Dissolved organic matter (DOM) release by phytoplankton in the contemporary and future ocean

Abstract

The partitioning of organic matter (OM) between dissolved and particulate phases is an important factor in determining the fate of organic carbon in the ocean. Dissolved organic matter (DOM) release by phytoplankton is a ubiquitous process, resulting in 2-50% of the carbon fixed by photosynthesis leaving the cell. This loss can be divided into two components: passive leakage by diffusion across the cell membrane and the active exudation of DOM into the surrounding environment. At present there is no method to distinguish whether DOM is released via leakage or exudation. Most explanations for exudation remain hypothetical; as while DOM release has been measured extensively, there has been relatively little work to determine why DOM is released. Further research is needed to determine the composition of the DOM released by phytoplankton and to link composition to phytoplankton physiological status and environmental conditions. For example, the causes and physiology of phytoplankton cell death are poorly understood, though cell death increases membrane permeability and presumably DOM release. Recent work has shown that phytoplankton interactions with bacteria are important in determining both the amount and composition of the DOM released. In response to increasing CO 2 in the atmosphere, climate change is creating increasingly stressful conditions for phytoplankton in the surface ocean, including relatively warm water, low pH, low nutrient supply and high light. As ocean physics and chemistry change, it is hypothesized that a greater proportion of primary production will be released directly by phytoplankton into the water as DOM. Changes in the partitioning of primary production between the dissolved and particulate phases will have bottom-up effects on ecosystem structure and function. There is a need for research to determine how these changes affect the fate of organic matter in the ocean, particularly the efficiency of the biological carbon pump.

Protein and Carbohydrate Exopolymer Particles in the Sea Surface Microlayer (SML)

“…It is likely that the collapsing diatom bloom was the major source of exopolymers that produced the TEP and CSP found in both water column and SML. Diatoms release a significant proportion of the carbon that they fix through photosynthesis as exudates, including as TEP and CSP, or their precursors (Engel et al, 2002;Passow, 2002b;Thornton, 2014;Chen and Thornton, 2015). TEP accumulate in cultures of diatoms (Corzo et al, 2000;Claquin et al, 2008;Fukao et al, 2010;Chen and Thornton, 2015) and in diatom-dominated blooms within mesocosm experiments using natural waters (Engel et al, 2002).…”

“…Pseudo-nitzschia is significant as it is a harmful algal bloom species that is known to bloom along the Oregon coast (McKibben et al, 2015). Phytoplankton, and to a lesser extent bacteria, are considered to be the source of TEP and the polysaccharide precursors that form TEP (Passow, 2002a,b;Thornton, 2014). It is likely that the collapsing diatom bloom was the major source of exopolymers that produced the TEP and CSP found in both water column and SML.…”

Cross-depth analysis of marine bacterial networks suggests downward propagation of temporal changes

“…Thus, at some times of year, it both releases more of some kind of dissolved organic matter (through processes reviewed by Thornton, 2014) and forms sinking aggregates (such as in Kiørboe and Hansen, 1993). Surface bacteria that are adapted to break down dissolved organic matter released by plankton X will increase in relative abundance.…”

Microbial Surface Colonization and Biofilm Development in Marine Environments

“…At the start of an algal bloom, when the algal population is small and growing, the algae are healthy and active in synthesizing labile DOM such as simple sugars, sugar alcohols, organic acids, amino acids, and DMSP, some of which may be released into the environment as algal exudates (209,272,758,827). In this phase, algal cells provide localized and concentrated labile organic substrate sources, so surface colonization is advantageous to certain marine microorganisms, such as many MRC bacteria (758), and allows rapid responses to the labile DOM substrates.…”