An Ancient Enzyme Finds a New Home: Prevalence and Neofunctionalization of Trypsin in Marine Phytoplankton

Trypsin is an ancient protease best known as a digestive enzyme in animals, and traditionally believed to be absent in plants and protists. Here, we surveyed the distribution, diversity, evolution and potential functions of trypsin genes in global ocean phytoplankton, the major primary producers in the aquatic ecosystem. Our analysis indicates that trypsin genes are widely distributed both taxonomically and geographically in marine phytoplankton. Furthermore, by systematic comparative analyses we documented lineage-specific diversity and expansion of trypsin genes in the evolution of marine phytoplankton. Genome-wide analyses revealed that trypsin genes were more prevalent in diatoms than in other lineages. Moreover, the expression of trypsin genes in diatom tended to be more responsive to environmental stimuli. The duplication and neofunctionalization of trypsin genes may be important in diatoms to adapt to dynamical environmental conditions, contributing to diatoms’ dominance in the coastal oceans. This work advances our knowledge on the distributions and neofunctionalizations of this ancient enzyme and creates a new research direction in the phytoplankton biology.

Deriving VIIRS High-Spatial Resolution Water Property Data over Coastal and Inland Waters Using Deep Convolutional Neural Network

The Visible Infrared Imaging Radiometer Suite (VIIRS) onboard the Suomi National Polar-orbiting Partnership (SNPP) satellite has been a reliable source of ocean color data products, including five moderate (M) bands and one imagery (I) band normalized water-leaving radiance spectra nLw(λ). The spatial resolutions of the M-band and I-band nLw(λ) are 750 m and 375 m, respectively. With the technique of convolutional neural network (CNN), the M-band nLw(λ) imagery can be super-resolved from 750 m to 375 m spatial resolution by leveraging the high spatial resolution features of I1-band nLw(λ) data. However, it is also important to enhance the spatial resolution of VIIRS-derived chlorophyll-a (Chl-a) concentration and the water diffuse attenuation coefficient at the wavelength of 490 nm (Kd(490)), as well as other biological and biogeochemical products. In this study, we describe our effort to derive high-resolution Kd(490) and Chl-a data based on super-resolved nLw(λ) images at the VIIRS five M-bands. To improve the network performance over extremely turbid coastal oceans and inland waters, the networks are retrained with a training dataset including ocean color data from the Bohai Sea, Baltic Sea, and La Plata River Estuary, covering water types from clear open oceans to moderately turbid and highly turbid waters. The evaluation results show that the super-resolved Kd(490) image is much sharper than the original one, and has more detailed fine spatial structures. A similar enhancement of finer structures is also found in the super-resolved Chl-a images. Chl-a filaments are much sharper and thinner in the super-resolved image, and some of the very fine spatial features that are not shown in the original images appear in the super-resolved Chl-a imageries. The networks are also applied to four other coastal and inland water regions. The results show that super-resolution occurs mainly on pixels of Chl-a and Kd(490) features, especially on the feature edges and locations with a large spatial gradient. The biases between the original M-band images and super-resolved high-resolution images are small for both Chl-a and Kd(490) in moderately to extremely turbid coastal oceans and inland waters, indicating that the super-resolution process does not change the mean values of the original images.