Nutrient Cycling

Nutrients are the inorganic building blocks that producers need to grow: nitrogen to build proteins and DNA, phosphorus to build cell membranes and energy molecules. In miniBIOTA, the total pool of biologically available nitrogen and phosphorus is fixed. No new nutrients enter from outside, and what leaves the biological cycle through burial in the substrate or loss as gas must eventually be replenished by decomposition or it is simply gone. How the system manages this finite nutrient budget, through uptake, excretion, decomposition, and the transformations driven by the substrate microbial community, shapes the growth rate and composition of every producer in the system.

Nutrient cycling describes the movement of essential inorganic elements, primarily nitrogen (N) and phosphorus (P), through the biological and chemical components of an ecosystem. Unlike carbon, which moves through a gas phase as CO2 and is tied to the energy cycle, nitrogen and phosphorus cycle primarily as dissolved ions in water and soil, with nitrogen also passing through a gas phase at specific transformation steps.

Nitrogen passes through several chemical forms with distinct biological roles and availabilities. Organic nitrogen in dead tissue is released as ammonium (NH4+) during decomposition (ammonification). Nitrifying bacteria in aerobic zones convert ammonium first to nitrite (NO2-) and then to nitrate (NO3-) through nitrification. Both ammonium and nitrate can be taken up by plants and algae. In anaerobic zones, denitrifying bacteria convert nitrate back to nitrogen gas (N2) through denitrification, removing it from the biological cycle entirely. N2 can only re-enter the biological cycle through nitrogen fixation, carried out by specific bacteria and cyanobacteria.

Phosphorus has no gas phase. It moves between organic forms (in living tissue and dead matter), dissolved inorganic phosphate (PO43-, available to producers), and adsorbed or mineral-bound forms (attached to substrate particles and temporarily or permanently unavailable). Decomposition releases phosphate from organic matter; plants and algae take it up; organisms excrete it in feces and urine; and it can bind to substrate particles, especially under high-pH or high-iron conditions. Unlike nitrogen, phosphorus cannot leave the biological cycle as a gas, but it can be locked in the substrate in forms that are not biologically accessible.

Nitrogen and phosphorus are the two nutrients most commonly limiting primary production in natural ecosystems. Freshwater systems are most often phosphorus-limited; marine and estuarine systems are more often nitrogen-limited; but both nutrients are important in complex systems with multiple producer types.

Florida's freshwater systems face a well-documented nutrient challenge. Agricultural and urban runoff loads Florida's lakes and rivers with excess nitrogen and phosphorus, driving the algal blooms and cyanobacterial mats that characterize hypereutrophic systems across the state. Florida's seagrass meadows are sensitive to elevated nutrients: high nitrogen availability favors epiphytic algae growth on seagrass leaves, shading them and reducing photosynthesis at a smaller scale than the water-column turbidity effect.

Florida's mangrove forests fix nitrogen through microbial activity in their root zones, adding biologically available nitrogen to nutrient-poor coastal soils. Seagrass roots and rhizomes stabilize sediments that would otherwise release bound phosphorus under disturbance. These nutrient dynamics are tightly linked to habitat health in Florida's coastal systems.

In a sealed enclosure, nutrient cycling operates under constraints that make certain dynamics far more consequential than they would be in open natural systems.

The total nutrient pool is fixed. All the nitrogen and phosphorus in miniBIOTA entered with the founding organisms, water, and substrate, and with any organisms or water added since. No new nutrients enter from outside. This means the system is running on a recycled nutrient budget; every molecule of nitrogen or phosphorus available to producers today has already cycled through the system at least once.

Denitrification is a potential nitrogen sink with no offset. In the developing anaerobic zones of the Freshwater Lake substrate, denitrifying bacteria convert nitrate to N2 gas. This N2 is biologically inert and escapes into the enclosure atmosphere, where it is not available to any organism in miniBIOTA. The only way to return atmospheric N2 to a biologically usable form is nitrogen fixation, which requires specialized bacteria or cyanobacteria. Whether nitrogen-fixing organisms are present and active in miniBIOTA is unknown. If denitrification is occurring and nitrogen fixation is absent or insufficient, the total pool of biologically available nitrogen in miniBIOTA is slowly declining.

Phosphorus can be locked in the substrate. Phosphate released from decomposing organic matter can adsorb to substrate particles, particularly under conditions of high pH, high dissolved oxygen, or high iron concentrations. If phosphorus accumulates in the substrate in bound form rather than staying in the dissolved pool, it becomes unavailable to producers unless substrate disturbance, pH change, or anoxia releases it. Crayfish digging and Malaysian Trumpet Snail burrowing may both periodically release bound substrate phosphorus back into the water column.

Nutrient concentration is higher than in comparable natural systems. Because there is no dilution by external water input and no export pathway, nutrients released by decomposition remain within the biome at high local concentrations. A pulse of decomposition following a die-off of plant material could temporarily elevate nutrient concentrations significantly, potentially favoring opportunistic algae and phytoplankton over macrophytes.

Cross-biome nutrient movement is real but uncontrolled. Rain drainage from the Lowland Meadow carries dissolved nitrogen and phosphorus from terrestrial decomposition and arthropod excretion into the Freshwater Lake. There is no mechanism to regulate this input; nutrients move with water following gravity.

Freshwater Lake is the nutrient processing hub of the aquatic system. Dissolved inorganic nitrogen and phosphorus in the water column are taken up by tapegrass, sagittaria, Amazon sword, phytoplankton, and biofilm. Nutrients released by consumer excretion and decomposition in the water column are immediately available for uptake. The substrate is the dominant nutrient reservoir: organic nitrogen and phosphorus accumulate in the detritus layer and are slowly released through decomposition. The anaerobic zone is the primary site where denitrification may be occurring, potentially acting as a slow drain on the biologically available nitrogen pool.

Lowland Meadow cycles nutrients rapidly through its grass and forb community. Plants take up nitrogen and phosphorus from the soil; grasshoppers and crickets concentrate nutrients in their biomass; cockroaches, isopods, and millipedes release nutrients from plant litter through decomposition. Arthropod frass is a concentrated nitrogen and phosphorus source that releases nutrients quickly. Rain drainage transports dissolved nutrients from the Lowland Meadow soil into the Freshwater Lake, representing a net export of terrestrial nutrients to the aquatic system with each rain event.

Mangrove Forest cycles nutrients slowly through its resistant leaf litter. Nutrients bound in mangrove leaf tissue are released gradually as the litter decomposes over weeks to months. The forest floor invertebrate community (cockroaches, isopods, millipedes) fragments litter and accelerates nutrient release, but the slow decomposition rate of mangrove material means nutrients are held in the litter layer longer than in the Lowland Meadow. Mangrove root zones in natural systems often support nitrogen-fixing bacteria; whether this is occurring in miniBIOTA is unknown.

Seagrass Meadow cycles nutrients through its seagrass-macroalgae-grazer community. Seagrasses take up dissolved nitrogen and phosphorus through both their roots and leaves. Macroalgae also compete for dissolved nutrients, and elevated nutrient availability in the water column can favor macroalgae over seagrasses by supporting epiphytic algae growth on seagrass leaves. Nutrient availability is therefore one of the variables influencing the seagrass-macroalgae competitive balance, alongside PAR delivery and grazing pressure.

Lakeshore and Marine Shore cycle nutrients at small scale through biofilm turnover and grazing by snails, periwinkles, and other edge-biome invertebrates. These biomes receive nutrient inputs from adjacent biomes through water movement and animal activity.

Primary producers (tapegrass, seagrasses, terrestrial grasses, macroalgae, phytoplankton, biofilm) are the biological uptake pathway: they remove dissolved inorganic nitrogen and phosphorus from the water and soil, incorporating them into organic molecules and making them unavailable to other producers until the plant tissue is consumed or decomposed.

Consumers and excreters (Slough Crayfish, snails, amphipods, Ghost Shrimp, insects) release dissolved inorganic nitrogen (primarily ammonium) and phosphorus through excretion and feces. Consumer excretion is a faster nutrient recycling pathway than decomposition: nutrients excreted by an animal feeding on plant tissue can become available to producers within hours, much faster than the days to weeks required for full microbial decomposition of the same plant material.

Substrate disturbers (Slough Crayfish, Malaysian Trumpet Snails) physically disrupt the substrate, releasing nutrients bound to particles or trapped in anaerobic layers. Slough Crayfish digging is particularly relevant: it can expose buried organic matter to aerobic decomposition and release phosphorus adsorbed to substrate particles.

Microbial community drives the nitrogen transformations that determine how much nitrogen is in plant-available form. Nitrifying bacteria in aerobic substrate layers convert ammonium to nitrate; denitrifying bacteria in anaerobic layers convert nitrate to N2; nitrogen-fixing bacteria (if present) convert N2 back to ammonium. These three processes together determine the nitrogen budget.

Rain System transports dissolved nitrogen and phosphorus from terrestrial decomposition and excretion into the Freshwater Lake with each rain event. The terrestrial nutrient pulse delivered by rain drainage is the primary cross-biome nutrient transfer mechanism.

Climate System affects nutrient cycling through its control of enclosure temperature. Nitrification and denitrification are both microbially mediated and temperature-sensitive; warmer conditions accelerate both. The chiller repair may be affecting nutrient transformation rates in the substrate, though no measurements exist to confirm this.

Lighting System drives primary production, which is the biological nutrient uptake pathway. Without producers taking up dissolved nutrients, inorganic nitrogen and phosphorus would accumulate in the water column. The rate of nutrient uptake is therefore directly linked to PAR delivery and the health of the producer community.

Nitrogen budget and potential denitrification loss (unresolved): The developing anaerobic zones in the Freshwater Lake substrate create conditions for denitrification, where nitrate is converted to N2 gas by anaerobic bacteria. N2 is biologically inert and cannot re-enter the biological cycle without nitrogen fixation. If denitrification is occurring and nitrogen fixation is absent or insufficient to compensate, the total pool of biologically available nitrogen in miniBIOTA is slowly shrinking. Over years of operation, this could progressively nitrogen-limit primary production. Whether this is occurring, and at what rate, has not been measured.

Phosphorus adsorption and substrate lock-up (unresolved): Phosphate released from organic matter in the Freshwater Lake substrate can adsorb to sand and clay particles, particularly under aerobic, higher-pH conditions. If phosphorus is progressively binding to substrate particles rather than remaining in the dissolved pool, it becomes unavailable to producers unless disturbance, pH change, or the shift to anoxic conditions releases it. Crayfish digging and Malaysian Trumpet Snail burrowing may periodically release some of this bound phosphorus, but the balance is unknown.

Nutrient pulse risk from decomposition lag (watch): If a significant fraction of organic matter in the Freshwater Lake substrate decomposes rapidly in response to a substrate disturbance or temperature shift, it could release a concentrated pulse of dissolved nitrogen and phosphorus into the water column. In a small closed system, this could temporarily elevate nutrients to levels that favor phytoplankton and opportunistic algae over macrophytes, driving the system toward a turbid state. The Slough Crayfish's digging behavior is a potential trigger for localized nutrient pulses of this kind.

Seagrass-macroalgae competition and nutrient availability (unresolved): In the Seagrass Meadow, macroalgae and epiphytic algae generally benefit more from elevated dissolved nutrient concentrations than seagrasses do. Whether current dissolved nutrient levels in the Seagrass Meadow favor seagrasses or macroalgae, and how this compares to the influence of PAR delivery and grazing pressure, cannot be determined without nutrient measurements.