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Algal Bloom Dynamics: From Cellular Mechanisms to Trophic Level Interactions

Final Report Summary - INFOTROPHIC (Algal Bloom Dynamics: From Cellular Mechanisms to Trophic Level Interactions)

Marine photosynthetic microorganisms (phytoplankton) are the basis of marine food webs. Despite the fact that their biomass represents only about 0.2% of the photosynthetic biomass on earth, they are responsible for nearly 50% of the global annual photosynthesis, and greatly influence the global biogeochemical carbon, oxygen, sulphur and nitrogen cycles. They can form massive blooms in ocean that can stretch over thousands of kilometres and be detected by satellites. These algal blooms are ephemeral events of exceptionally high primary productivity that regulate the flux of carbon across marine food webs. These blooms are regulated by environmental factors such as abiotic stress (nutrient availability, light regime) and biotic interactions with grazers and viruses. Since algal blooms exert a global-scale influence on the atmosphere and climate, we are interested in understanding the cellular mechanisms that govern their response to microbial interactions with pathogens (viruses, bacteria, grazers) that control the fate of these blooms on large scale. Despite of the huge importance of marine algae, relatively little is known about the molecular basis for their ecological success. The wealth of recent genomic information from marine microbes, coupled with availability of a suite of molecular resources and chemical analytical tools, provide an opportunity to address fundamental questions about their unique evolutionary history and ecological success in contemporary oceans. Nevertheless, there is a critical need to “decode” these genomic resources and translate them into cellular mechanisms, community structure and, eventually, to determine their role in ecosystem function. Our work aims at elucidating the cell signalling pathways that regulate cell fate decisions and uncover the chemical “language” (infochemicals) involved in the complex microbial interaction in the oceans. Infochemicals are the information-conveying chemicals that have a functional role in regulating intercellular signalling, cell fate of microorganisms.
We explore three major biotic interactions that have profound significance in controlling algal bloom dynamics in the oceans; cell-cell communication, predator-prey and host-virus interactions. Using diverse approaches, we dissect unexplored signalling pathways employed by algae during these biotic interactions and reveal the role of fundamental cellular mechanism of programmed cell death (PCD), autophagy, and sexual life cycle as possible defense strategies.
Major achievements:
A. Elucidating the role of redox metabolism and ROS signaling in sensing environmental stress conditions
To explore redox-based mechanisms that mediate algal sensing and acclimation to diverse environmental cues, we combined mass-spectrometry based quantification of the whole redox proteome (redoxome) and in vivo mapping of subcellular redox microenvironments. We revealed a metabolic redox-sensitive network that regulates the cellular response to oxidative and nitrogen stress. We provided a systems-level mapping of key redox-sensitive “hubs” in essential metabolic pathways which we currently plan to subject to functional studies. Using the organelle-based redox sensors, we identified distinct compartmentalized signaling in response to light regime, nutrient availability and infochemicals that are derived from biotic interactions. We further identified intriguing correlations between early oxidation patterns specifically in the mitochondria and subsequent induction of cell death signaling cascade and eventually regulation of cell fate decision.
B. Insights into host-virus interactions in the marine environment on multi-scale levels
Viruses that infect marine algae are recognized as major ecological and evolutionary driving forces, shaping community structure and nutrient and energy cycles in the marine environment. We explore the molecular basis for host-virus dynamics and the signal transduction pathways that mediate host resistance and susceptibility to viral infection that are major drivers of massive algal demise in the ocean. Major efforts in our lab are focused on understanding the metabolic demand of infection by these giant dsDNA viruses with high burst size. Our recent work integrated transcriptomic and metabolomic data collected during the course of viral infection revealed a rapid transcriptome remodelling targeted towards de novo fatty acid synthesis. Fuelled by glycolytic fluxes, elevated fatty acid synthesis is essential to support viral assembly and the high demand for internal lipid membranes. A profound metabolic shift towards production of virus-derived sphingolipids was observed during infection by down-regulation of host de novo sphingolipid genes and induction of the virus-encoded homologous pathway, never before described in any viral genome. Products of the virus-encoded pathway lead to accumulation of virus-encoded glycosphingolipids (vGSLs) enriched in viral lipid membranes. vGSL also acts as a bioactive lipid that can induce host PCD during lytic phase of infection. We further identified a novel role for specific sterols derived from the mevalonate-isoprenoid branch in the viral life cycle. Based on this metabolic approach, we recently successfully established the use of specific lipid-based biomarkers for detection of viral infection in natural E. huxleyi populations in the ocean. Furthermore, hallmarks of autophagy, including up-regulation of autophagy-related genes (ATG genes), formation of double-membrane structures (autophagosme) and detection of the host encoded Atg8 protein within purified virions, demonstrated the pivotal role of an autophagy-like process in viral assembly and egress. In addition, this was the first report of the presence of autophagy in marine protists.
The most famous infochemical produced during infection is DMS, a bioactive gas that is released to the atmosphere from algal blooms (~109 tons annually) where it gets oxidized to sulfur aerosols acting as cloud condensation nuclei. Although DMS production was known since the 1930’s, its algal enzymatic source was unknown till now. In collaboration with Prof. Dan Tawfik, we recently revealed a new and critical piece in the puzzle of the oceanic sulfur cycle, by identifying the algal enzyme DMSP lyase in the E. huxleyi, known to produce large amounts of DMS during its massive blooms. This work may therefore allow future studies to quantify the relative biogeochemical contribution of algae and bacteria to the global DMS production and to reveal its signaling role in mediating microbial interactions in the ocean.
In an attempt to achieve large scale quantification of the role of viral infection in the ocean, we collaborated with Prof. Ilan Koren and developed remote sensing tools derived from satellite data that are able to track the dynamics of blooms and potentially quantify its biological control by pathogens (e.g. viruses and grazers) at scales of hundreds of kilometres. This new approach allowed, for the first time, quantification of the turn-over of ~1.6·1010 grams of carbon within one week due to viral infection. Since the phenomena of algal bloom stretches over more than 10 orders of magnitude, relative to a single cell size, we aim to bridge the gap between the cellular level and the macro scale level by studying the mechanism of viral propagation over large scale of algal blooms. We recently conducted a scientific expedition to the North Atlantic Ocean, to track and sample one of the biggest algal blooms, in order to test our lab-based hypothesis on host-virus dynamics under natural population setting. Data collected during this cruise allowed us to discover two novel mechanisms of viral dissemination (by grazers and aerosols) over large scale in the ocean and its feedback to the atmosphere.