We have successfully completed the second year of field samplings of the project “Beach biodiversity and the ecological role of the invasive species Rugulopteryx okamurae in beach ecosystem functioning” Project Excel_01050, lead by Ivan Franco Rodíl.

During the second year, we contributed in measuring the bacterial diversity and leaching of dissolved organic carbon associated with accumulation of the invasive algae on different beaches of the coast of Cádiz.

The samples are currently being analysed but they show already how the organic matter of the beach racks alters the microbial benthic community along the beach transect. Hopefully the results will contribute to the management of this environmental issue that affects the local economy (fisheries, tourism).

Andalusia has a high diversity of wetlands. These include intertidal shallow systems, which are important both ecologically and economically, given that the microalgae that grow at the sediment surface are an important source of food for higher-level organisms such as fish, oysters etc.

The microphytobenthos (MPB) is the photosynthetic community living on the sediment surface. They play an important role in shallow estuaries as they can be responsible for a large part of the primary production. This community is dominated by diatoms (algae covered in a silicate casing, called the frustule) and can achieve very high densities giving a golden colour at the sediment surface.

In order to deal with the highly fluctuating environments conditions that characterize the intertidal areas, such as light, temperature, tide cycle and seasonal variations, diatoms have developed several strategies. For example, they have the capacity to migrate vertically during the day to adapt to the light conditions and tidal variation.

Marika Mecca, one of our PhD students, is trying to understand the effects of climate change on the physiology and behavior of these benthic microalgae. Under normal conditions, the sediment surface undergoes large changes in temperature between night and day, under water (immersion) or exposed to the air (emersion), and between seasons. Thus, under a climate change scenario where heatwaves are more frequent, the shifts in temperature will be even larger and the effects on the physiology of the microalgae can be detrimental.

To study these effects, Marika uses various instruments and sensors in the lab, such as oxygen microsensors to measure the amount of oxygen produced, temperature sensors to measure the variations vertically, reflectance sensors to detect the migration rhythms etc.

The principal area of study is the Guadalquivir River. Once the cores are collected in the field, back at the laboratory these sediments are subjected to different conditions simulating the changes observed in the field under normal and extreme weather conditions.

The results show an effect of prolonged high temperatures on the migration rhythm of the microalgae (avoidance of the sediment surface), although differences in total net metabolism are less probably due to a stimulation of rates by the higher temperature. These results were presented at the ASLO 2023 conference in Mallorca.

Oxygen measurement

Oxygen is the most advantageous electron acceptor during the process of respiration. That is why all higher organisms use oxygen to breathe. The oxygen is also involved in the re-oxidation of most reduced compounds produced by other types of anaerobic respirations (e.g. H₂S, Fe²⁺, Mn²⁺). In addition, during daytime, photosynthesis by benthic microalgae produces oxygen Therefore, by quantifying oxygen consumption rates, we can have a net measure of both aerobic and anaerobic respiration processes, and of photosynthesis during light, occurring in the sediment.

How do we measure it?

In our laboratory, we usually use two different type of techniques, one using microsensors to measure vertical profiles of oxygen across the sediment water interface and with depth in the sediment and another by measuring the changes in oxygen concentrations in closed core incubations over time.

These methods provide similar but complementary information.

Microsensors

Profiles made with microsensors provide important information as to the total flux of oxygen across the sediment water interface, but also to the deeper layers, the depth of maximum oxygen production in light corresponding to the layer of microalgae, and the depth of the oxic zone (when oxygen concentration goes to zero). However, given that this method is based on a microsensor, fluxes measured are solely diffusive (movements of oxygen at a microscale) and are very localized to a small area around the tip of the sensor (a few microns).

Whole core incubations

Whole core incubations and measurements of the oxygen changes, provides a more integrated estimation of the oxygen flux as it includes a larger area, the consumption due not only of the microorganisms but also of larger macroorganisms (worms, molluscs, crustaceans etc). It also allows to include the effect of another process, that of advection that moves the oxygen molecules at larger spatial scales (for example by waves, the ventilation of burrows etc). This is why, fluxes measured by whole core incubation are called sometimes total fluxes.

To do this, we close the core (cylinder) containing our sediment ecosystem. On the inside of the lid there is a gel that contains a special dye that reacts with oxygen. Using an optical fibre we can measure through the transparent lid the changes in the fluorescence of the compound as the oxygen concentration changes inside the core. Quite cool..

However, both of them allow us to study the sediment metabolism and depending on the specific aims of our experiment, we use one or the other, or both. The more tools, the better!