Heterocapsa rotundata is a mixotrophic dinoflagellate that can ingest picoplankton, including bacteria, and is known to form large blooms in temperate estuaries during wet winters, particularly when grazing pressure on phytoplankton is low. We hypothesized that phagotrophy gives H. rotundata an advantage over other phytoplankton species during low light conditions. We used laboratory and field experiments to investigate changes in phagotrophy by H. rotundata in response to changes in light availability. Prey removal experiments with a non-axenic culture of H. rotundata were used to determine changes in H. rotundata's ingestion rates in response to changes in irradiance. Fluorescent microspheres were used to measure in situ ingestion rates of H. rotundata collected on 20 different occasions from the Choptank River during the winter of 2016. In situ H. rotundata ingestion rates were tested for correlation with inorganic nutrient concentrations and irradiance levels. Ingestion rates measured with cultured and in situ H. rotundata followed similar patterns and ingestion rates increased as irradiance decreased. H. rotundata has the potential to obtain nutrients from multiple nutrient sources, switching from phototrophy to partial heterotrophy as irradiance decreases. This response may allow H. rotundata to survive and to potentially form blooms when growth rates of most other estuarine phytoplankton species is low.Heterocapsa is a genus of dinoflagellates that contains numerous bloom forming and toxic species (Salas et al. 2014). One particular species, Heterocapsa rotundata (Lohmann) Loeblich (Hansen 1995), is ubiquitous and occasionally forms large blooms. H. rotundata has been reported in a range of environments all over the world including Chesapeake Bay, U. . H. rotundata tends to either dominate or be a prominent part of the phytoplankton community for at least part of the year in some of these areas (Seong et al. 2006;Balzano et al. 2015;Millette et al. 2015). Yet, relatively few studies have focused on the ecology of H. rotundata.Some studies have focused on the formation and decline of H. rotundata winter blooms in Chesapeake Bay tributaries (Cohen 1985; Sellner et al. 1991;Millette et al. 2015). These researchers found abundant H. rotundata blooms in wet, cold winters when salinity is low (Cohen 1985) and when there is a release in grazing pressure from microzooplankton and copepods (Millette et al. 2015). Although wet, cold winters and a release in grazing pressure are factors that impact every phytoplankton species, it is unknown how H. rotundata can take advantage of these conditions over other species to bloom. One possibility is that H. rotundata may use their capacity as a mixotroph to overcome light limitation. This would give H. rotundata an advantage over other phytoplankton in the winter. H. rotundata has been shown to consume heterotrophic bacteria, cyanobacteria such as Synechococcus, and small diatoms (Seong et al. 2006;Jeong et al. 2010). Their bacterial ingestion rate is known to incr...

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Herein, we characterized the digestive gland (‘gut’) bacterial community (microbiome) of the Ohio pigtoe, Pleurobema cordatum (Rafinesque, 1820), using 16S rRNA gene sequencing. Two populations were compared: wild P. cordatum (n = 5) from the Tennessee River and P. cordatum (n = 9) relocated to artificial mesocosms and exposed to various thermal regimes for 2 weeks. We also characterized the bacterial communities from the habitat (water and sediment) of these wild and mesocosm-held populations. The gut microbiome of wild P. cordatum was dominated by members of the bacterial phylum Tenericutes (72%). By contrast, the gut microbiome of mesocosm-held P. cordatum was dominated by members of the bacterial phylum Proteobacteria (64%). We found no temperature-associated difference in the gut microbiome of mesocosm-held P. cordatum. The bacterial communities of water and sediment from the Tennessee River were diverse and distinct from those of the studied mussels. By contrast, the bacterial communities of water and sediment in the mesocosms were dominated by Proteobacteria. These results suggest that when the studied mussels were moved into artificial rearing environments, their gut microbiome shifted to reflect that of their habitat (i.e. an increase in Proteobacteria). Moreover, the abundance of Tenericutes (also previously reported in other unionids) was reduced from 72% in wild mussels to 3% in mesocosm-held mussels. As a result, we think that mesocosm-held P. cordatum became dysbiotic, which could explain the observed wasting syndrome and associated trickling mortalities in captive P. cordatum.

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