American lobster postlarvae alter gene regulation in response to ocean warming and acidification.

Anthropogenic carbon emissions released into the atmosphere is driving rapid, concurrent increases in temperature and acidity across the world's oceans. Disentangling the interactive effects of warming and acidification on vulnerable life stages is important to our understanding of responses of marine species to climate change. This study evaluates the interactive effects of these stressors on the acute response of gene expression of postlarval American lobster (Homarus americanus), a species whose geographic range is warming and acidifying faster than most of the world's oceans. In the context of our experiment, we found two especially noteworthy results: First, although physiological end points have consistently been shown to be more responsive to warming in similar experimental designs, our study found gene regulation to be considerably more responsive to elevated pCO2. Furthermore, the combined effect of both stressors on gene regulation was significantly greater than either stressor alone. Using a full factorial experimental design, lobsters were raised in control and elevated pCO2 concentrations (400 ppm and 1,200 ppm) and temperatures (16°C and 19°C). A transcriptome was assembled from an identified 414,517 unique transcripts. Overall, 1,108 transcripts were differentially expressed across treatments, several of which were related to stress response and shell formation. When temperature alone was elevated (19°C), larvae downregulated genes related to cuticle development; when pCO2 alone was elevated (1,200 ppm), larvae upregulated chitinase as well as genes related to stress response and immune function. The joint effects of end-century stressors (19°C, 1,200 ppm) resulted in the upregulation of those same genes, as well as cellulase, the downregulation of calcified cuticle proteins, and a greater upregulation of genes related to immune response and function. These results indicate that changes in gene expression in larval lobster provide a mechanism to respond to stressors resulting from a rapidly changing environment.

Coccolith dissolution within copepod guts affects fecal pellet density and sinking rate.

The most common biomineral produced in the contemporary ocean is calcium carbonate, including the polymorph calcite produced by coccolithophores. The surface waters of the ocean are supersaturated with respect to calcium carbonate. As a result, particulate inorganic carbon (PIC), such as calcite coccoliths, is not expected thermodynamically to dissolve in waters above the lysocline (~4500-6000 m). However, observations indicate that up to 60-80% of calcium carbonate is lost in the upper 500-1000 m of the ocean. This is hypothesized to occur in microenvironments with reduced saturation states, such as zooplankton guts. Using a new application of the highly precise 14C microdiffusion technique, we show that following a period of starvation, up to 38% of ingested calcite dissolves in copepod guts. After continued feeding, our data show the gut becomes increasingly buffered, which limits further dissolution; this has been termed the Tums hypothesis (after the drugstore remedy for stomach acid). As less calcite dissolves in the gut and is instead egested in fecal pellets, the fecal pellet sinking rates double, with corresponding increases in pellet density. Our results empirically demonstrate that zooplankton guts can facilitate calcite dissolution above the chemical lysocline, and that carbon export through fecal pellet production is variable, based on the feeding history of the copepod.