How The Environmental RNA Signature of Nitrate Reductase Can Be Used as a Predictor for Phytoplankton Distribution

Abstract

With anthropogenic climate change altering the ocean, it is critical to understand what variables control phytoplankton distribution so we can predict how distributions will be altered as result of climate change. This is vital because phytoplankton are important for primary productivity, as well as acting as the base of the marine food web supporting fisheries. This study aimed to determine whether the environmental RNA signature of Nitrate Reductase (NR) from phytoplankton is correlated with in-situ nitrate concentration. First, a phylogeny of phytoplankton NR amino acid sequences was generated to determine if the sequences separate into monophyletic groups matching their taxonomic identification. Secondly, RNA transcripts for NR derived from samples collected in the North Pacific Ocean were placed on the tree to determine which clades expressed NR. Finally, heat maps were constructed to show RNA abundance for each phytoplankton clade by latitude and nitrate concentration to establish distribution trends by phylum. All phytoplankton, no matter phyla or size, were more abundant in higher nitrate concentrations. When RNA abundance was normalized by chlorophyll concentrations, there was little separation in distribution based on plankton size, and different trends based on phyla emerged. Alveolata, Archaeplastida, and Stramenopiles were found in similar low to moderate nitrate concentrations (0.0023-0.8052μM). Cryptista did not display a consistent trend across the phylum, as all clades displayed different abundance patterns. Haptophyta, large and small, made up a significantly higher proportion of phytoplankton found in low nitrate environments (0.0009μM). These results indicate there is a separation of phytoplankton phyla by nitrate concentration, supporting the hypothesis that these phyla have evolved to utilize different ecological niches, however further research is needed with higher taxonomic resolution to fully quantify the factors controlling the distribution of different phytoplankton clades.