Ideas and Perspectives: Value and efficiency of ocean sample return and laboratory analysis: Key analytes are orders of magnitude rarer than a needle in a haystack

Abstract. There is a pressing need to understand how the oceans are responding to human perturbations, yet their vastness poses a considerable challenge. The development and deployment of biologically and ecologically relevant in situ sensors are frequently invoked as a means to improve this understanding; however, there are still relatively few sensors for these parameters due to the analytical challenges involved in their measurement. Specifically, many analytes of interest are orders of magnitude rarer by mass than a needle in a haystack (e.g., dissolved iron and eDNA are 10⁴ to 10⁸ times rarer), and this scarcity creates challenges involving preconcentration, interfering elements or compounds, and analytical methods difficult to perform underwater. While sensor development is important, the role of sample collection and return is also an important, yet oft overlooked, complement for studying ocean change and is ripe for additional development. Laboratory analyses of returned samples can be high throughput for challenging analytes like micronutrients and omics parameters, and these techniques are operational now. Moreover, sample return allows parallel analyses and archiving through splitting of sample material. Similar to human health diagnostics, laboratory analyses can provide ocean health diagnostic capabilities showing how ecosystems are responding, complementing sensor-based vital sign measurements indicating environmental change. Together, these properties make sample collection and return a critical component for achieving the global ocean coverage needed to capture the environmental changes well underway today, and this approach should continue to be factored into ocean observing strategies.