Darwin Welcomes Visiting Postdoc Subhendu Chakraborty

A warm Darwin-welcome to Subhendu Chakraborty, a Postdoctoral Research Fellow in the Marine Biological Section of the Department of Biology at the University of Copenhagen in Denmark working with Lasse Riemann to model marine nitrogen fixation. 

Subhendu will be visiting the Darwin Group through the end of November with the goal of developing a size-resolved particle model to predict the amount of global nitrogen fixation by heterotrophic bacteria associated with sinking marine snow particles.

He holds a PhD in Mathematical Biology (2009) from the Indian Statistical Institute for work with Prof. Joydev Chattopadhyay looking at the nonlinear dynamics of nutrient-phytoplankton-zooplankton systems in the presence of toxin-producing plankton. His BS is in Mathematics (2002), and his MS in Applied Mathematics (2004), both from theUniversity of Calcutta, India.

In his spare time, Subhendu enjoys cooking, singing, chatting with friends, and playing cricket. During his visit, you can find him in 54-1422. His email is schakraborty@bio.ku.dk

From Subhendu:

My work seeks to understand and quantitatively model the constraints and activity of heterotrophic nitrogen fixers in the ocean. These organisms are ubiquitous – their genetic signature is found throughout the ocean – but their ecology and biogeochemical impact remain unclear.

Each year, the ocean loses more than 200 Tg of nitrogen from its bioavailable nitrogen (N2) pool through denitrification processes. If this loss wasn’t compensated by nitrogen-inputs, the ocean would quickly lose its fertility, because fixed nitrogen is the most important nutrient fueling phytoplankton growth in the ocean. A special group of marine plankton, known as N2-fixers, compensates a substantial part of this loss by converting dissolved nitrogen gas into bioavailable nitrogen such as ammonium and organic nitrogen. Diazotrophic Cyanobacteria appear to be the major contributor to marine N2 fixation in surface waters of tropical and subtropical oligotrophic oceans, where it may fuel up to ~50% of new primary production. Although this process was previously believed to be rather insignificant in temperate or cold pelagic oceanic regions, recent culture-independent studies reveal the presence of non-cyanobacterial diazotrophs in marine and estuarine environments and suggest its ecological importance throughout the oceans. Our ignorance about the regulation of N2 fixation by non-Cyanobacteria in their natural marine environments currently prevents an evaluation of their importance in marine N cycling and budgets.

In this context, sinking particles, which are responsible for carbon and nutrient transport from the surface ocean into the deep sea and serve as an important part of the food chain in marine ecosystems, are considered hot spots for bacterial activity and N2 fixation, however, the extent to which these N2 fixers are present and exported on sinking particles, amount of fixed N2, and their role in regulating the composition of the particle is not well known.

Heterotrophic bacteria degrade marine snow particles as they descend in the water column, an activity that causes changes in the C:N ratio of the particle, in the amount of available carbon, and in local oxygen conditions. A number of studies have hypothesized that marine snow particles are important loci for nitrogen fixation by heterotrophic bacteria. Using trait-based modeling approach, I am trying to answer the following questions: Can nitrogen fixation be reconciled with the dynamic environmental conditions on particles? How does the microbial activity change the composition of the particle and the local oxygen conditions? When does it become profitable to perform N2-fixation for the bacteria? Which conditions are suitable for N2-fixation, and for how long are these conditions maintained?

During my visit, I look forward to collaborating with Mick Follows. With his interest in understanding how the interactions of physical, chemical and biological processes modulate the structure and function of marine microbial communities and regulate the oceanic cycles of carbon and nutrient elements on global and climate scales, and his interest in understanding the processes and importance of N2 fixation in the ocean, I am excited to be able benefit from his experience of using idealized theory, numerical models and observed data to connect cellular-scale processes to global microbial community structure to predict the amount of global N2 fixation by heterotrophic bacteria associated with sinking particles.



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