Abstract
Even though the uptake and assimilation of organic compounds by phytoplankton has been long recognized, very little is still known about its potential ecological role in natural marine communities and whether it varies depending on the light regimes the algae experience. We combined measurements of size-fractionated assimilation of trace additions of 3H-leucine and 35S-dimethylsulfoniopropionate (DMSP) with microautoradiography to assess the extent and relevance of osmoheterotrophy in summer phytoplankton assemblages from Arctic and Antarctic waters, and the role of solar radiation on it was further investigated by exposing samples to different radiation spectra. Significant assimilation of both substrates occurred in the size fraction containing most phytoplankton (>5 µm), sunlight exposure generally increasing 35S-DMSP-sulfur assimilation and decreasing 3H-leucine assimilation. Microautoradiography revealed that the capacity to take up both organic substrates seemed widespread among different polar algal phyla, particularly in pennate and centric diatoms, and photosynthetic dinoflagellates. Image analysis of the microautoradiograms showed for the first time interspecific variability in the uptakes of 35S-DMSP and 3H-leucine by phytoplankton depending on the solar spectrum. Overall, these results suggest that the role of polar phytoplankton in the utilization of labile dissolved organic matter may be significant under certain conditions and further confirm the relevance of solar radiation in regulating heterotrophy in the pelagic ocean.
Highlights
The ability to take up and utilize dissolved organic matter (DOM) as a source of carbon and energy was demonstrated for a wide variety of algal cultures more than three decades ago, yet it was initially thought to be ecologically irrelevant due to the inability of the algae to compete with bacteria at the low substrate concentrations found in natural environments [4,5,6]
Owing to their high surface to volume ratio and their efficient uptake systems, heterotrophic bacteria are regarded as the most efficient consumers of DOM [5], and phytoplankton osmoheterotrophy is neglected in most geochemical models of carbon flow [7,8]
While most studies on phytoplankton osmoheterotrophy have focused on algal cultures and on freshwater or benthic systems, pelagic marine environments have received less attention and very little is known about the role of algal osmoheterotrophy in natural marine communities
Summary
The ability to take up and utilize dissolved organic matter (DOM) as a source of carbon and energy (hereafter ‘osmoheterotrophy’) was demonstrated for a wide variety of algal cultures more than three decades ago (see refs. in [1,2,3]), yet it was initially thought to be ecologically irrelevant due to the inability of the algae to compete with bacteria at the low substrate concentrations found in natural environments [4,5,6]. It was discovered that a variety of marine phytoplankton taxa can take up the ubiquitous algal synthate dimethylsulfoniopropionate (DMSP) [15,16] and assimilate its sulfur [15], influencing the cycling of organic sulfur in the surface ocean These evidences, together with the phagotrophy described for many algal groups [17,18], suggest that algae may play a more diverse role in aquatic biogeochemical cycles than just supplying heterotrophs with autotrophically synthesized organic matter. While most studies on phytoplankton osmoheterotrophy have focused on algal cultures (which may not be representative of ecologically relevant organisms) and on freshwater or benthic systems, pelagic marine environments have received less attention and very little is known about the role of algal osmoheterotrophy in natural marine communities
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