation of human rights serves paradoxically to curtail (migrants') human rights, justifying interdiction (‘to save lives’), and impeding access to safety in Europe. The result is a double reification of ‘boat migrants’ as threats to border security and as victims of smuggling/trafficking. Through a narrative of ‘rescue’, interdiction is laundered into an ethically sustainable strategy of border governance. Instead of being considered a problematic (potentially lethal) means of control, it is re-defined into a life-saving device. The ensuing ‘rescue-through-interdiction’/‘rescue-without-protection’ paradigm alters the nature of human rights, which, rather than functioning as a check on interdiction, end up co-opted as another securitization/humanitarianization tool