Use of phytoplankton‐derived dissolved organic carbon by different types of bacterioplankton

TLDR

Phytoplankton and heterotrophic prokaryotes form a tightly linked microbial food web, yet the quantitative carbon flow between them—captured as interaction strength—has not been measured. The study aims to quantify how selectively bacteria respond to specific algal dissolved organic carbon. Microautoradiography was employed to measure the preferences of major heterotrophic prokaryote groups for DOC from various phytoplankton species, yielding interaction strength values. Interaction strengths were skewed toward weak links, with occasional high specificity, indicating that weak interactions are common and help sustain the high diversity of marine bacterioplankton.

Abstract

Summary Phytoplankton and heterotrophic prokaryotes are major components of the microbial food web and interact continuously: heterotrophic prokaryotes utilize the dissolved organic carbon derived from phytoplankton exudation or cell lysis (DOCp), and mineralization by heterotrophic prokaryotes provides inorganic nutrients for phytoplankton. For this reason, these communities are expected to be closely linked, although the study of the interactions between them is still a major challenge. Recent studies have presented interactions between phytoplankton and heterotrophic prokaryotes based on coexistence or covariation throughout a time‐series. However, a real quantification of the carbon flow within these networks (defined as the interaction strength, IS) has not been achieved yet. This is critical to understand the selectivity degree of bacteria responding to specific algal DOCp. Here we used microautoradiography to quantify the preferences of the major heterotrophic prokaryote phylogenetic groups on DOC derived from several representative phytoplankton species, and expressed these preferences as an IS value. The distribution of the ISs was not random but rather skewed towards weak interactions, in a similar way as the distributions described for stable complex non‐microbial ecosystems, indicating that there are some cases of high specificity on the use of specific algal DOCp by some bacterial groups, but weak interactions are more common and may be relevant as well. The variety of IS patterns observed supports the view that the vast range of different resources (different types of organic molecules) available in the sea selects and maintains the high levels of diversity described for marine bacterioplankton.