Pelagic tunicate grazing on marine microbes revealed by integrative approaches

Abstract

Marine microorganisms comprise a large fraction of ocean carbon and are central players in global biogeochemical cycling. Significant gaps remain, however, in our understanding of processes that determine the fate, distribution, and community structure of microbial communities. Protists and viruses are accepted as being part of the microbial loop and a source of microbial mortality. However, pelagic tunicates (salps, doliolods, pyrosomes, and appendicularians), which are abundant in oceanic and coastal environments and consume microorganisms with higher individual grazing rates than other common grazers, remain underappreciated in their role controlling microbial communities, distributions, and flux through ecosystems. In spite of sampling challenges owing to their fragile nature and patchy distributions, recent developments in methodology have deepened understanding of grazing rates and selectivity of these ubiquitous grazers. Next-generation sequencing, quantitative polymerase chain reaction, high-resolution videography, improved microscopy, biomarkers, and in situ approaches are transforming our knowledge on the role of pelagic tunicates in determining the fate and function of microbial communities. Here, we review recent research on pelagic tunicate grazing with a focus on newer methodologies and their application across pelagic tunicate taxa. Synthesis of these studies points to a major role for pelagic tunicates in the control of marine microbial communities. Comparisons between pelagic tunicate taxa indicate important differences in prey selectivity, which will impact how these grazers are incorporated into global models. Application and integration of these methods will produce continued insights with the ultimate goal of illuminating the unique role of pelagic tunicates in the microbial loop and biological pump.