Demonstrating the roles of Ni availability on controlling nitrogen fixation and hydrogen production in marine diazotrophic cyanobacteria. Ni is an essential cofactor in Ni superoxide dismutase (SOD) & Ni-Fe uptake hydrogenase, two enzymes responsible for removing superoxides and regulating hydrogen cycling in some marine diazotrophic cyanobacteria, respectively. Applying trace metal defined culture techniques (Ho et al. 2003), my group has demonstrated the essential role of Ni on nitrogen fixation and hydrogen production (Ho 2013; Rodriguez and Ho 2014; Rodriguez and Ho 2017; Tuo et al. 2019). Our most recent study found that Ni limitation results in one order of magnitude higher hydrogen accumulation rates in the low Ni than high Ni treatments (Tuo et al. 2019). We propose that Ni deficiency decreases hydrogenase expression and leads to hydrogen accumulation and nitrogen fixation reduction in marine diazotrophic cyanobacteria. We are currently evaluating our custom-made antibodies for Ni-SOD and Ni-Fe hydrogenase and shall apply them to investigate how Ni availability or other environmental factors influence cyanobacterial nitrogen fixation and hydrogen production in the ocean.

Proving the importance of anthropogenic aerosol deposition on trace metal cycling in the surface water of the Northwestern Pacific Ocean (NWPO). We have pioneered studies for the impacts of anthropogenic aerosol deposition on trace metal cycling in the oceanic regions. Illustrated by the distribution patterns of trace metal composition and their ratios in seawater, plankton, suspended and sinking particles, and aerosols, our series field studies have demonstrated that anthropogenic aerosols are the major trace metal source in the surface water of the South China Sea, Western Philippine Sea, the Kuroshio, and the NWPO (e.g., Ho et al. 2007; Liao et al. 2017; Liao and Ho 2018). The impact shall not only be on trace metal cycling but also on phytoplankton community structure and material cycling in the ocean.

Applying trace metal isotopic composition as proxies to study trace metal and material cycling in the ocean. Trace metal isotopic composition are powerful tracers and proxies to study physical and biogeochemical processes in the ocean. For example, Yang et al. (2018) found that Cd isotope fractionation can match either a closed or open system Rayleigh fractionation model, depending on the relative contribution of physical and biogeochemical processes on its cycling. Liao et al. (2020) found that anthropogenic aerosol deposition may play an important role in causing the variations of its elemental and isotopic composition in oceanic surface water globally. A series of trace metal isotopic studies on Fe and Ni are currently ongoing in our laboratory.

Investigating trace metal requirement in Symbiodiniaceae. Our systematic and seminal studies have obtained fundamental understanding for trace metal requirement in Symbiodiniaceae (Rodriguez et al. 2016; Rodriguez and Ho 2017, 2018; Reich et al. 2020). We found that Symbiodiniaceae possesses high Fe and Zn requirement. Our most recent studies have indicated that Fe requirement in Symbiodiniaceae may be significantly elevated to sustain their growth at relatively high ambient seawater temperature (Reich et al. in prep.). We are carrying out experiments to demonstrate whether Fe availability is an important factor affecting coral bleaching in coral reef ecosystem under global warming.